Air Change Rate (Ventilation System) 3

[aida2011]

Dear expert,

I want to size ventilation fans in a chiller machinery room. The heat gain by the room is 136 kW and the volume of room is 1830 m3. Temperature must be maintained between 35 to 40 deg.C. After calculation, the ACH required is 47, which is insanely high. I read in literature that the typical ACH for chiller room is less than 15, and of course it does not tally with my calculation due to the size of the room. So, what figure should I take and why?

Thanks

Isaac

 

[DRWeig]

What entering (makeup) air conditions did you assume?

Best to you,

Goober Dave

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[chicopee]

I am assuming that you want outside air to cool down the machinery room, so before you can calculate the air change per hour, you need to know to outside air design temperature than there would be an appropriate formula to use.

 

[KiwiMace]

When you have open drive chillers + pumps with a lot of heat load in the room, you can quickly get to a point where it is easier to install an AHU to manage the heat load than deal with a huge ventilation rate. 15 ACH is 27500 m3/h (16000 cfm) = lots of big propeller fans with a wall of your building dedicated to letting air in and stopping noise getting out. You must have mechanical ventilation (EN378, ASHRAE 15) but it need not be so big.

 

[MintJulep]

Please share your calculation, so we could perhaps point out any error you may have made.

Otherwise we're just guessing.

 

[ProcessHVAC]

Your calc is definetly wrong, not sure your exact OA conditions, but assuming 86 F, I get about 13-20 ACH

 

[317069]

Two fans of the type used in paint booths with two make up air units or louvers of 15sq.ft. free area each can provide 16000 cfm,
Room load is about 50 tons, set point is between 95 to 104F. if this is the outside conditions too, then we consider your equipment as they are installed in outside under a shade.

 

[317069]

Ooops load is not 50 tons

 

[willard3]

The blind leading the blind.........

 

[317069]

Bible studies

 

[ChasBean1]

If you want to keep the room at 85°F and design conditions outdoors are 91°F db/73°F wb, the cooling air exchange requirement is up to infinity. If it is a mechanical space with cold piping and potential for condensation on components, I would suggest instead dehumidifying the room (providing slight air conditioning from nearby 24/7 AHUs or via a dedicated DX coil). Provide slight positive pressurization and dehumidification by cooling any makeup air to about 55°F. Why? Condensation will greatly reduce on components, sweat on piping and insulation will reduce, there will be a higher degree of comfort (even if it is 82°F dry bulb) and all cold components will last longer. If you don't, it's like all your pieces and parts will be exposed to rainfall whenever the outdoor dew point is high. I constantly tell my kids to get their bikes in the shed when there is rain coming so I don't have to continually repair the bikes by replacing pieces and oiling chains. You have the same opportunity here. The plant will degrade when parts are constantly wet. Dehumidify your mechanical spaces. It will pay back by the truckload. Even if it's +500 cfm of dehumidified air in a very large mechanical space, the reduction in humidity will pay back dividends.

 

[chicopee]

A sample of a simplified evaluation of your OP is in the attachment. I have used similar values that you presented and the assumptions may not be consistent with your situation but at least this may assist you as a guide. I got eight (8) air change per hour. Regardless on the numbers crunched, the air flow distribution throughout the building is important as you do not want dead air pockets.
Good luck

 

[chicopee]

The final air change value should have been 1 air change in 8 hours or 1/8th of an air change per hour.

 

[ChasBean1]

Aida, can you pls. show you calculations?
thx

 

[aida2011]

Hi folks

I would like to thank you all of you who posted very insightful comments, and my special thanks to chicopee for providing me his calculation.

My calculation is as follows:

Machinery room ventilation air required for chillers can be estimated by using the formula:

V = H/(ρ×C_p×∆T)
= 136/(1.099×0.017×11)
= 661.8 m^3/min

Where

V = Ventilating air (m3/min)
H = heat radiation by 3 chillers (kW) – data given by GEA
ρ = Density of air at 38oC (1.099 kg/m3)
Cp = Specific heat of air (0.017 kW.min/kg.oC)
ΔT = Permissible temperature rise in machine room (oC)

The ventilation air required is 661.8 m3/min or 39,708 m3/h

Since the volume of room is 1,830 m3

ACH = 39,708/1,830

= 21.7 ACH

Maybe something wrong that I got a very high figure. Please help.

Best regards

 

[ChasBean1]

Understood. I was being facetious when I asked about the calcs. 21 ACH is (based on your calc.) on an 11° delta-T. If it's 73° outside, the calc is for an 84° room. If it's 85° outside, the calc is for a 96° room. If your room set point is 85°, but you're using 90° outdoor air to cool, your airflow requirement is infinity...