Coil coil load calculations

[offshorehvac]

Hi friends

Can someone please verify and clarify?

Application: Offshore Rig
Area of operation: North Sea
System X

Ambient : 72*F/80%RH
Inside required in Summer : 75*F/50%RH

Loads/Airflows

Room Sensible Load : 187000 BTU/Hr
SHR : 0.89
Supply air: 15000 CFM
Fresh air : 6100 CFM
Return Air: 8900 CFM

Air flows are based on minimum air changes requirements as per offshore regulations

Mix air conditions : (6100x72+8900x75)/15000=73.78*F/68%RH
(RH for mix air is from psy chart)

Supply air temp
187000=1.1 x 15000 (75-Ts)
Ts = 60.7*F/68%RH

When RSHR line is extended on saturation curve it gives an ADP of 47.5*F

For simplicity I have neglected Bypass Factor/Fan heat gain etc..

My Questions are:

1. What is the meaning of ADP 47.5*F and supply temp of 60.7*F. Do we have to cool down the air to 47.5*F and then raise the temp to 60.7*F.

2. Coil capacity should be calculated based on Enthalpy difference for Mix temp and ADP or Mix and Supply Temp.

Thanks

 

[imok2]

I get mix air temp = 73.777 and RH = 61.58
Even if the ADP is 47.5*F you will not get there because at 61*F DB leaving air you will still be above the dew point of the entering mixed air of 59.73*F or a dry coil. Sensible Cooling Btu/hr = 214,331 or about 18 tons

1. "What is the meaning of ADP 47.5*F and supply temp of 60.7*F. Do we have to cool down the air to 47.5*F and then raise the temp to 60.7*F". (NO)

2. Coil capacity should be calculated based on Enthalpy difference for Mix temp and ADP or Mix and Supply Temp.

Mixed and Supply Temp.

 

[offshorehvac]

Thanks Imok2

If we use only supply air temperature and still we can get room RH 50% if the process follow the SHR line, then, what is the significance of ADP.

Thanks

 

[AbbyNormal]

ADP is like the average surface temperature of the coil. Some air contacts the coil surface gives up moisture and leaves saturated at the ADP, some air sails through the coil and does not make contact with a tube or fin.

The supply air temperature is a mixture of the air that contacts the coil with the air that does not make contact with the coil

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[offshorehvac]

Hi

I have to give this info to a coil supplier for coil selection. would it be usefull if I give them the Supply air temperature as well as ADP info.

Does ADP plays any role in coil selection?

Thanks

 

[KiwiMace]

Not since computer coil selections programs came about.

 

[AbbyNormal]

The way I read your parameters you have no ADP.

Cool air down to the supply dewpoint you need and reheat it

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

Cool the air down to 54 or use a dehumidifier

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

maybe just cool down abOut 8500 CFM of outside air to 54F and go 100% outside air be 12 to 13 tons



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[offshorehvac]

Hi AbbyNormal

What could be the issues if I keep the system as it is. (for room temp and room RH)

As I understand (theoretically). If the procees starts at with a supply air of 15000 CFM, @61*F/68% RH, then follow the RSHR line, absorb 187000 BTU/Hr of sensible load and will give 75*F DB/50% RH

I have uploaded the Psy Chart and Report. Please have a look. Is this thing really going to work.


Thanks

 

[AbbyNormal]

Your parameters changed a bit, but there is still no ADP possible.

Extending the line from Mix to CC will not cut the saturation curve, there is no ADP that will work. This means a cooling coil by itself cannot convert the Mix Condition to the CC condition.

Why move all that air?


Your supply dry bulb is too low for 15000 CFM and 187000 sesnible room load, it would be more like 75-187000/(1.08x15000)= 63.5, the dewpoint of the supply air is going to be about 54 or os in that ball park.

Just cool off the outside air to 54F, or whatever your required supply air dewpoint is.

187000/(1.08 x ( 75-54)) = 8245 CFM

fan power will be about (8245/15000)^3 = 17% of what you will use for 15000 CFM, 15000 CFM could be a 10 to 15 hp motor, 8000 could be 5 hp.

You wind up 15000 CFM, you will see a couple degrees of fan heat.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[offshorehvac]

Air flow is based on minimum air changes requiements. There is nothing that I can do on this.

But now I understand that if I keep the system as it is, I can get 75*F but there could be problem in getting 50% RH. Right?

 

[AbbyNormal]

If you went all outside air you would get more fresh air than you require. Do something like face and bypass then if you want to waste fan power. Cool off the outside air bypass the return air if you absolutely have to have the high air flow. Winter you could throttle down the outside air and move more return.

With your present scheme I do not see any problems controlling the temperature, but I do see a problem controlling humidity.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[imok2]

If you ae not able to manipulate the osa and return air dampers to vary the cfm then with your stated conditions you will always be at the mercy of the OSA as far as the humidityis concerned, but if you can adjust the CFM of OSA and return air then you may have a better chance of controlling the humidity to a better degree