View Full Version : Temp. Rise at duct outlet

04-05-2008, 05:04 AM

While performing a test in a low temp air chiller, the outlet temp from the coil was -25F.

At the fan inlet (Plug Fan), which is 10" from the coil, the temp is 6F.

At the outlet of the 10" dia duct, which is only 6" long, temp is again 6F.

Air qty -- 500 CFM.

Can someone suggest, where and what we are doing wrong.

Checked the temp display and the sensor and all found OK.

Tried testing with different tester and same results, when sensor is at the coil outlet, temp is -25F and at the fan inlet temp is 6F and same at the fan outlet.

05-05-2008, 01:05 AM
What is the air inlet temperature ?

Is that -25F an air temperature or a pipe temperature?

What size is the coil in relation to the duct size?

05-05-2008, 01:16 AM
Is it possible that air somehow bypassing coil? Could you describe little more that evaporator-fan-duct setup?

05-05-2008, 01:18 AM
Is the fan running backwards?

06-05-2008, 03:14 AM
Air Inlet Temp (Coil Inlet) is 70F.

Air coming out of the coil is -30F.

Air outlet at fan is 6F.

Coil dimension are 12" X 15".

Fan -- 10"dia centrifugal, direct coupled- 0.16 KW.

Duct outlet -- 10".

CFM -- 400-500.

Fan is not running backward.

The air temp is measured at coil outlet, between coil and fan inlet venturi ring.

Did smoke test in the coil/fan chamber and found no leaks.

07-05-2008, 12:55 AM
Can you confirm the air flow rate is actually 400-500cfm?

What is the fan motor temperature as this is in the airflow it will affect the air temperature.

07-05-2008, 03:14 PM
I checked the air flow rate and is actually 450cfm.

Fan is enclosed and motor temp cant be checked.

07-05-2008, 10:19 PM
This should realy not be possible at all. The only solution I see is air by-passing the coil. Are you sure of your -30F reading? Can you move the sensor to different locations, at the very end or side of the coil, not in the middle. Where exactly do the temp rise occur?

08-05-2008, 03:40 PM
I am getting -30F after 4" of the coil outlet.

Temp rise occours at the fan outlet.

08-05-2008, 06:57 PM
Ok, is the temperature -30F at the very fan inlet and 6F at the outlet? If so, this indicates the fan is responsible for the temp rise. I suppose absolutely no outside air is drawn into the fan inlet. If the air flow is very low, then it will be a substantly temp rise over the fan. You have 30-35F temp rise over the fan. 160W ejected into an air flow of roughly 21 CFM give a temp rise of roughly 35F. Are you absolutely sure of your air flow? Is the fan consuming substantly more power than it should be? You should maybe measure the amperage of the fan. How is the ducts insulated? Porly insulation also adds to the increase of temp.

09-05-2008, 03:46 PM
Thanks SteinarN,

As U mentioned about the poor insulation, I think that could be the reason as I see lots of condensation on ONE side of the Insulated panels.

I'll double check this again.

09-05-2008, 09:53 PM
SteinerN is correct,this is not possible.In your old money at the flow rates and temperatures described the coil is removing 54,000btus/hr ( 15.83Kw ) so if the fan is only rated at 160watt it cannot be responsible for the heat gain occurring at the fan outlet.The heat gain you describe equates to 36% of what has been removed so that is 19,444btus/hr ( 5.7Kw ) Also unless the coil is minimum 6 row I fail to see how the coil size given can provide the cooling effect anyway so suggest you provide some more detail on the system ie refrigerant,application,compressor details,other components fitted etc.How about a jpeg even.

09-05-2008, 10:06 PM
Hi, jwasir!
You say "low temperature air chiller"! Does that evaporator coil have defrost heaters. If you have defrost heater, is it possible that heater is on?

10-05-2008, 04:22 PM
The Evap is 10 rows deep and it has HG defrosting...

I still beleive the temp rise is due to poor insulation.

Thanks everyone!!!