Calculating Coolinig Load for Gas Stream (any HVAC engineers out there?)

[puttyME]

We have a digester gas stream of 2,000 CFM flow rate. It has an entering temperature of about 200 deg F and we need to cool it down to about 75 deg F (125 degree delta T). How would you calculate the BTUH of heat needed to be removed from the gas?

My first instinct was to apply a modified version of Qs = 1.08*CFM*deltaT or Qs = Cs*Qs*deltaT and Ql =Cl*Qs*deltaW (from ASHRAE book)

But im not sure where the 1.08 factor comes from and if these equations would even apply to a gas other than air with a heavier molecular weight.

I've got a delta t and a flowrate, you'd think this would be easy, what am I missing?

Any help is appreciated, Thanks.

 

[RossABQ]

Q = M * Cp * delta-T
1.08 comes from substituting the appropriate values for air into this equation with CFM as the variable

 

[cry22]

Think Google

http://www.scribd.com/doc/13839504/HVAC-Constants

 

[ElSid1]

The number assumes a .24 thermal conductivity for air. Yes, you can use a modified version of the equation, you just need to know what the ratio is when comparing air to digester gas. A thorough example is in the HVAC Rules of Thumb Handbook. PLEASE note, the rules of thumb are based on STANDARD Conditions!. This is a basic mistake in almost ALL calculations I see. The density of air at 0 ft (sea level) and 70F is diffent than 100 F at 2000 ft elevation by 12%.

Off of my soap box (just did a presentation on air density correction), for the digester gas, going through a heat exchanger, use the LMTD method to calculate the BTU/hr.

 

[trashcanman]

Note that the 0.24 Cp is an average and is okay for AC purposes. You should use the more accurate integral form since you are cooling from 200 to 75 Deg.

 

[MTES93]

They have all hit it.

1.08 = 0.24 x 60 x 0.075 or specific heat x 60 min/hr x standard air density. Air density (if that is your product stream, varies all over the map with elevation, temperature and humidity and even fan inlet pressure (it is compressible)). As mentioned, specific heat can vary as well. In order to determine the mass flow of your process stream you need to know the conditions at the inlet to your fan (assuming that is what is moving it). Assuming a simple system with an inlet, a fan, then a coil and an outlet, and assuming 200 deg F inlet is what your fan sees, then the mass flow hitting your coil would be determined there. Your CFM at 75 degree F (once you cool it) would be different than what your fan was seeing, however the mass flow would be the same.

Remember fans are constant 'volume' devices meaning they will try to move the same volumetric airflow regardless of density (as long as the motor can handle it, but once the air is in YOUR system we should then revert to the conservation of mass... ie.. mass flow is constant in the system.

This appears to be an industrial type application, a great reference is the American Conference of Governmental Industrial Hygienists, Industrial Ventilation Handbook. Chapter 5 talks specifically about this.

 

[EnergyProfessional]

You need to know heat capacity of the gas mixture. I assume methane and CO2? that way you either derive a factor to adjust the "1.8 equation" or you calculate it.