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





RE: Calculating Coolinig Load for Gas Stream (any HVAC engineers out there?)
1.08 comes from substituting the appropriate values for air into this equation with CFM as the variable
RE: Calculating Coolinig Load for Gas Stream (any HVAC engineers out there?)
http://www.scribd.com/doc/13839504/HVAC-Constants
RE: Calculating Coolinig Load for Gas Stream (any HVAC engineers out there?)
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.
RE: Calculating Coolinig Load for Gas Stream (any HVAC engineers out there?)
RE: Calculating Coolinig Load for Gas Stream (any HVAC engineers out there?)
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.
RE: Calculating Coolinig Load for Gas Stream (any HVAC engineers out there?)