Where to use AirGreen systems?
The basic building block of the AirGreen system is the AG unit. This is a Dehumidifier with heat removal, or when used as a Regenerator it is a Humidifier with heat added to reconcentrate the desiccant.
Cooling or heating are done with water supplied to the heat exchangers that chill or heat the desiccant. In turn, the desiccant cools or heats the air via Absorber or Evaporator (these look identical).
The main choices on using the AG units are:
1. Standalone 100% Outside Air System
The complete air conditioning system comprises 2 AG units (Air Conditioner + Regenerator) and a Heat Pump or other supply of 60°F chilled water and 130°F hot water.
For example, the AG-2000 supplies 2,000cfm of Outside Air and approximately 1,800cfm of Return Air - to allow for building pressure. The Heat Pump is sized to provide the maximum enthalpy removal from the Outside Air. At 95°F and 55% RH, 19 tons of chilled water are required.
At 95°F and 70% RH, 24 tons are needed.
In both cases, 8.5 tons of internal cooling are provided (5 latent, 3.5 sensible). By lowering the chilled water temperature to 45°F, and additional 4 tons of internal cooling can be provided.
Thus, the AG system provides internal sensible and latent cooling up to approximately 50% of the load in addition to the Outside Air treatment.
(hover to enlarge)
2. Added as DOAS to Chiller System
In this scenario, an AG system is added to the chiller system to handle the Outside Air more efficiently.
Say the chiller is 100 tons and the ventilation air is 4,000 cfm (10%). With 95°F, 55% RH Outside Air the system is at its maximum latent capacity of 21 tons. The building inside condition would be on average 72°F, 60% RH without the AG system and with the AG system it can be raised to a more comfortable 78°F, 45% RH.
With a 4,000 cfm AG system the latent capacity increases to a minimum of 26 tons and can be raised to 31 tons if either the Outside Air becomes more humid or the internal humidity load increases. Thus, the whole system has flexibility to handle increased loads without operating outside its capability (20% latent).
The efficiency of the system increases from three factors:
The overall improvements are 55% more net cooling and a much more comfortable and healthy indoor environment. The improved conditions would be maintained even when a conventional system would be put under stress and the ventilation would have to be reduced to below mandated levels.
3. Added as Pre-treatment to Chiller System (Make-up Air Unit)
This is similar to example 2. above except that the conventional system has a large AHU instead of fan coils. Outside Air is pre-conditioned by the AG unit and fed to the AHU dry and slightly cooled.
The calculations are almost the same as application 2. One variation shown is that the AG unit can be supplied with tail water from the AHU since it is not required to cool as much. This further improves the efficiency of the chiller.
A 4,000 cfm AG system would remove 270 lbs of water an hour at the condition shown and would go up to around 380 lbs an hour if necessary.
4. Packaged Rooftop Unit
The largest Commercial market is in RoofTop Units. Last year about 800,000 were sold at an average of 10 tons. AirGreen units may be packaged into a standalone RTU complete with compressor. The only difference is that the AG units will supply 100% Outside Air whereas most RTUs will handle no more than 20%, usually 10 to 15%.
Most RTUs are on flat roofs and feed directly into the conditioned space via stub ducts. As with most conventional systems the indoor environment is cold and damp because the chilled air is supplied at around 55°F and saturated. In Florida type climates (95°F, 70% RH) the discomfort is well-known.
The AG system would be placed near a group of DX RTUs – let us say four 10 ton units – and supply dry air in the direction of each of them. The DX outside dampers would be shut in all conditions (except genuine economizer mode). A 1,500 cfm AG system will handle the entire latent load so that the DX units will run with dry coils.
The DX system without AG can handle only 1,000 cfm of Outside Air and the net cooling to the building is only 22 tons. With AG the ventilation increases 50% and the indoor air improves dramatically to 78°F, 45% RH. The net cooling doubles around 45 tons. As the DX units are retired only half of them will need to be replaced. Their lives should, however, be lengthened because the compressors will not be stressed and the coils will not at risk of corrosion from condensate.
As building insulation standards increase the need for separate handling of the Outside Air and humidity increases as it becomes the major part of the load.
5. AirGreen DOAS with VRF System
VRF systems are very common in the rest of the world and are catching on in the US. They claim 30% cost savings most of which come from diversity in the building. However, they are no better at handling Outside Air than any DX system and the cassettes have to be piped with condensate drains.
Thus, the addition of an AG unit to a VRF system to handle Outside Air will have many of the improvements stated in 1-4 in addition to the diversity savings of the VRF. Let us take for example a 30 ton VRF system with 1,500 cfm of Outside Air. The AG system can be integrated very easily by adding refrigerant-to-water heat exchangers to the refrigerant lines. At the OA condition of 95°F, 55% RH, 12 to 14 tons of capacity will be needed for the AG system and the remaining 16 to18 tons for the cassettes. In most cases the number of cassettes can be significantly reduced – by as much as 7 tons in this case.
If an ERV is used, as would be prudent in a VRF system without AG, then the AG system can be smaller as the OA load is reduced - in this case to 1,000 cfm. Note that it is not reduced by as much as half because the building still has the same internal latent load.
6. AirGreen DOAS + Dehumidifier
The AG system does not have to be installed with 100% Outside Air. One AG unit must have OA but a second (or even a third) can be installed as dehumidifiers that also cool. In this diagram, the water lines are not shown for clarity.
The main difference is that one regenerator will be run using Outside Air instead of Return Air. This has the effect of reducing the concentration of the desiccant that is then piped to another regenerator that does have the drier Return Air to use. The green lines show the desiccant flows.
7. Pre-treatment for Large RTU
Another way of using AG units with larger rooftop units is as pre-treatment devices. A 25 ton RTU would need, say, 1,500 cfm of Outside Air. The AG units would be placed on either side of the RTU to pre-condition the ventilation air and to use the Return Air for Regeneration. Note that there is not always as much Return Air as Supply Air and some of the condenser air can be added to the available Return Air for Regeneration.
In this example, the AG system would require a 15 to 20 ton dedicated heat pump to supply the cooling and dehumidification needs. In places where freezing is not an issue these can be regular AG units supplied with chilled and hot water. In places where freezing is a possibility when the AG units are in use the cooling and heating may be refrigerant-based using the modified AG units shown in example 4, for a packaged system.
8. Packaged Rooftop Unit 2
This packaged system uses refrigerant instead of water in AirGreen units.
Variable amount of Outside Air.
9. Variable Outside Air
Similar to 1. but with variable % Outside Air.
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