question about reheat
i was rescently working on an addison unit that was a little more technical than i am used to. On of the things that this unit has is reheat and i would like someone to explain to me exactly what reheat is and how it works.
there are quite a few different applications of reheat, but picture a reheat coil as being the last coil before the air gets to the customer ........ Sometimes you will see an air handler set up something like this.
Preheat Cooling Reheat
Preheat section will maintain a given temp, as will cooling , and the reheat will do alot of the work in terms of temp control, along with VAVs VFDs and other airflow control.......... VAVs commonly include a reheat coil.
Its not an easy explanation for me, but the actual definition is :
"The heating of air that has been previously cooled by cooling equipment or an economizer. A heating device, usually a hot water coil, is placed in the zone supply duct and is controlled via a zone thermostat."
Lets say your unit is set to maintain a constant discharge temp of 55 degrees...you use the reheat to maintain your space temp. How its controlled depends on your control system (pneumatic, electronic)
If your unit controls humidity by turning on the cooling, your need to reheat the air so you dont drive your space temp to low
you will usually only find reheat on DX type equipment (but not always). DX type equipment can not give great control over discharge air temperatures...they only have so many stages and it may be more than you need. so you add a reheat coil, so that the space is not overcooled or the compressors are not cycled on/off so much. some reheat coils, like in packaged units, will use hot gas as the reheat. there are many different ways to get reheat.
another use for reheat is if the system is to maintain a specific humidity level. you may use a very cold chilled water/glycol or DX coil, which will wring more humidity out of the air...but it will also overcool the air. a reheat coil will bring the temperature back up to desired conditions.
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Reheat is used for a couple reasons, the addisions I have looked at all had a lot for outside air involved and they used hot gas reheat.
Using reheat in cooling seems to go against nature but there are reasons for it. The main reason being you have a large latent load, you need to remove a lot of moitsure. This could mean a system that had a lot of fresh air or a building with a dense occupancy and very active people, like a dance club or a Pentacostal Church
To control the temperature of a place, you can work out how much air at certain temperature (say 55 degrees) you need to supply to that space.
To control humidity you have to work out how much moisture is in that supply air also. In many cases a cooling coil can cool and dehumidify air to the correct temperature and the correct moisture level, to control both temperature and humidity in a place.
BUT, in some situations when you need to remove a lot of moisture, it becomes impossible for a cooling coil alone to give you air at both the correct temperature and mositure level. It can only do one and not the other.
If it can supply air at the right temperature, it still has too much mositure in it, and the place is too humid.
In order to supply air with the correct mositure level in it, it would have to overcool the air to get the mositure out. The problem now is the supply air is so cold the thermostat gets satisfied real quick and the cooling shuts off. When the cooling is off it stops removing moisture and the place ends up too humid.
You can use reheat to warm up this air that had to be super cooled so that the system keeps running and removing moisture.
The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.
Another reason for reheats is for the different cooling loads in a building. Some areas will need more cold air than others and reheats be they variable air volume or constant volume will add more heat to raise the temp of supply air to the areas that don't need so much cold air.
Same applies for heat loads in a building just vice versa.
Coil placement is what makes a dehumidifier out of an air conditioner.
DX Evap coil or Chilled Water coil then Condenser or Hot Water coil in the same air flow path . Reheat the air cooled below dew point for return to zone for comfort levels.
Oh, almost forgot 1 minute in a microwave does wonders for cold pizza too!
Last edited by Snoring Beagle; 10-20-2007 at 03:36 AM.
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electric reheat is very costly to operate!
heat pipe (reheat) is the mac daddy of reheat (free reheat)
thanks for the help
I really appreciate the help and it gave me a better idea of what reheat really is.