heating coil or cooling coil?

[botching]

Hi guys,
My first post. In a horizontal draw though AHU, is the heating coil recommended to be configured after the cooling just before the supply fan? or the heating coil first, then the cooling coil then the supply fan.

thanks

[Brian_UK]

I recognise that you are in a warm/hot location so this may sound silly but....

Following airflow through the AHU from outside to discharge ductwork...

1 Heat - Frost coil, to maintain minimum air temperature.

2 Heat - reheat coil, to bring the air temperature up to that required.

3 Cool - to reduce temperature to that required.

Also to reduce humidity if required.

4 Humidifier - to maintain humidity level after air cooling, if moisture removed.

5 Heat - second reheat coil to raise temperature after dehumidification if required. Some may use the first reheat coil to raise the temperature/dew point and not use this coil.

[chillin out]

Do you know this, I look at these things every day but can't remember....lol

Thinking about it, I would put the cooling first then heating then fan.

If the heating comes before the cooling then there is a chance the pressure would build up inside the cooling coil and burst.

Chillin

[botching]

thanks guys. Now its clear to me why the heating coil should come after the cooling coil

[botching]

on the other hand, in case a blank section say 500mm is configured between the cooling and heating coils, would it lessen the danger of damage on the cooling coil and would that be acceptable then?

[chemi-cool]

In DX AHU, you also have to protect exsesive heat with a thermostat that have a manual reset.

Especialy if you use electrical heating.


Chemi

[walden]

With temperature only control it does not matter which way round the heating or cooling coils are placed. When there is de-humidification control, using a cooling coil, that cooling coil must be followed by a heating coil so that the air can be re-heated, enabling it to carry more moisture and hence reduce the relative humidity.

I have been told that it is better to have the fan on the outlet of the AHU drawing air through the coils rather than on the inlet of the AHU, blowing air into them. Can someone explain to me why?

[chemi-cool]

When the air is drawn from the coils, it is even on the surface.

Also less loss of air pressure.


Chemi

[Brian_UK]

Good point about the extra heat onto the DX coil but not so critical on a chilled water unit perhaps.