Calculation KW ouput from a chilled water unit

[Thermo01]

Hi there,

I'm looking for some help on calculating the duty of a chilled water computer room air handler

I've taken some readings (from calibrated equipment)on both the water and the air side and calculated the duty from these figures but the figures are no where near the same.

Air on 29'C
Air off 17'C
Density 1.2Kg/m3
Airflow 6m3/s

Using Q = M x Cp x DT this gives me a duty of 6 x 1.2 x 12 = 86.4Kw

However taking figures on the water side from the same unit at the same time I get the following figures

Water inlet 12'C
Water Outlet 7'C
Specific heat capacity 4.19Kj/Kg
Flow rate 2.483L/s

Using Q = M x Cp x DT this gives me a duty of 2.48 x 4.19 x 5 = 51.956

Can anyone advise on to where I'm going wrong.

Thanks

 

[LittleInch]

well in the air case the figure used is density, where as what you need is Cp or heat capacity.

however as this is approx. 1 kJ/kg/K then it doesn't actually impact the result

How did you measure air flow? that's a lot of air moving around?
Water flow looks too accurate (number of digits)

but basically it's check the data to see if you've missed something or assumed something which isn't correct. The numbers seem to work out. We can't see your system so can't check anything.

Two chillers?

Also is there appreciable condensing of water vapour?

Are you at 7,000 ft?(much lower air mass / density)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Thermo01]

Hi,

Yes, sorry I missed the density off due to it being 1 or 1.02Kj/Kg and due to the fact it doesn't really change the outcome.

This unit is inside the building and recirculates the room air, no fresh air at all, Just a bunch of servers.

I measured the airflow using a vane anemometer, there are 3 large grills and a mean reading was taken across each grill, then the air volume of each grill was added together.

The water flow was taken by a digital water manometer and is correct.

The relative humidity in the room was at 30%, if this helps.

 

[LittleInch]

Airflow measured that way isn't great. Depending on the size of the grill it will / can vary across the whole square area of the grill.

how did you get the "mean" value?

A water manometer measures differential pressure. How was this converted to flow?

I'm guessing there is an orifice plate involved somewhere but how do you know what it's parameters are?

Sorry to question your readings, but when you're a long way adrift there must be something seriously in error.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Thermo01]

I know, it would be better with a balometer but I've used the vane anenometer in the past and the figures are usually similar. The vane anemometer has an option to use mean, making it a bit more accurate imo.

My digital manometer has the Kvs values stored for various valves, I input the valve type and size and that auto calculates the flow. ( maybe this is something that I should check the old fashioned way)

I have the design airflow and water flow rates for this unit so I know what they should be.

 

[EnergyProfessional]

For cooling, you also have water phase change, latent load. You need to measure dewpoint of incoming air and account for that. Your equation just works for sensible heating or cooling.
This is quite a coincidence, since just yesterday I wrote a program to calculate water (or glycol) flow based on air enthalpy and liquid temperature measurements. One byproduct of that calculation is the cooling capacity.

 

[LittleInch]

I suspect your water flow is the crucial one here and one which is harder to check.

Not sure how a DP across a valve is going to give you accurate flow as when it's fully open there should be very little DP to measure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Thermo01]

Energy professional, that's a good point, I was only measuring sensible. I suppose I should have used enthalpy DT for total cooling load but these units are high sensible cooling units and have a high SHC of 0.97 or so.
Interested to hear more about that program.

 

[Thermo01]

LittleInch.....Yep, you guessed it.

I'd inputted the valve into the meter as one size smaller meaning the Kvs value was 48, when it should have been 82.6, obviously this had an effect on the flow rate.

using the correct flow rate now means that both duties are within 10%.

With the cooling flow rate I also didn't remove the fan gain which was 6kw or so.

 

[moideen]

air enthalpy difference is need to calculate the air side heat capacity.

4.5*CFM* AIR ENTHALPY DIFFERENCE= BTU/HR