The first trick is to determine where to put the mechanical cooling coil. The temptation might be to think it would be a mistake to put it in as the last component in the air flow. This is because it seems you would then simply be removing a lot of the latent effect that you are putting into the air in the direct evap portion of the system. However, inspection of psychrometric processes indicates that this is not the case. In fact, anytime the dew point of the air leaving the indirect evap is lower than the desired design air drybulb, you will see an advantage in running the direct evap section if it is located upstream of the cooling coil. A quick example will demonstrate this effect: Let's look at a 12,000 cfm system providing 60º supply air at an extreme sensible ambient weather condition in Seattle, WA. We will first look at this system with a standard mixed air (20% OA) arrangement with a traditional cooling coil. Then we will superimpose a three-stage indirect-mechanical cooling-direct system and compare the energy performance:
(click for larger image)
In the above image, the traditional mixed-air psychrometric process is indicated in red and the evaporative process in purple (with blue indicating the mechanical cooling portion). Both systems start at the ambient OA condition of 95º/68º. The standard cooling system then mixes the OA with return air in a 20%/80% proportion and then cools sensibly to a 60º leaving air temperature. (The cooling process in the traditional system is shown as purely sensible, but in reality, the system would likely use a return air bypass configuration to allow the portion of the supply air to be super-cooled to achieve latent cooling and thus prevent any latent load in the space from steadily building humidity through multiple passes through this pyschrometric process.) Note that the sensible cooling load in this system requires about 20 tons of mechanical cooling.
In the three-stage evaporative system, 100% OA is first indirectly cooled to to the condition at point I/D evap + R2. Then, mechanical cooling takes over to point I/D evap + R3, after which the direct evap section evaporatively cools to a 60º LAT condition at I/D evap + R4. I have then shown a sensible heating process from the LAT to represent the zone load to demonstrate that this will provide a very comfortable resultant air condition in the space at I/D evap + R5. Note that this is true even if there is a significant latent component to this load (The resultant room temperature is approximately centered in the pink zone that represents the ASHRAE summer comfort envelope). For this analysis, the direct evaporative system is operating at full capacity, and the cooling coil is modulating to provide the desired LAT DB. (Since this system is 100% OA, we are not concerned about humidity levels building up in the space as in the recirculating system.)
The first thing that should just jump out of this is that the evaporative system requires less than HALF the mechanical cooling of the traditional system--while providing the increased ventilation benefit of 100% OA! And this is neglecting the additional latent cooling load that would probably be needed to maintain humidity levels in the space with the traditional recirculating system. To add to the IEQ benefit, the direct evap section works as an air washer and effectively increases the filtration of the air to improve IEQ beyond that of a traditional 100% OA system.
Now lets compare the three-stage system we just examined (with a indirect/cooling coil/direct arrangement) to that of a three-stage system with an indirect/direct/cooling coil configuration:
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Two things should be obvious in this example. First, the mechanical cooling load is even lower than in the previous example: down to under 5 tons! Thats about a quarter of the load for the traditional system, and a little more than half of the load of the evaporative system with the cooling coil before the evap section. Second, and this is the secret behind the reduction in mechanical load, the air leaving this system is significantly closer to saturation than the previous example. In other words, despite the fact the leaving dry bulb temperatures are the same in both cases, in the latter case there is more latent heat in the supply air. The evaporative cooling process before the coil allows the system air to hold more latent energy but yield the same sensible condition for conditioning the space. A quick inspection of the comfort zone indicates this air is perfectly suitable to provide an acceptable comfort condition, even with a reasonable latent load.
Earlier, I said there should be an advantage to running the direct evap upstream of the cooling coil if the dew point of the air entering the direct evap was colder than the design air DB. In this case the dew point of the entering air is 53 degrees--which is quite a bit cooler than the 60 degree design point we are looking for, so thus we gain the advantage seen. What about the case where the entering air is too moist? Let's look at a system where the entering air dew point is well above the supply air temperature:
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In this case, the OA enters with a 62º dew point. It cools through an indirect section to about 72º/65º, and then directly to a cooling coil to reach the leaving air temperature of 60ºF db. Since the enthalpy and WB lines are nearly parallel, it seems there is very little advantage to using direct evaporation to get the air leaving the indirect evap section to saturation and then cooling it. But, importantly, there is certainly no disadvantage, (other than the electrical draw of the pump). Also notice that again, this system provides a significant load reduction compared to a standard system even while providing 100% OA!
Running through that process actually shows a slight advantage for using the direct evap section:
This cooling advantage should be confirmed for your specific system since it is highly dependent on the latent capacity of the cooling coil, and is offset by the pump energy and some small increase in system static pressure when the direct evap media is wet.
However, keeping the direct evap pump running even in these conditions provides several advantages besides energy savings:
- Simplifies the control scheme
- Provides IEQ benefit of air washing
- Increases the life of the direct evap media by reducing cycling of the evap pump
Whether or not it makes sense to use the direct evap portion of your system in times of high ambient moisture is a decision that can change depending on the particulars on any given project. But if there is a net energy penalty for using this system when the OA dew point is high, one can see from this analysis that the penalty is slight and that it would only occur for very few hours a year.