View Full Version : 100 outdoor air units
06-14-2005, 09:10 PM
how do 100% oa units work in theory. if i put 100 degree air into my home ac the superheat will not be the 15 degrees. will i see normal readings when checking these out????? i am working on a aaon unit.
06-15-2005, 06:32 AM
not exactly sure what you are asking but, if you bring in 100 degree air then you will have to remove the heat and humidity. If you are bringing in outside air, where is it going? If you are exhausting air, then you are making up the exhausted air by bringing in air from outside. So 100% outside air would mean that you are bringing in so many CF and exhausting so many CF in order to maintain a positive pressure in your building. In this case you may have a make up air handler to pre-condition the air before you bring it to your air handler that would maintain temperature of the set point. Most of what I see is not 100% more like 10% depending on what the oa temp is you can get free cooling by bringing in cold air from outside. This all depends on what you want to do in the building you are in. Again I am not sure how all this applies to residential, I havn't seen one in a house yet.
06-15-2005, 08:59 PM
I'm not sure I understand your question either, but that rarely stops me from speaking.
100% OA units that are refrigeration only are still air conditioners. But they condition air from the outdoor conditions instead of from the return air conditions. Since the outdoor conditions change from cool/dry to hot/humid, they have to handle more heat load, higher latent fraction, and more variable inlet conditions. This takes more cooling for the amount of airflow (typically 250 CFM per ton) and more controls to figure out what to turn on when. Usually the evaporator will be be thicker (more rows) and the air flow lower than you usually see.
If it has a desicant wheel, it adds to the complexity, but is still trying to cool and dry the inlet air.
You should follow the manufacturer's instructions very closely. "Beer can cold" will cause trouble as will 15 degree superheat at 100 degrees inlet temperature.
06-15-2005, 09:12 PM
I think I know what you are asking for and why:
The 100% OSA units here in the Los Angeles, CA area (OSA conditions between 25 and 115 degrees F) either use liquid injection in the suction line through the use of a desuperheat valve by themselves, or more commonly a combination of hot gas bypass and a desuperheat valves to work efficiently through both the low and high dry bulb intake temperatures.
Hope this helps.
John J. Dalton
06-15-2005, 09:23 PM
You will first off most likely be dealing with a low CFM per ton, like 200 or lower.
There will be staged compressors and possibly a reheat system. The reheat would quite off be a subcooling circuit or by hot gas.
I have a few projects in a hot humid environment where there are three or four compressors on deep row coils.
There is then secondary cooling coil in series with a small compressor and immediately down stream of this secondary cooling coil is its condensing coil which is used for reheat.
The heat removed by the deep coil is dumped outside. The extra dehumidifying/reheat system kicks on and off as required.
The lead compressor circuit of the main coil has hot gas bypass.
The compressors could be staged by discharge air control. You need to get serial number and talk to Aaon
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