AHU Cooling Coil

I assume from you question you are talking about AIR- you need to let us all know as much specifics as possible.

Basically, the coldest leaving air temperature you can get is about 4F higher than the entering cooling liquid (or refrigerant temperature). The air temp cannot go much below freezing or the coil will plug with condensate freezing on the face of the fins.

There is no theoretical maximum temperature, it is limited only by the metallurgy of the coil.

The maximum pressure drop is probably about 1 inch WC (water column) because higher than this means that the coil has such a condensate loading that it will likely begin to spit the condensate due to air transport. Additionally- by about 2 inches WC differential the pressure would begin to deform the coil, especially if it is a large coil.

Thanks for your reply. I think my question was not clear. Let me explain the situation,

The pressure drop accross the coolintg coil is zero and the temperature difference is too high (Inlet - 9 deg C and outlet is 21 deg C). I am suspecting the return air temperature is too high, eventhen I am having doubt that the cooling coil can make this much temperature difference ??

I think that your question is:

You have an AHU in a chilled water system. The water pressure drop in and out is zero (according to you.... there is allways a difference otherwise there will be no flow... the differential pressure might be too small but htis is another story). The temperature in is 9°C and the out is 21°C making a differential temperature = 12°C.

Answer: You are right: the temperature difference should be ~ 5 to 7°C max

Causes: 1- the air flow is substantial (very high...too high) and very warm. This might happen at the start of the cooling of a certain area that is very hot but should reduce quickly to the range mentioned.

2- The water flow is restricted or not enough volume flowing through. The AHU must have a regulating 3 way valve controlled by a thermostat-controller system. Check the valve opening and ensure that it is opening fully to direct the flow into the AHU otherwise, the flow could be going back (most of it) to the chiller. Also, if there are other AHU in parallel (or else other configurations), check them to see if they are operating properly in comparison. Pipe work, Valves, Circulating Pumps etc are all involved. ... Is this a new or old installation that worked OK before?

Yes this is the exact answer what I was looking for. Thanks for the same.

Please clarify, If the AHU serving area is too high or the particular AHU is designed for low Tonnage will it create the high differential temperatue across the cooling coil ?

If the AHU is undersized for the room it is serving, the differential temperature should remain the same (cooling water side) but:

1 - Either the target temperature will never be achieved for the room,

2 - Or It will take too long for it to get there. This will depend on the amount undersized and the Heat load coming into the room at the time it is tested.

By the way, the cooling water inlet temperature should be checked against the design requirement (at the time it was designed, the engineer will state the water temperatures and differential required with the Volume for the AHU, or use the AHU OEM tabled data and criteriae to select the AHU for the BTU/h or kW for each room. The in/out temp will be specified for the ahu to achieve the cooling required.)

if the Diff temp is specified at 7 or 5 deg C then this should be achieved by the chiller water circuit and piping and control valves etc. This is a job for experienced technicians in the field. For anyone to go about it requires a good knowledge of the equipment installed and the performance at each stage. Balancing the flow amongst all the units is one of these...

Cooling coil is a heat exchanger. It may be of DX type or be of chilled water or brine as the secondary refrigerant. Sensible cooling of air takes place when it flows over dry cooling coli whose surface temperature is lower than the dry bulb temperature of the air. Dehumidification will take place if the surface temperature of the coil is below the dew point temp of the air.

Many factors influence the Temp drop and pressure drop across the coil. Depth of the coil, number of rows, fin spacing, by pass factor etc are few. More bend more pressure drop. Usually copper is used for tube and Aluminum is used for fins. Fins shall be of smooth or wavy or louvered type. Depending upon the type pressure drop varies. As a thumb rule across the coil 4 C temp drop will take place.

Check ur system ( chilled water line ) u might have air lock in the system since ur Delta T is very High or u hav wye Strainer block or Slime in ur cooling coil it might be the reason for poor Heat Exchange

is there any other cause of poor efficiency of cooling coil like scale in copper tube and fouling of fines ?.

and if so please need guideline to over come this situation.