HVAC System Selection

[Zeroman]

I am working on a project where I have to put a new HVAC system in a building full of pipe transporting hot bitumen and steam. The building is H5.2m x L15m x W7.2m and is at a temp of 40 deg C. How should I approach this problem? I have been told that there are not many people in this building for a long period of time. Does that mean that I can simply fit it with a make up air unit (no coolant) and exhaust grills? How would I calculate how much CFM the fan should be if I wanted the room temperature to be about 25 deg C? Am I going about this problem incorrectly, I would appreciate any comments.

Thanks !

 

[ChrisConley]

The first thing you need to do is get a handle on the actual heat gain of the space. You should be able to do a reasonable estimate of the heat loss if you know the temperature of the fluids, and the insulation value. You would be able to get an even better idea of the heat loss if you knew what the temperature of each fluid was going into the building, and then leaving.

Once you have a heat loss you then need to look at what the practical maximum temperature you'd like to maintain is, and then look at your summer design condition. If you intend to cool using only make-up air the volume of air can get very high if you are trying to maintain a setpoint too close to your outdoor temperature.

The formula for outdoor air required for cooling is:

cfm = Heat Gain/(1.08 * delta T)

- Where delta T is the difference between summer design temperature and your desired indoor temp.

This is where you have to make the judgment call on how hot is too hot. You might consider making personal cooling equipment available for workers and increasing the maximum allowable temperature.

 

[Zeroman]

Hi Chris, Thanks for the info. So what your saying is if I know the R value of the insulation around the piping and the inside temperature then I should be able to calculate the total heat leaving the pipes using equation Qout=h*A*deltaT where h is the convection constant, A is the surface area and delta T is the difference across the pipe, where T2 is 30deg C and T1 is the temp of hot bitumen (approx. 280deg C). So all I need is to get the h value, right? And using this value of Qout I can calculate the cfm using the information you gave me, is this correct? Thanks again Chris.

Dave

 

[MarauderX]

Chris Conley has the right start - find the heat gain and go from there.

You may want to know a lot more about the piping layouts and processes that are going on in the building. The best design would be to place your air delivery/exhaust as close as you can to any heat/fume generation as you can. If it is simply a plant for heat exchange and you only need to cover the losses, the calcs become easier. If there is heat generation or chemical mixing going on, you will most likely need a good ventilation system to deal with excess combustion gases or emergency spills. If there aren't workers in the building doing maintenance for extended periods you should be ok to provide a bare minimum, although codes might dictate you give the space more.

For a difficult layout requiring a lot of coordination from the contractor, I would keep as much of the HVAC units outside of the box by putting them on the roof, if possible.