sizing a/c for exhaust hoods 1

[gjack007]

I am trying to get an idea of how much additional tonnage is needed for cooling of a commercial kitchen (pizza parlor). A computerized heat gain calls for 6 tons which includes the building, people, lighting, etc. How much additonal tonnage is needed basedon the following:

3 exhaust hoods @ 4000 cfm each - the are estimated to exhaust at 4000 cfm and provide make-up air (un-tempered) @ 70-80%

Can someone help with this?

Thanks

 

[DouginMB]

A few questions must be answered before i can help:

1. Where is this pizza parlor?
2. Are you sizing A/C for the whole pizza parlor, or just the kitchen?

 

[ChrisConley]

In general though, the way to calculate the additional heat gain is using:

Q = 1.08 (cfm) deltaT

If you have no heat recovery the additional cooling load will be (assuming around a 25 degF delta T)

Q = 1.08 (12,000) 25 = 324,000 Btu/hr

The question I have is what do you mean that the exhaust fans will provide untempered make-up air...@70-80% of what? How are the exhaust fans providing make-up air?

Regardless, due to high exhaust values I'd recommend examining some form of heat recovery on the exhaust.

 

[tlhoo]

Is the make-up air being introduced at the hood? With your numbers of 70%-80%, it sounds like you may be using a preforated plenum type of hood. If not, look into these. They provide good capture and containment, and have a low velocity make-up air stream that dumps air into the space right around the hood. The untempered air then directly goes out through the hood, having little effect on the comfort of the rest of the kitchen/building. In this case, you can discount the effect of the heat gain from the untempered air(at least partially). Are you providing any heating on the make-up air? I'd look into including direct fired heat at the make-up air unit. You might have gas service in the area from the cooking equip., and it's not that expensive an add to the unit cost.

Couple other comments:

1 - Short circuit hoods don't introduce untempered air into the space, but they're horrible at capture and containment.

2- Don't use directional high-throw diffusers near the hood. They can blow the effluent right out of the hood. Use perforated diffs that actually just dump the air at low velocity. These will have little/no effect on capture and containment.

3- If you have heating at the make-up air unit, set the discharge temperature @ 50-55 F in the winter. A cook at a hot grill would rather feel 50 degree air on his head than 72 degree air (i.e. don't make it neutral air, take advantage of the fact that it's cold outside).

4- Most kitchen complaints result from poor capture and containment at the hood. If the hot effluent and cooking- generated water vapor (humidity) is spilling out into the room, then you're not going to have a comfortable kitchen.

5-Look into end panels on the hood. They can lessen the amount of required exhaust airflow without affecting capture and containment. (And in turn, lessen the amount of make-up air)



Good luck.

 

[gjack007]

The pizza parlor is in Providence, RI. The whole restaurant is 2000 square feet and the cooling would be for the whole building. It is an open style kitche so the area is one big zone. According to the equipment sales rep, the fans exhaust 4000 cfm and make-up 70-80% of the air from outside. I am not sure how this is done but I will have more info. tomorrow. They told us the fans only replace the 70-80% to keep some type of negative pressure to insure that the exhaust products are removed adequately.

I did use the formulas as shown and came up with some staggering tonnage for such a small place. Thanks for the input - I'm still; open to any suggestions.

Gregg

 

[imok2]

Seems to me the best solution is to isolate the kitchen area

 

[jherbert]

hi,

one must be very careful chosing systems
for kitchens with open flame.

If you are thinking about a DX roof unit,
it is worthwhile to remember that leakage
from DX systems could be fatal.

People have died. Refrigerant and open
flame = phosgene gas.

cheers,

 

[quark]

6 ton seems to be ok for a 2000 sq.ft restaurant. Are the cooking stoves totally covered? If not they will extract the air from the conditioned area and you will have to increase the tonnage. I will consider Imok's suggestion of separating the kitchen.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[DouginMB]

Not having a plan of your restaurant, i have to make some assumptions and apply code values:

1. 1/3 of floor area is kitchen, 2/3 is dining. So dining area equals 1,333 sqft. Building is rectangular, roughly 30' by 66'.

2. Based on code maximum occupancy, dining should seat about 90 people.

3. 90 people at 20 cfm/per is 1800 cfm of ventilation air introduced into the dining.

4. For Providence RI weather, about 5 tons of cooling is needed to cool the ventilation air.

5. Lights (1.5 W/sqft.) and people load add another 3 tons.

6. Ceiling height is 10'. Percent of wall area that is glass is 20%. Assuming R-13 insulation in walls and R-20 in roof (code values), and clear, insulate low-e windows, envelope load add about 2 tons.

For 3 hoods at 4000 cfm each and 80% make-up, that equates to 2400 cfm make-up. The ventilation to the dining (1800 cfm) can be applied to this make-up, so only 400 cfm needs to be brought in through the kitchen unit. This equates to about 1 ton. Not knowing the heat gain of the kitchen equipment and frequency of use, I would estimate about 20% of the listed gas input btuh. (in other words, a 100,000 btuh oven would require 20,000 btuh of cooling).

In summary, for this pizzaria in Providence RI, it looks as if the dining will require a 10 ton unit. The kitchen cooling will depend on the amount/use of equipment, but if three 100,000 btuh ovens are used (constantly), a minimum of 6 tons would be needed.

Note, all of the usual disclaimers apply as I hastily did these calculations on scratch paper during my lunch break. Any load calculation should be verified by a design professional familiar with local codes and building plan.

 

[DouginMB]

Oops, sorry about all of the underlining. i can't find a way to edit the post.