Tempered of Untempered Make Up Air?

[lumbee]

I'm doing the hvac design for a commercial kitchen. I have used the formulas in the International Mechanical Code to find the required exhaust cfm. I'm trying to decide on how much make up air I need. The code says the make up air should be approximately equal the exhaust air. I know you want the kitchen to have a slightly negative pressure but what is the best way to determine the amount of make up air? Also, the code says that the temperature difference between the make up and the air in the conditioned room shall not exceed 10deg.F unless the air does not decrease the comfort level. How do you go about figuring the quality of the make up air?

 

[DouginMB]

If possible, use the actual exhaust and make-up air requirements for the hoods being installed. You can get this information from the kitchen design consultant. This is allowed by the code if the hoods are rated. The code values are usually conservative (high). Depending on the type of hood, you might have to heat the air if you're in a cold climate. If you're in a warm climate, I would insist on a back plenum type make-up. This will decrease the amount of make-up that needs conditioning under the assumption that all of the make-up supplied to the hood gets captured by the hood. The remaining make-up air (usually 10-20% of the total exhaust) should get made-up by a combination of transfer air from the dining space and direct ventilation through the kitchen HVAC system.

 

[KGRoberts]

Quite often kitchen exhaust make-up air is unconditioned, and for energy reasons needs to remain unconditioned. By using a short circuit hood you would make up 60-80% of exhaust. In very cold climates you might want to temper the make-up air.

You might also want to factor in the required ventilation requirements for the surrounding spaces when sizing your make-up air. Dining areas require a fair amount of OA and should be recognized in your air balance.

 

[lilliput1]

Note that the OA ventilation requirement in the dining & servery area may dictate if you can use kitchen hoods with integral makeup or not, since the hood direct OA makeup will not contribute to the ventilation requirement. In large dining areas w/ large OA for ventilation, you may need to reheat (thermostat sequence reheat after turndown to minimum volume if Variable air Volume System with fixed offset between supply & return) the supply air (mix of conditioned OA & return air).
Coordinate also w/ hood manufacturer the control for the exhaust fans & makeup air fans. Under fire condition, should the exhaust fan continue to run while the makeup air fan & room supply air fan system shut down or should all shut down?
Don't forget exhaust & transfer air makeup for dishwasher
Typical CFM transfer air for pressurization would be about 200 CFM per door. Provide transfer air openings sized at maximum 300 FPM free area velocity. If not enough free are space available, use transfer air fan interlocked with hood exhaust.