'Greening' Lab Design

Laboratory fume hoods are energy intensive. In order to provide safety for their operator, they need to ensure a constant face velocity of air at the sash--air that must first be conditioned to keep the space temperature acceptable for comfort, moved via mechanical means to the lab, and then exhausted out of the building.

A common comparison used to highlight the energy costs of these systems is to compare the energy impact of a single fume hood with that of a typical US household. On average a single lab fume hood uses as much energy as three typical US houses. And when you consider that a given facility may have many lab hoods in a single laboratory space, you can see how these energy impacts quickly add up.


In order to minimize the wasted energy associated with these laboratories, high-precision VAV lab controls have been developed to ensure operator safety, and to only provide the minimum amount of air necessary--And great savings have been realized by this sort of measure. But the energy efficiency of these systems can be improved even more.

Once the airflow has been taken down to a minimum, the energy associated with conditioning that air has been greatly reduced. But the energy associated with moving that air still can be reduced further. ASHRAE 90.1 states:

ASHRAE Standard 90.1 - 6.5.3.2.3:
“For systems with direct digital control of individual zone boxes reporting to the central control panel, static pressure setpoint shall be reset based on the zone requiring the most pressure; i.e., the setpoint is reset lower until one zone damper is nearly wide open.”

This calls for static pressure reset for VAV systems to minimize fan energy--ensuring that only the minimum amount of static pressure is provided to move the air. And this strategy is perfectly applicable to laboratory VAV systems as well as commercial air conditioning--as long as the system components are selected appropriately.

Tek-Air has published a white paper entitled Demand Based Static Pressure Reset Control for Laboratories That explores the energy benefits of this type of control scheme.


This paper analyzes system component selection, including control valves and sensors and illustrates the impact of these decisions on the overall energy use of the VAV system. In an analysis of a 50,000 cfm exhaust system, the reduced static from a pressure reset strategy can result in nearly $9,000 per year savings in fan energy (based on 0.75" savings, and $0.06/kwh electric costs).

These sorts of static pressure savings are easily attainable with a wise selection of air valve components. The commonly specified venturi-type valve has a minimum operating pressure that prevents these savings from being realized, and this added pressure drop often creates objectionable noise, which requires even more pressure drop for the system in the form of sound attenuators. This pressure reset strategy requires valves that can operate accurately and safely at low pressures.

The Tek-Air PRD valve provides unmatched pressure performance, and a quick examination of a cross section of the valve shows why:


Each blade of the damper is a smooth airfoil, greatly reducing turbulence and keeping the pressure and acoustic profile of the valve to a minimum.

If pneumatic air is not available, Tek-Air's new Accuvalve provides very similar performance with the convenience of electronic actuation. (And it won an innovation award at the 2008 AHR expo!)



A peek at the cross section of this valve shows how it attains these low pressure drops:


The airfoil shape of the valve assembly assures minimal pressure drop and sound generation for great efficiency in the fan system.

Energy savings cannot come at the cost of safety, and it is imperative that systems utilizing this method of pressure reset have sensors that can operate accurately and effectively in a wide range of pressure regimes. Tek-Air uses vortex shedding flow sensor technology to ensure the most accurate and linear control on the market.

Energy conservation is only going to become a bigger and bigger issue for designers of all building systems, and fume hood systems are a large opportunity for savings. It is important that designers and owners consider all the impacts of their design decisions and their system selections.

(Don't forget about checking the fan for stability: See this article for a review on this issue.)

Rethinking Air-Cooled Chillers

Air Cooled Advantages

Air cooled chillers offer many advantages to owners and designers. The first, and perhaps most compelling for many jobs is lower installed cost. Lower installed costs (compared to water cooled chillers) are driven by the following advantages:

• No Cooling Tower, Tower pumps, Tower and Pump Starters
• No equipment room required for the chillers
• Mounted starters

They also are easier to maintain, since the systems are significantly simpler than water-cooled systems:

• No on site Systems Engineer required
• No water treatment or make up water required
• No leaks on the roof
• No cooling tower, condenser pumps, associated starters

Generally, however, these advantages have come with significant trade-offs: Efficiency and Sound performance.

However, the introduction of Variable Speed oil-free air-cooled chillers by Smardt changes the balance.


First off, the Smardt Chiller is efficient. With IPLV's as low as 0.65 kw/ton, these chillers rival water-cooled system when the parasitic loads of the condenser pumps and cooling tower are considered. These chillers gain their efficiencies both from the inherent efficiency of the Turbocor compressor and the elimination of oil return issues that prevent other air-cooled chillers from capitalizing on the reduced head pressures available at low ambients.

This means these chillers use about 60-65% energy of other air-cooled chillers for the same load, and can nearly eliminate the energy benefit typically provided by moving to water-cooled systems. When you consider the cost of water (nearly $15/1000 gallons in Seattle, including sewer charges) this means the yearly cost of operation of these units is unrivaled. And energy conservation rebates are extremely attractive for these chillers.

The other major traditional trade off with using air-cooled equipment is sound. Screw chillers especially are known for their unfavorable sound characteristics. In most municipalities, sound ordinances are driven by occupancy and time of day. The most stringent criteria must be met during evening hours, typically when the units are not at their peak load. However, with constant-speed systems, the compressor is either on or off. This means it is either putting out its full sound or none at all. At full speed, such compressors can often exceed the evening sound criteria--even if they are on only momentarily. And the staging between on and off can be objectionable in its own right, regardless of sound level.

The Smardt chiller minimizes the problems with compressor sound in two ways. First, the variable speed drive allows the compressor to ramp slowly up and down to match the required output, eliminating the objectionable switching between compressors that constant-speed chillers exhibit. And secondly, they are just extremely quiet to begin with. Since no moving mechanical part is in contact with the chiller casing, very little mechanical noise is transmitted. Ninety-ton Turbocor compressors have been tested at 72 dBa at one meter, compared to screw compressors that can be as high as 80 dBa or higher in the same test. Five of these compressors operating together yield a sound level of 75 dBa at 10’.

More Benefits

But efficiency and sound are not the only benefits from using the Turbocor technology on air-cooled chillers. Other, less obvious ones exist.

Turbocor compressors have only one moving part, yielding un-matched reliability.

Reliability is enhanced by the elimination of oil in the refrigerant system. And the frictionless bearing requires almost no maintenance.

Since Turbocor compressors are variable speed driven, they provide an inherent soft-start on the compressor. Instead of kicking the motor up to full speed when power is applied to the system, the VSD slowly ramps the compressor up to the required speed for the load sensed by the system. This reduces stress on the already greatly simplified system to reduce wear and tear on the components.

But this soft start has another, very important advantage over standard air-cooled chiler systems--the use of the VSD eliminates inrush amperage. When an electrical motor is at rest, there is very little inductive resistance to current flow through the windings. As the motor starts to turn, this inductive resistance increases with the increase in RPM. What this means is when power is applied across the line (or even with a reduced voltage starter) to a stopped motor, there is a spike of electrical current far greater in amplitude than the design amp draw of the motor:

(example graph of inrush on a well pump motor)

This temporary increased amp draw heats the motor beyond where it is designed to operate for extended periods. This forces the chiller designer to provided anti-recycle timers to prevent rapid re-starts that could fatally overheat the motor. In practice, this usually means constant speed compressors cannot be started more often than every half-hour or so.

Additionally, this increased amp draw has effects that need to be addressed electrically. This becomes even more significant if the chillers are being served by emergency power. The emergency generators that serve the chiller must be sized to handle the inrush amperage. This can be a very costly addition, especially since the added amperage is only required for the first 30 second of operation or so.

Generators = $$$

Turbocor compressors on the Smardt air-cooled chillers eliminate inrush and provides a soft-start. This both heightens reliability and reduces electrical costs. For jobs where reliability is a primary concern, like data centers, this technology makes a lot of sense. First, it eliminates the need for increased generator sizing, it is an inherently more reliable compressor, and it frees the cooling system from reliance on a water utility service that could be disrupted.

Aaon Rolls Out 410a Digital Scrolls!

Digital Scrolls offer great advantages over standard scroll compressors for rooftop packaged systems. Aaon was the first manufacturer to offer digital scrolls for their R-22 systems--but the phaseout of that refrigerant is looming.

The latest version of the Aaon Ecat32 software includes new offerings of digital scrolls for selected units using R-410a. These advanced compressors are available in the following units for the HFC refrigerant:

• 230/3/60 – RM-006; RM-013; RN-026
• 460/3/60 – RM-006; RM-007; RM-013; RM-015; RM-016; RN-026; RN-031

Stay tuned for more offerings as Copeland rolls out their R-401A digital scrolls!

Greening Small Rooftop Packaged Units: Heat Recovery

This article on 'greening' rooftop packaged units is the third of the ‘Greening Small Packaged Units’ series and addresses the use of exhaust air heat recovery in these types of systems.

Heat recovery is a well-understood and accepted method of energy conservation. However, the energy saved comes at a cost. Generally, an air conditioning system that has heat recovery capabilities operates with higher pressure drops than a system without heat recovery, and there may be other parasitic loads that are required to run the heat recovery equipment.

Energy codes generally require heat recovery on systems that use a significant amount of outdoor air, since it is a reasonable assumption that on such systems, which have very large ventilation loads, the amount of energy saved will greatly outweigh the additional energy required to operate the heat recovery equipment. However, depending on the operating conditions, there usually are energy benefits for systems that operate with even very minimal outdoor air requirements.

For an owner or designer trying to decide whether heat recovery is right for a particular application, it is important to know what these benefits are in terms of energy cost reductions, payback or return on investment, and, more and more frequently, carbon emission reductions.

For rooftop packaged units, the heat recovery product of choice is the heat wheel. The industry has settled on this product for many reasons, including first cost, footprint, efficiency and layout considerations. Aaon uses the Airxchange wheel, which is an ARI 1060 certified heat recovery device.


As with their rooftop economizers, Aaon provides this efficiency option as an integrated, factory installed option. This greatly reduces on site labor, eases commissioning, and ensures the owner of the energy benefits of their investment.


(If field-installed RTU economizers have a high rate of failure, imagine how often field installed heat recovery wheels are a commissioning problem!)

To aid in the heat recovery analysis, Airxchange has provided a free software program (registration required) to calculate the energy and cost benefits of applying their heat wheels on air-handling systems. This makes it very easy for an engineer to do a bin-data analysis of the benefits of this option. Given a particular heat wheel and some basic information about the RTU it is serving, it will calculate the gross heat recovery for cooling and heating hours, as well as calculate the additional fan energy required to operate the wheel. It will also perform a simple economic analysis calculating a net dollar savings when using the heat wheel.

An analysis of a 16 ton Aaon RM unit (pdf) shows the net energy savings available using a wheel on this type of unit. In the above analysis, a 5,200 CFM supply air system is compared looking at conditions of 100% OA and 30% OA. In both cases the analysis (using Seattle bin data, a 5 day week and typical office hours of operation) shows net energy cost savings, about $500/year on the 30% OA case, and about $1,700/year on the 100% OA case. Almost all of those savings come from the heat required to offset the ventilation load during the winter—the cooling savings are small by comparison.

However, the effect of the wheel on cooling is important in one respect--the use of the heat wheel may allow the designer to reduce the cooling (and, of course, heating) capacity of the RTU. In this example, the wheel adds 1.4 tons and 84 MBH to the cooling and heating capacity of the 30% OA system, and 3.7 tons and 230 MBH to the 100% OA system.

These ‘free’ tons of capacity that you gain by using the heat wheel effectively allows your cooling system to operate at a higher actual IPLV than is calculated in the ARI rating of the unit. ARI has acknowledged this in the publication of ARI Guideline V (Calculating the Efficiency of Energy Recovery Ventilation and Its Effect on Efficiency and Sizing of Building HVAC Systems). This guideline basically defines an efficiency rating for the heat recovery system (RER) and a ‘combined efficiency’ rating (CEF) for the entire system, accounting for the EER of the RTU and the RER of the heat wheel. This CEF is calculated in the Airxchange software linked above


If the goal of a design is not just energy savings, but carbon emission reduction, the wheel’s advantage is obvious. Every btuh that is recovered from the exhaust air is less natural gas that would need to be burned in a gas burner (the most common form of heat for these units in this region). But there is one other powerful way in which wheels can leverage energy savings or reduce carbon emissions: they can be used to greatly increase the applicability of a heat pump cycle for heating operation. In an Aaon unit, the entering air into the refrigerant coil needs to be 45º F or higher for the heat pump system to provide any heat. In the example reviewed above (RM16) the mixed air at a design heating day in Seattle is pre-heated to nearly 50 º F for the 100% OA case—well above the minimum needed for HP operation! And although capacity drops off, an air-source Aaon heat pump will still operate at conditions as low as 17 º F ambient. Converting the system to a water-source HP greatly improves the heat capacity at even the coldest days—and by reducing the amount of heat required from the ground, the use of the heat wheel can help keep ground loop costs down, too!

Converting a system from gas heat to heat pump operation has a large energy and carbon reduction benefit. First, it transfers the heating energy source from a high embodied-carbon fuel to electricity, which in the Pacific Northwest is considered a nearly carbon-free energy source. And it provides an advantage over electricity because, even with heating COP’s on the order of 1.5*, it greatly reduces the amount of utility electricity required to do the same amount of heating.

*at extreme conditions—moderate conditions greatly improve this performance

Greening Small Rooftop Packaged Units: Variable Air Volume

Second in the "Greening Small Rooftop Packaged Units" series.

Variable air volume systems are an accepted energy conservation strategy that has gained wide acceptance in the HVAC industry. And HVAC systems provide other benefits, too, including improved occupant comfort and flexibility.

The energy benefit of VAV systems comes primarily from the ability to reduce fan energy use when the full capacity is not needed. Since the fan system is typically sized at peak load, using a constant volume system means that you essentially waste fan energy for 95% of the operating hours of your system. Since fan power decreases with the cube of the speed (theoretically--motor amp draws at low speeds plateau, reducing savings in practice), the fan savings can be significant.


In fact, ASHRAE considers the potential for energy savings with variable volume systems so great, that they are considering revising standard 90.1 to require this feature on single-zone systems, in addition to the current requirement on multiple zone systems.

But there is a catch for designers using rooftop packaged DX units. Very few manufacturers provide VAV enabled units for smaller tonnages. Below about 20-50 tons, there is very little on the market to service this need. Aaon, on the other hand, offers a full line of VAV units down to capacities as low as 2 tons. And they configure their units to use either air-cooled DX refrigeration, water-cooled DX refrigeration, or chilled water cooling!

Part of the problem with using VAV at smaller tonnages is that for DX systems, the size of the smallest compressor in the system is a considerable portion of the entire cooling load--as much as 100% for single-compressor systems. This means that as you vary the leaving air volume, the capacity of the cooling system stays the same, greatly decreasing the leaving air temperature. In most cases, this will cause the DX coil to frost, which leads to all sorts of problems for the system. This drawback is generally dealt with by installing a hot gas bypass on the first cooling circuit. However, this strategy works against the energy conservation intent of using a VAV system in the first place, since the HGBP imposes a false load on the compressor system, and the compressor draws full amps even at partial load.

In the example below (click here for full pdf of selection), the compressor on a 5-ton VAV unit draws more energy than the supply and exhaust fans together--nearly twice as much!

(click image for larger view)

You can easily see that in some systems a VAV unit operating with a hot gas bypass could actually use more energy than a constant-volume system with simple on-off compressor control. Of course the latter system may cause some comfort problems that the VAV system would avoid, but it would cost you energy to gain the added comfort.

Aaon has elegantly addressed this drawback by their use of digital scroll compressors allowing you to vary compressor capacity linearly to match system load and avoid freezing your coils--and to do so in an extremely energy-efficient manner.

In 2004, the ASHRAE Journal published a study (pdf) that examined possible advances in energy efficiency in rooftop packaged DX units. In it the researchers created a high-efficiency 10 ton unit configuration:

Based on the initial energy and cost analyses, we developed a design configuration incorporating the best design options:
• Increased heat exchanger size to achieve an EER of at least 10.3, consistent with the ASHRAE 90.1-1999 requirement for 10-ton electric-heat rooftop units;
• Variable air volume using an induction motor and inverter;
• Energy recovery wheel (ERW); and
• Economizer.

This unit was also tested in a configuration that included a variable speed compressor similar to the Aaon digital scroll. The researchers concluded that the base unit, without the variable speed scroll, reduced energy costs by 25% compared to a constant-volume unit. The variable speed compressor was shown to further improve the part load performance.

The proposed unit configuration, significantly, is extremely similar to the example unit above. In other words, the 'future energy-efficient unit' of 2004 is available as an Aaon catalog unit today!

High Efficiency VAV Unit of the Future

Aaon High Efficiency VAV unit of Today

Greening Small Rooftop Packaged Units: Economizers

Introduction: Small rooftop packaged air conditioning units are sold in staggering numbers in the United States. As such, they represent a very large portion of the installed and future energy use in the built environment. This article on 'greening' rooftop packaged units is the first of a series that will address opportunities to increase the efficiencies of these units, and highlight JB products that can address these opportunities. Each article will discuss a different facet of efficient rooftop packaged unit design. This first installment will discuss the impact of effective economizers for rooftop packaged units

It is well established that air-side economizers save energy in the Pacific Northwest. And this stands to reason when you look at a graph of where the bulk of Seattle weather bin data lies:

(click for larger image)

The majority of the bin hours per year lie to the left of the 55º line, indicating that an economizer system would eliminate the need for mechanical cooling altogether during these hours. And nearly all of the hours are located to the left of the 75º line, where ambient temperatures would be lower than return air temperatures in a cooling system--allowing the system to offset some mechanical cooling load by using outside air.

When you consider that the use of outside air also brings IAQ benefits, it is clear why air-side economizers are such a compelling strategy for Northwest mechanical systems.

But there is a problem with air economizers in small packaged units: Too many of them don't work properly in the field. The reason for this is that for most small rooftop packaged cooling units do not have factory installed economizers. The standard of the industry is a bolt-on option that is shipped as a separate assembly to the jobsite for installation by the installing contractor. In some cases, they may not even be available at all for some duct configurations.

Typical small packaged unit economizer instalation

In practice, these economizers have a high rate of failure. The issue of non-functional economizers for small rooftop packaged units is significant enough that Puget Sound Energy includes re-commissioning of these devices in their Commercial HVAC Rooftop Unit Premium Service Rebate (program developed with the assistance of NEEC). And the Califorina Public Interest Energy Research program (PIER) goes further, recommending to owners and designers:

Specify reliable, factory-installed and -tested economizers with direct-drive actuators and low-leakage dampers.

That's exactly what Aaon provides on all of their units down to 1 ton.

(click for larger image)

Aaon's rooftop unit design provides inherent energy advantages over the competition. And factory-installed economizers are just one of many.

Extra: PIER software to estimate economizer savings.
Want free psychrometric software? See our offering here.

Introduction to Modular Chillers

Modular chillers are a product innovation that has recently gained wide acceptance in the HVAC industry. But since they cost more than standard chillers on a per-ton basis, it might seem a unlikely that this equipment would be a very popular cooling solution. However, modular chillers offer advantages that are not available with standard chiller equipment.

These advantages can be summarized in a few points:

• Ease of Installation
• Compact footprint
• Redundancy
  

Ease of Installation

Modular chillers were originally developed as replacement chillers for existing building chiller plants. Many chillers are located in the bowels of the buildings they serve. It is often far easier to remove the existing equipment in pieces than to find a rigging path suitable to take it out of the building in one piece. Of course, this only helps if it is also possible to move the new chilling capacity into the chiller room in pieces!

Modular chillers were designed to fit through standard doors and to have a small turning radius to negotiate internal corridors without requiring demolition of existing walls.





Additionally, since these modules are light enough to ride in a freight elevator, it is usually possible to avoid crane costs for the installation project. Further cost savings are realized once the modules are in the room. Since each module has very low refrigerant volumes, the retrofit usually does not trigger codes requiring refrigerant monitoring or emergency ventilation.

Compact Footprint

In order to fit through doors and down corridors, modular chillers are designed to be extremely compact. They use highly efficient brazed plate heat exchangers to minimize their size as much as possible:



The result is a chiller plant with the smallest footprint per ton of any current option available--even if you are installing the modules in a new project instead of a retrofit.

330 ton chiller plant comparison (click for larger image)

Redundancy

With multiple, independent modules, modular chillers provide unmatched redundancy. If a single circuit is down there are always multiple other circuits operating. And providing N+1 redundancy to a modular chiller plant is far less expensive in first costs and mechanical room space than for any other chiller type. This inherent modularity allows fantastic turndown capabilities, and the part-load efficiency of a modular chiller plant is comparable with that of a large constant speed centrifugal or screw chiller.

ClimaCool Advantages

ClimaCool modular chillers were designed to take full advantage of the modular chiller design. For example, some manufacturers design their chillers for modular installation, but not modular operation. These chillers are designed with an electrical bus bar system to power all of the modules from a single power source. This may mean a slight savings at installation, but significantly degrades the redundancy advantage of this type of chiller. With a bus bar system if one chiller needs to be worked on, the entire array needs to be powered off.

ClimaCool avoids this disadvantage by powering each module independently of all the others:



Similarly, ClimaCool offers full redundancy on the water side, too, by providing isolation valves for the heat exchangers as a standard feature. Some manufacturers offer these valves as a first-cost add and they may significantly affect the chiller's footprint dimensions if added. Providing these valves as standard provides for yet another ClimaCool advantage: Easy conversion to a variable primary flow system! Modular chillers have a tight flow envelope on the brazed plate heat exchangers--each heat exchanger should essentially be considered a constant-flow device. By providing electric actuators controlled by the chiller controller on these isolation valves, the modular chiller plant can easily adjust for variable primary flow.

Another way in which ClimaCool offers advantages over other modular designs is in heat exchanger protection. Brazed-plate heat exchangers are highly efficient and very compact, but they demand very clean water to prevent clogging. All manufacturers of modular chiller equipment require straining of the system water before it enters the exchanger. Some manufacturers provide large-mesh strainers that are mounted in the headers serving the heat exchangers at each module. This approach requires an annual back-flush of the heat exchangers to clean out the debris that inevitably passes through the mesh. It also discourages proper maintenance, since the strainers are hard to get to and are therefore often ignored until clogging causes flow problems. ClimaCool takes a different approach, using small-mesh basket-type strainers outside of the headers to prevent heat exchanger fouling. This eliminates the need for annual back-flushing, and greatly eases maintenance. They also offer a deluxe 80-mesh high-capacity strainer option for especially dirty water or for systems where maintenance man-hours are limited:


Additionally, ClimaCool provides, as standard, convenient back-flush hose-bibs to allow this sort of maintenance as needed without requiring disassembly of the chiller header or taking the other modules off-line.



And, of course, ClimaCool offers chillers that comfortably exceed minimum energy code requirments:



Efficiency, redundancy, compact size and ease of installation: All reasons to consider ClimaCool modular chillers for your next chiller project.

After 10 years, Johnson-Barrow and JCI/York Announce Split

JCI Worldwide services has recently announced a new HVAC products distribution strategy and support for its Building Efficiency business division which includes York international and JCI controls. In an effort to integrate its marketing efforts towards owners, contractors and engineers, JCI will be reorganizing its go-to-market strategy in 15 plus major markets around the nation, including Washington State. The plan is for York to be more directly marketed through the JCI offices with the assistance of an independent rep organization as a support service to the controls division. This will also include the integration of the unitary (Consumer Products) division into their new business strategy.

Wayne Garret, Western Regional Sales Manager for JCI, says the move is designed to better integrate the three aspects of the company. “We needed to get a single face to the customer regarding who JCI is. Unitary, Controls, and Engineered products needed to be more closely tied than was presently the case. This will help our customers get a better understanding of the JCI depth.”

Mark Johnson NW sales manager was asked what things will happen as a result of these changes. “In Markets such as Seattle, major changes are already underway to integrate controls, engineered products, and unitary equipment. Air Cold, a division of Ferguson has discontinued its representation of the York Unitary products. Johnson Barrow will be terminating it’s relationship with the Engineered products division, and finally JCI in Bothell, WA will direct marketing strategies for the Washington and other Pacific NW markets.”

Patrick Hollister of Johnson-Barrow commented that this announcement did not surprise their organization. “For years we have been figuring that York would reposition itself to better integrate the unitary and controls division into a more uniform marketing organization. Thus, over the last couple of years we have been positioning ourselves to be more diversified in our product and service offering. Evidence of this can be seen with the addition of AAON, Smardt Chiller, and Climate Cool. We want to maintain our independence as a solutions oriented company focused on unique products that provide value for our customers”.

When asked to comment on the JCI announcement, Gary Bodenstab of Johnson-Barrow echoed Hollister’s observations. “Look at the magnitude of change around the country. US Air has replaced Air cold in the major SW markets, Ferguson has dropped York. The controls division is in flux trying to regain market share in the NW markets. We figured some major change was underway in the NW—it was only a matter of time. We wish the best for JCI and its new strategy. It’s now time for Johnson-Barrow to focus on our roots of independent companies dedicated to market innovation, energy conservation, and customer value.”

A Giant Golf Ball in Your Outside Air Opening

"What the heck is that thing"

That's often what we hear when we introduce people to the Tek-Air IAQ-Tek outside air monitor.

Sometimes elegant solutions to difficult problems are a little surprising.

The Problem: Outside Air Measurement

Getting an accurate measurement of outdoor air flow is a vexing problem for HVAC professionals. It's about the most difficult airflow measurement situation around.

The air is often moist, dirty, and at extreme temperatures. Most air inlets, especially on packaged air handling units, are poorly designed for accurate flow measurement. The airflows in the inlets are usually highly turbulent, non-uniform and at very low velocities. Wind impinging on the inlets can cause large flow fluctuations. This is tough duty for any flow measurement system.

To make matters worse, the outdoor air flow is one of the more important measurements in an building HVAC system with big implications to the indoor environmental quality and energy use of the building. Understandably, LEED® guidelines encourage the use of outdoor air monitoring.

Generally, the air flow velocities available at an OA probe need to be slow enough to prevent moisture carryover--this makes traditional pressure measurements with pitot-type sensors very unreliable, because the signal from these probes varies with the square of the velocity. At low velocities, the noise from turbulence, wind and other sources simply drowns out the signal with a very low signal-to-noise ratio.

This has led to the use of hot-wire anemometers (thermistors) in this application. These products provide excellent low velocity air flow measurement, but this application provides challenges unique to this technology. In particular, dirt and moisture build-up on the sensors will cause the calibration to stray and upstream filters are usually recommended. Additionally, since the sensors measure the velocity at a discrete point in the air opening, a large number of sensors are required to adequately provide a representative flow measurement for large openings. And even with a large number of sensors, the turbulence and non-uniformity of the airflow in an outdoor air hood or behind a louver makes it very difficult to get a useful reading, no matter how accurate each sample measurement is.

A Different Way

Tek-Air saw the above difficulties and looked for a new solution. And that's why they developed this unique airflow sensing device.

Most flow sensors are designed to minimize the disturbance they create in the airflow. Tek-Air realized they needed to take a different approach for this difficult challenge:



The IAQ-Tek probe is large--really large. In fact each sensor body is about 8" in diameter and has over a dozen pressure ports in it. It dampens out the effect of localized turbulence on the airflow measurement by forcing a large-scale diversion of the airflow in the inlet. The measured variable is the average pressure difference between the ports on the front of the sensor body and the ports on the back of the sensor body. The 'golf-ball' dimples in the face of the sensor ensure stagnation of the airflow to significantly decrease the effect of localized turbulence and ensure a steady, accurate reading.

The unique design of this probe allows accurate readings at 6-8" behind an oudoor air louver, and directly in front of dampers. No prefilters, air straighteners or sections of straight duct are required.

So what does this give you?

• Accurate and stable low velocity readings from 75 to 750 fpm
• Immunity to signal noise
• Great flexibility in application

These probes can get accurate measurements in places you wouldn't even consider other OA probes:






The units come with a temperature and density compensating transducer (-40º to 120º F), for accurate measurement in all conditions. And each system comes with a Nema 4x monitor with LCD readout for local observation. They are rugged devices that need no significant maintenance requirements and can even be hosed down, if needed, for cleaning.

Can they really be accurate in such tight conditions? A test with the unit installed 4" behind a louver outlet, with 18" between the louver and an OA damper yielded the following results:



That's from -6% to +4% (of full range) error at velocities of 100 to 700 fpm with damper positions from full open to 45º. That's fantastic accuracy in an extremely difficult measurement condition.

So maybe you do need a giant golf ball, after all.

Rethinking Air Handler Pressure Testing Specifications

Pressure testing is one of the most important ways to ensure the quality of the cabinet of an air handler provided on your job. Leakage costs money and energy. Every CFM that leaks out of an air handler is air that energy has been expended on that is now lost. Likewise, every CFM of air that leaks into an air handler displaces air that has been conditioned, requiring more air volume to do the same duty. And air leakage can have other negative effects, like causing condensation on surfaces or allowing unfiltered air to enter the system.

So to prevent leaks as much as possible, an experienced engineer will specify leak testing on the air handlers provided on their jobs. This is accomplished by blocking off all openings into the unit and pressurizing it (positively or negatively) with a pressure blower and then measuring the airflow into (or out of) the unit to maintain a test pressure:


Several decisions have to be made when deciding how to test the unit:

1. When to test (At factory or at jobsite)
   
2. How many sections to test
3. Positive or negative test pressure
4. What pressure to test to
5. How to set the failure criteria

When to test the unit

There is a very strong argument to be made that the only pressure test that matters is the pressure test performed on the site. After all, it really matters little to the final built out project if the unit performed flawlessly on the factory floor. The actual performance in the field is all that anyone really cares about. If only a single pressure test can be fit into the budget, it stands to reason that the field test is the one most critical to the overall quality of the delivered project.

However, there is a case to be made for a factory test, too. The field pressure performance of an air handler is not only a function of the manufacturing process, but is also strongly dependent on site conditions, including the flatness of the support the air handler sits on and the rigging and mating procedures used by the contractor. Once a unit is on site, it is sometimes very difficult to determine where the failure lies if it doesn't meet the specified leakage rates. In a worst case, you might have the manufacturer, the shipper, the installing contractor and the general contractor all pointing fingers at each other. If the fault actually lies in a factory defect, the problem could be found and corrected in the most controlled environment possible with a factory test.

So the real answer? Both, if you can afford it. And if you catch a problem before it gets to the field, you might feel you couldn't afford not to do both.

How many sections to test?

Many pressure tests specifications treat the entire air handler as a single section, and require a single test for the whole unit. Some break the unit up into positive and negative pressure areas (upstream and downstream of fans, respectively) and call for them to be tested separately. Each method has its advantages and its disadvantages.
The first consideration is cost. Each test costs time and energy that will be reflected in the overall price for the job. Requiring multiple tests on a single unit will raise the cost of the air handlers to the owner. This will also require more time at the factory and on site, and could affect overall completion dates in some instances.
A second consideration is accuracy. Since multiple tests allow the unit to be tested to the actual pressure condition the sections will see, presumably this will give you a better idea of the leakage than a single test. However, there is an appreciable amount of leakage within the air handler at the internal wall that will be factored into this measurement (and double counted!) that will unrealistically penalize the performance of the unit. This is especially significant, since the internal openings within an air handler (at fan walls, usually) that need to be blocked off to perform the test are rarely built in a fashion that allows for an effective air seal to be created for these temporary tests. Even a small amount of leakage at these internal walls can mean the difference between passing or failing a tight leakage criteria.
Consider a single test for units unless special requirements drive a need for multiple positive and negative tests.

Positive or negative test pressure?

If you are making multiple tests on multiple sections of an air handler, the answer to this question is simple: Test to the conditions each section will see in operation. If you are performing a single test on the entire unit, then you may want to carefully consider how you wish to test the unit. In general, there are some leaks that will open up under one pressure condition and will close under the other. Generally it is accepted that leaks at panel seams tend to close under negative pressure and tend to open under positive. Doors that swing out tend to behave similarly, while doors that swing in behave in the opposite fashion. Seams at test closures can do either based upon the method of construction of the closure. So there is no easy rule of thumb that says one method is preferable to the other. In many cases it makes sense just to find the point of extreme pressure in the unit under normal operation, determine if this is positive of negative, and test to that condition. This condition is easy to find by simply calculating the pressure condition at each section in the unit by starting at the external static pressure and working towards the inlet, adding back pressure losses at each internal component, and subtracting the fan static pressure increase at the fan wall. Each open section of air tunnel will have a pressure associated with it, with the extremes usually falling at the inlet or discharge plenum of the supply fan. These are the maximum pressures the air handler will see in operation--and usually one will be significantly further from ambient pressure than the other.

What Pressure to test to and
How to set failure Criteria

The above two considerations go hand in hand, so I will deal with both of them together. Traditionally, specifications are written so that a certain percentage of the total air flow is allowed in leakage (usually around 1-2%) at a specific test pressure. How the leakage percentage and the test pressure are determined varies from engineer to engineer and job to job. Sometimes the test pressure is based on the total fan static, sometimes it is based on the actual cabinet pressure, and sometimes it is based on a nominal test pressure (like, say 10"). In either of the first two cases, there is usually a sizable safety factor applied.
The allowed leakage percentage varies, but it is usually in the low single digits.
While this has been the standard in the industry for many years, there are some significant weaknesses in this approach to testing. First, the actual leakage rate measured in the field is determined essentially by the total face area of all the leaks in the system--this is a function of cabinet size and construction quality, not of supply air flow. By tying success or failure of the test to the fan capacity of the system, you are favoring simpler, smaller air handlers over larger, more complicated air handlers.
Imagine a simple 10,000 CFM air handler operating at 8" of total static with just a fan, a heating coil, prefilters and a mixing box. Then imagine that same air handler, but with a cooling coil, high efficiency final filters, return fan and air blender:

(click for larger image)

In the example above, you have two 10,000 CFM air handlers, one with 297 square feet of cabinet area, the other with 630 square feet of cabinet area. The large air handler has more than double the cabinet area, and more leak points such as doors, dampers, coil penetrations and shipping splits--yet both would be required to meet the same leakage rate in a test--in this case, say, 200 cfm at a 2% leakage criteria.

A further complication would arise with the pressure selection. A common pressure test criteria is 1.5x the maximum fan static pressure--or in this case, 12". It is often difficult to find a pressure blower with a static capability in this range--It might be impossible to effectively provide this test in a timely manner on a job site. Additionally, many components (especially doors, when tested in a pressure condition opposite that they would see in operation) leak uncharacteristically at higher pressures. And the unit would never see pressures anywhere near 12" in real operation, anyway, since the fan will typically create an area of negative pressure in the inlet plenum, and positive at the discharge. The maximum amplitude of either pressure is, by necessity, less than the total static pressure capability of the fan. Thus the leakage rate measured in the test will be a very significant overestimate of the actual leakage that will be experienced in operation.

A different way of specifying pressure performance can address both of these complications--and that is to tie the performance to the cabinet itself, as opposed to the air flow. There is already a criteria to do exactly this. ClimateCraft recognized the difficulty with specifying pressure test criteria to arbitrary pressures and airflow percentages. They realized that SMACNA already had a pressure test criteria, the SMACNA leak class rating. The leak class of a pressure plenum (or air handler, in this case) is calculated using the following formula (from ANSI/ASHRAE 111-1988):

Leak Class	=	(leak CFM) x 100
		(Area sq.ft.) x (Test Pressure) ^ 0.65
OR
Leak CFM	=	(Leak Class) x (Area sq.ft.) x (Test Pressure) ^ 0.65
		100

ClimateCraft has adopted this method of rating pressure performance and builds their units to meet or exceed a leak class of 6. For the units above, that equates to about 146 CFM at 8" for the larger unit, and 69 cfm for the smaller (or 1.5% and 0.7% of the supply air volume, respectively).

In practice, we have found that a leak class of 6 represents excellent performance for high-quality custom air handling units of any of the manufacturers we represent and quite often can be met by the high-quality foam panel semi-custom air handlers by Aaon, too. This is, however, a very high bar for traditional commercial-grade batt-insulated air handlers.

Perhaps the biggest advantage of the leak class specification is that it encompasses both the allowable leakage critiera and the test pressure in a single number. A leak class 6 air handler will perform to the same leakage class whether it is tested at 4" or 10"--the test pressure can be chosen to meet realistic pressure conditions and to facilitate testing of the unit. It is purely determined by the design of the air handler cabinet and the execution of assembly.

Digital Scroll Compressors: Just Plain Cool

Copeland Compressors (now a Division of Emerson) has recently introduced their digital scroll compressor technology. This is one of the most interesting products to come out in a long time for the DX cooling market. But to understand why it is so cool, you first need to understand a little bit about scroll compressors in general.

Scroll compressors have essentially displaced the older reciprocating compressor designs for small-tonnage air conditioning systems. Which is much to the operator's benefit, because scrolls are inherently more reliable and require none of the maintenance that the piston-type reciprocating compressors required. But this advantage comes with a price: comprehension. Reciprocating compressors were so much easier to understand--since the compression stroke in a piston is easy to grasp and most people are familiar with this process from the similar function of pistons in gas engines.

Some smart guy had to come along and invent a highly efficient and low-maintenance compressor technology that no one can describe easily--even using curious arm gestures and words like "orbit"!

The secret to a scroll compressor is two high-precision spiral "scrolls" that are designed to mesh with each other to extremely close tolerances:


The upper scroll is stationary and the lower scroll 'orbits' in a rotary fashion:

Comparison of scroll to piston compressors showing relationship of upper and lower scroll (click for larger image)

The upper and lower scrolls continually 'pinch' off volumes of low pressure gas and move them towards the center of the scrolls, compressing the volume further and further as they work. This compression requires extremely close tolerances between the sides and ends of the scroll surfaces, since the only seal is the lubricating oil in the refrigerant circuit. If tolerances are to great, no seal is effected and the compression is lost.

Still hard to picture? This animation should make things a bit clearer:

So, great: We have a highly efficient compressor with two capacity settings: 'On' and 'Off'. If you are trying to meet a close control spec, this may not be close enough control. You would typically overshoot and then undershoot the required cooling capacity as the compressor kicks on and off. And since anti-recycle timers are required to prevent overheating the compressor motor, there is a limited number of times the cooling capacity can be switched on and off in an hour.

A new innovation allows two-step unloading to 66% capacity--but this can still be a pretty big step of control on a small refrigeration system--especially ones with only a single compressor. Wouldn't it be nice to get a fully modulating compressor with all of the advantages of the scroll compressor?

That's where the digital scroll comes in. Copeland's engineers were clever enough to realize that they could achieve this performance out of a scroll, not by modulating its capacity directly, but instead by modulating the time during which this capacity is provided. They found that if they quickly turn on and off the compression cycle, without having to turn off and on the compressor motor, they could modulate the output very closely to meet the needed capacity. The trick was finding a mechanism by which this rapid switching between active and inactive compression could be accomplished.

The solution they arrived at was elegant. They found that by merely moving the scrolls apart axially, they could defeat the oil seal between the scrolls, and turn off the compression. Then they simply needed to move the scrolls back together and compression would immediately restart.


The above graphic shows the scrolls separated to cancel out the compression cycle, and a visual representation of how the scroll would operate to provide 50% capacity--Operating 10 seconds on and then 10 seconds off in a repeating cycle.

What's the result? Very efficient operation down to 10% of full capacity:

(click for larger image)

See the savings noted above? That's compared to the commonly used DX modulation method of hot gas bypass (HGBP). It's important to note that the HGBP works by applying a false load to the compressor--it does not reduce compressor energy at all! As far as the compressor motor is concerned, it is doing just as much work as when the compressor is providing full output. In fact, the HGBP system is even more of an energy hog than is suggested by the graph above--since compressors with this device will operate at full load for extended periods of time, drawing full amps all the time, as opposed to a standard system where the compressor would turn on and off to match the load.

The digital scroll gives a DX system all the fine control capability of a chilled water system, without sacrificing energy performance like HGBP systems do. It allows effective operation of VAV airflow systems without frosting coils. It provides efficiencies unmatched in the DX market. For these reasons Aaon was quick to incorporate these compressors into their RM and RN rooftop packaged AHU lines. Unfortunately, digital scrolls are not available in all standard scroll compressor sizes and voltage ratings. And they are currently only available in R-22 compressors. This handy chart indicates where the digital scrolls are available in each RM/RN model size and for which voltages. This file is valid as of 9/22/07, and is definitely subject to change in if/when new digital scrolls are released. Additionally, R-410a compressors are expected out in the near future for 6, 7, 13, 15, 16 and 25 ton sizes in 460/3ph electrical services only. Stay tuned for the availability of those units!

Update: Aaon has rolled out units using R-410a digital scrolls, as well as new software to calculate the efficiency benefits of these compressors.

Advanced Tower Nozzle Design Eliminates Clogging

Anyone who has operated cooling towers for any significant amount of time knows that a common maintenance point is clearing clogged water distribution nozzles. This is especially a problem for gravity-fed cross-flow towers, where there is very little pressure to force debris through the nozzle orifice, and debris such as leaves, paper and ferrous 'throw' from the pipes can clog the nozzle. This reduces the effectiveness of the water distribution, and in turn the efficiency of the cooling tower.

Typical cross-flow tower gravity distribution pan

Pressurized distribution systems, as are found on Evapco counter-flow towers, eliminate a lot of clogging problems by utilizing pressurized large-orifice nozzles which use the force of the water pressure to keep the nozzles clear. But even these types of systems can clog periodically.

So that is why Evapco developed the Evapjet nozzle.



That's a nozzle?

Yes, it is, but probably the best way to appreciate it is by watching the video of it in action: Evapjet Video (may require Microsoft Internet Explorer to view).

(Pretty cool, huh?)

Importantly, this nozzle can pass a 1" ball, and reduces the total number of nozzles required for a tower by 66%! So you have a much reduced chance of clogging, and many fewer nozzles to maintain.

This nozzle is provided on new Evapco cooling towers, and is available for retrofit on many existing towers of most major manufacturers. If you are interested in retrofit, call Fluid-Tek for a quote!

UPDATE: 9/23/07

Don't be discouraged if you have a fluid cooler and not a cooling tower. Evapco offers their unique ZM (Zero Maintenance) nozzles (pdf) with similar anti-clogging properties as the Evapjet--they just don't make for as cool a video!

Read more about spray header and nozzle replacements here.

Saving Water With Evaporative Condensing

Using evaporative cooling makes energy sense, not just in directly cooling the air, but also in cooling the heat rejection portion of a compressorized cooling system. This is because refrigerant compressors can be thought of, simplistically, as pumps. In a pump, two things govern energy use--the flow rate of fluid through the pump, and the head that pump needs to overcome to move the water. The same is essentially true for compressors.

In a compressor, the mass flow rate of the refrigerant essentially determines the cooling capacity it is providing. So for a given cooling load, we can't reduce the flow rate to increase our efficiency. All that is left is the compressor head. And that is something that we can effect.

In a refrigerant system, the condensing pressure of the refrigerant vapor is determined by the temperature of that refrigerant.

Let's look at this relationship for R-410a (from DuPont .pdf here):




We can see that the condensing pressure for 120º R-410a (which is around where an air-cooled condenser would operate) is about 450 psi. Compare that to a condensing pressure of about 280 psi or so at a temperature of 90º-which is an easily attainable condensing temperature in an evaporative condenser in the Pacific Northwest. When you consider that a reasonable suction temperature might be about 42º (or 150 psi) reducing your condensing temperature from 120º to 90º represents a head reduction of nearly 60%! In practice, moving to an evaporatively condensed piece of equipment from a comparable air-cooled piece of equipment can improve NPLV's by about 20% or so.

So, obviously, this makes sense from an energy conservation point of view. But what about water use? The obvious trade-off is that you are now using water where before, in the air-cooled case, you weren't. So how can we reduce the use of water while still benefitting from the reduced head pressure on the compressors?

To understand how how to improve water utilization in these systems, it is important to know what the state of the industry is for evaporative condensing. A typical system is illustrated below:


What you typically have is an induced draft evaporative unit that sprays water over a refrigerant coil. The water evaporates, and that evaporation cools the refrigerant in the coil. The remaining water falls into a basin where a pump then sprays the water back up over the coil again. All the while, a fan operates to draw air past the water and enhance the evaporative process. Importantly, every 1000 btu rejected by this system reflects about pound of water evaporated.

So what can we do to reduce water use? Well, the obvious thing is to reject less heat via evaporation. However, this might seem problematic because we want to maintain the low condensing temperatures that we can reach using evaporation. This is where it is important to understand a little about how refrigerant systems really work.

One of the main concerns with refrigerant compressors is they are designed to move gas--not liquid. A very effective way to break a compressor is to introduce liquid into it. So to be sure that no liquid enters the compressor, refrigeration systems are designe to operate with a few degrees of superheat. This takes the refrigerant safely away from the saturation line, and assures that the compressor will not see any liquid. However, this adds some extra heat into the system that then need to be rejected. Then, through the operation of the compressor, even more heat is added into the system, taking the system even further away from saturation.

However, the additive effect of the intentional pre-compressor superheat and the heat added by the compressor itself means that there is a significant amount of heat in the refrigerant that needs to be rejected before refrigerant condensing can even start. This de-superheat process is illustrated below:

(click for larger image)

If this heat can be removed without requiring evaporation, a significant amount of the water use can be eliminated. And this is exactly the approach that Aaon has taked with their evaporative condensing design. Their solution is illustrated below:



The main difference between the first system and this one is the addition of a finned desuperheater coil located above the spray system, in the cool, saturated air stream above the wetted portion of the condenser. This coil allow the system to reject the superheat without using any water--saving, on average, about 20% of the water use at peak load.

However, the benefits extend beyond there, since at about 70º ambient, this coil can reject about 50% of the total heat in the system, and it can reject 100% at about 30º ambient. So the true water savings range from about 20% to 100% depending on the operating profile and ambient conditions of the unit.

There are still other benefits: If chemical water treatment is being used on this system, the lower water use will translate to lower chemical use. And, since the tube surfaces in the wetted portions are at lower temperature in the Aaon system, there is a corresponding lower chance of creating scale--which is formed primarily from calcium carbonate which exhibits inverse solubility, depositing much more readily at higher temperatures. This lower fouling, in turn means the system will operated more efficiently for years to come, since less scale means better heat transfer at the tube surfaces which means lower head pressure on the compressor!

ECM Motors and Heat Pumps

What are electronically commutated (ECM) motors?

These are single-phase motors running off of a DC power signal (rectified, if running off of an AC power source) that use an electronic method of switching power to alternating coils around the rotor in order to induce a rotating magnetic field. (More information available here).

They are much more efficient than traditional electro-mechanically commutated motors, and local utilities offer rebates for their use. But ECM's also provide additional benefits on heat pump applications.

Because the motor speed is controlled by the speed at which the magnetic field rotates around the shaft, and that speed is controlled by the electronic switching of the ECM, these motors are inherently variable speed devices. When applied in equipment, this variable speed capability is either used directly, providing a variable speed capability on the fan, or, more commonly on packaged refrigeration equipment, as a self-balancing mechanism to provide a fixed discharge airflow, independent of external static pressure.

Additionally, the electronic switching in these devices allows for more torque to be delivered to the rotor, allowing for greater static pressure capability on the fan it is driving.


Florida Heat Pump has capitalized on these advantages by offering ECM motors on their heat pump products. FHP ES and GS series heat pumps are supplied with ECM fan motors for efficiency, ease of air balancing, and unmatched static pressure capacity.

Let's consider those last two further. FHP uses the ECM to self-balance their units to a set leaving air flow (low, medium and high settings controlled by jumpers on the control board) regardless of external static on the fan (within a given range). Let's look at the fan table for these units (2 1/2 ton ES030 shown):

(click for larger image)

You either get 850, 1000 or 1150 CFM at anywhere from 0.10" to 1.2" of external static pressure. All you do is set the jumper to the flow rate you want to see and then the fan motor will automatically adjust itself to provide the airflow desired.

But look at that range of static pressure! Typical heat pump fans might only provide three tenths of an inch or so external static. Having over an inch available (smaller units offer about 0.80" external) is a game changer, making it very possible to have improved filtration (LEED® points) or air side economizers (energy code requirement) on these inherently efficient heat pump systems!

And these advantages are above and beyond the improved energy efficiency that is realized by using the more efficient fan technology.

ECM motors improve WSHP efficiency, reduce balancing labor and widen the applicability of these already efficient systems.