Ventilation Requirements of Warehouse with LPG Forklifts

To answer your first question, your building pressure will greatly affect the natural ventilation through the hoods. When your rooftops are in economizer mode, you may very well be relieving air to the outside through those hoods. Did the warehouse use to be unconditioned? If so, it may have just had exhaust fans bringing in air through the hoods.

Regarding existing hood capacities, if you were designing a new one, for outdoor air intake or exhaust, you'd want a design face velocity of 400 to 500 fpm. So you could take the free area of the hood and multiply by these face velocities to determine a rough CFM capacity at each hood--it looks like your 6600 CFM is in that range. It does, of course, depend on the building pressure differential as to how much will actaully be coming in or out of these at any given time.

I agree with BronYrAur that these may be remnants of a intake/exhaust system prior to the current warehouse mechancial cooling RTUs. Perhaps these can be closed off now that the 30-ton units have intake and relief capabilities, unless there are off-season times when they want to exhaust/intake without running the RTUs. Are there other independent mechancial fans for intake or exhaust at the warehouse? At any rate, if these gravity hoods are always "open", and they are needed at certain times, a revised set-up where you can positiviely open and close dampers at these devices would probably save some energy.

Regarding forklift ventilation, a good book for your arsenal in addition to the ASHRAE stuff should be the ACGIH Industrial Ventialtion--A Manual of Recommended Practice (I have Edition 22). In here they give some rules of thumb for dilution ventilation for lift trucks:

5,000 CFM/propane-fueled lift truck
8,000 CFM/gasoline-fuled lift truck.

Hope that helps.

CountOlaf
Thank you for reminding me of the ACGIH Industrial Ventilation Handbook. I have access to one.

I think that you both are right in that the gravity hoods are left over from when the warehouse was not conditioned. I have estimated from the power bills that the facility uses 235,000 kWh for air conditioning and have modeled in eQUEST that it should only use 35,000 kWh so I am going to suggest that motorized dampers be installed on the gravity hoods. Also intake and exhaust fans could be installed on some of them and then a night air cooling system could be used for cooling instead of using the air conditioners.