How to calculate temperature rise in a compressor room

[CobusvanJ]

Hi all,

I am more a pressure vessel engineer, but I am trying to do a pre-design check on the heating taking place in a compressor room. I am trying to figure out if cooling of the room is needed as the intake air temperature should not exceed 46deg C.

My Heat Transfer/Thermodynamics is very rusty, but I tried anyway and now my answer seems way too low so I expect I made a mistake somewhere.

Details are:
2 air compressors each 22kW
Outside Air temp 32deg C
Altitude above sealevel 1500m
Room size 6m x 4.5m by 2.5m high

Assuming almost all energy is converted to heat, I used Q=cmΔT to calculate delta T (the expected temperature change) but I got an answer of 0.93deg C which seems too low...

Can someone please assist?

 

[BronYrAur]

What are you using for your mass, or more specifically your mass flow?

 

[willard3]

Check manufacturers for this information, heat load is equipment dependent.

 

[chicopee]

Your equation Q=MCvdT is not quiet right. Using English units: Q in(Btu/hr); m s/b in (Lbm/hr); Cv is specific heat air (Btu/Lbm-dT); dT in (dF); Once you have determined your air mass flow rate; then you can determine your flow rate in (CFM) for the altitude of the project using the ideal gas law. The cfm will be free air flow rate another word there is no restriction to the air flow. If the are restrictions such as duct losses you'll need to calculate the system characteristic curve and get a copy of the fan characteristic curve to revise your CFM.

 

[IRstuff]

Your "details" seem to leave out the thermal characteristics of the walls, so that's one issue.

The second issue is that your calculation using the thermal mass is incorrect, since you only have heat FLOW, which is in watts, while your calculation must have used ENERGY, which is in joules.

Using simple convection, your 22 kW heat flow would result in a 56C rise on the OUTSIDE wall temperatures of the room --> 21m*2.5m, which makes the interior even hotter. Assuming the same convection process internally, the inside air temperature is at least another 56C higher, and possibly quite a bit higher, depending on the thermal conductivity of the walls. The converse is to assume that the heat loss through the walls can only allow 5C rise, resulting in a maximum of 2 kW heat loss. That means your A/C must supply at least 20 kW of cooling. Since one ton of A/C is 3.5 kW, you need at least a 6 ton A/C

TTFN (ta ta for now)
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[chicopee]

Oops! made a mistake it should not be Cv, it should be Cp(specific heat under constant pressure) for the equation Q(btu/hr)= m (lbm/hr) x Cp (Btu/lbm-dF)x dF(degrees Farhenheit). Cv is normally used for solids and liquids and Cp is normally used for gases and vapors.