HAP 4.41 Heat Loss/ Heat Gain Calculations.

[raptors076]

Hi,

I have trouble understanding the output of the HAP 4.4 Carrier software.
I did calculations for heat loss and found out that cooling load comes out to be 5 Ton but the CFM required for the space shown in the result sheet is 2400.

How do I interpret these results? Maximum CFM per ton you can get is 400. How can I size AHU based on this calculation. Should I go for 6 Ton unit or is there any error in the calculations?

Is is possible that load is 5 Ton and the CFM required is 2400? If so, can someone explain it. The only case I can think of is number of people in the space are considerably more than normal occupancy, which is not the case in my prjoect.

Thanks

 

[walkes]

400 CFM per ton is a nominal flow rate. In the results you will have to look at your coil leaving temperatures as well as the determine if you have a blow through or draw through coil. The coil has to account for motor heat gain. The draw through coil will have a lower leaving air temperature to account for pick up from the motor.

You will also have to check to see what you have put in your system for duct heat gain to see if you are accounting for that.

Check your system for supply temperature as well.

The total coil tonnage will take into account both the sensible and latent loads.

 

[cdxx139]

I had the exact same question when I started doing loads and using HAPS.

400 cfm/ton is a Rule of Thumb, I dont remember exactly where it came from (and if anyone knows Id love to relearn it). So dont consider it a maximum. BTU=1.08 CFM dT right? So if my cfm is 400 and my delta T is 35, you get over a ton!

Use HAPS as a guide. But always do a cooling load by Psych chart. Get your mixed air, look up your delta enthalpy, and get your tonnage. Then compare to HAPS, and you will have a better idea if you should up to 6 tons.

knowledge is power

 

[imok2]

A rule of thumb in air conditioning work is that 400 cfm of air is equal to 1 ton. If 400 cfm of air and a delta T of 20 degrees is achieved, this is equal to only 400X20X1.08=8640 btu/h. However, this neglects the latent heat portion of the cooling load.

The latent heat (3360 btu/hr) is absorbed at the coil by condensing moisture from the air. The formula taking into account the latent heat is: multiplied by X (CFM) * ?T (oF) results in or X*1.08*?T Btu/h to get total heat. Another equation is Q = 4.5 x cfm x Delta h to get total heat. What you are taking into account with these numbers is humidity control.

 

[DrRTU]

Post the output so we can review it. Include air system and spaces.

 

[317069]

400cfm/ton is rule of thumb.
cfm is calculated regarding to the air status on both sides of the cooling coil.
as for your program, check the entering temperature and leaving temperature then apply the equation mentioned above, you will know why the program gave you 2400 cfm.
also if you check the performance section in the cooling unit manual you will see more than one value of cfm for the same unit at the same capacity.

 

[MechEngNCPE]

Don't use the 400cfm/ton rule of thumb. I heard that as a designer and it has never been applicable in any project I have done. Go by BTU/h/12000 to get your tonnage, like mentioned above Q=4.5*CFM*(DH). Thats the equation, not a "rule of thumb" Always chech with the phsych chart as well.

 

[317069]

you can check the help file in HAP program. I believe that it explains the calculations as the block load does.
also you can check the supply air temperature you have selected, the resulting RH, and SHR. but in general, cfm is related to sensible load only not total load.

it is not nessessary to go with 2400cfm as your program said or with 2000 cfm as you think you should do, you have to go with the cooling unit manual performace tables and get you loads to be satisfyed

 

[raptors076]

I have uploaded my heat load calc. and Carrier units. Please review them and provide your comments.

I didnt mean to use 400 cfm it was just there for reference. Each cooling unit has different cooling capacity. For some of the units I provided for Carrier suggest this 400 cfm per ton number.

My question, I guess I didnt put it correctly was, how can I size the cooling unit if you have to meet both CFM and BTU/hr for the spaces. My cooling load is 3 ton and total supply air required is 1378 cfm.
Either I can go for 3 ton unit which meets my load requirement or I can size my unit based on CFM required. 1.e 1378 CFM.

 

[raptors076]

1

 

[raptors076]

2

 

[walkes]

Check the System Sizing Summary printout. It will show the tonnage of your cooling coil and the coil leaving air temperatures that should correspond to the values on the air system design load summary.

With the additional air flow your coil leaving temperature may be higher, then you will lose the ability to dehumidify.

 

[317069]

The air system design load summury you have uploaded shows:
1- your total coil load is 36086 Btu/hr. which is 3 tons, not a 5 tons as you said in your first post.
2- your supply fan 1378 cfm, not 2500cfm
3- there is no any window or door
could you please upload the first page of the program print out.

 

[EngAzat]

Hi raptors076
I think the attached pdf file will be helpful
Good Luck