Kitchen Exhaust Make-up/direct fired

[carlosgw]

Kitchen exhaust hood suppliers have been providing direct fired gas makeup air units for the makeup air in kitchens. We need to get some of the outside air to the dining room for ventilation. I do not like to use direct fired natural gas units for ventilation because 1. of the added humidity & 2. I just never liked the idea of direct fired units. Of course the owner does not want the added expense of the indirect fired unit. Am I the only one who does not use direct fired for ventilation?

 

[SAK9]

One solution to cut down the outside air make up to kitchen is to draw part of the make up air from dining rooms or adjacent corridors etc.You could mix this with outside air to achieve comfort conditions in the kitchen.This would maintain kitchen negative with respect to dining rooms and would help contain the kitchen odours.

Some amount of reheat will still be required depending on your minimum winter temperatures.Direct fired gas heater is an economical option and is no diffrent from the gas hob in the kitchen!

I would not recommned any kind of heat recovery system on the kitchen extract as you will be spending all the money saved in mainatining it.

 

[lilliput1]

I also don't like to use direct fired units. You can not use a filter with it. There is posibility of smelling the products of combustion if combustion is not complete.

 

[HVAC68]

I would suggest to keep the kitchen under negative pressure, thereby inducing some air from the adjoining dining areas (I suppose the dining areas are adjoining the kitchen) - this would also ensure that the kitchen gets some comfortable conditions. The make-up air or outside air can be added to the dining area AHU.

HVAC68

 

[carlosgw]

That's what I am talking about. How to heat (you could say pre-heat) the ventilation air to the dining room which is then drawn back though the kitchen as part of the exhaust make-up. 9 times out of 10 the building is small and there is not rest-of-the-building to draw air from.
Has anyone experience using direct fired units in occupied spaces? Is moisture a problem?

 

[GMcD]

I'm also on the "No Direct Fired MUA Bandwagon". I have never specified, or accepted as an alternate, a direct fired make-up air unit for a commercial kitchen installation in the last 20 years. I usually get no argument when I ask if the owner/Architect would like flue gases spewed onto their dinner? The Building Codes are a MINIMUM Standard, and "good practice" should be at a level above the Codes. You also have to consider the cooks who breath this mixture of air for long periods of time, and their occupational exposure to flue gases.

Use an indirect fired heating section, but make sure it is stainless steel of some kind, as 100% outdoor air in colder climates will create flue gas condensation in the burner/heat exchanger section and rot a normal galvanized/aluminized steel hex section out in no time.

 

[cme]

We need to get some of the outside air to the dining room for ventilation.
......... I wouldn't dump unconditioned air in the dining room.

You want to keep the kitchen area min 20% negative relative to the dining area.

 

[Geof]

Being in the business of manufacturing both indirect and direct gas-fired heating equipment, my perspective is a little different.

Direct-fired is cheap. That's the bottom line. We sell more of those units than anything else. The construction is simple, pressure losses through the unit are low, and they last forever with a little maintenance.

Indirect-fired is more expensive, both to buy and to operate. The exchangers have a limited life span - we warranty for ten years, some other suppliers won't give you that much. Pressure drops through the unit require more horsepower. There are more components to break down (good for our parts department ).

Both are safe. We've never had anyone injured by a direct-fired unit. However, IAQ is a consideration. If the moisture or traces of CO and NOX can cause problems, indirect is the way to go.

Next, if you are the specifier - stick to your spec. We've lost many jobs because we were spec'ed, bid our own equipment, and someone came along with a low-cost alternative that the contractor liked and the engineer approved....

 

[cme]

In 10 years the casing will be rusted out, the motor junk and the bearings shot.

That's its useful life.

 

[NNACons]

I see your op tions as rather limited.

1) retrofit existing dining room system to accomodate for additional outside air in order to pressurise dining room in relation to kitchen while actually acting as makeup air for kitchen exhaust. This can introduce some rather large outside air loads depending on how much you are exhausting from kitchen.

2) if you cant upgrade your dining room system, introduce make up air supply as close to kitchen hood as possible without affecting the capture performance of the hood. this will limit the level of discomfort of kitchen staff during winter applications associated with dumping of cold preheated makeup air.
(We usually specify kitchen hoods with built in makeup air paths).

3) I dont see a problem with gas fired heating equipment to serve as heating for kitchen make up air so long as the following points are abided by.

a) kitchen exhaust system and a/c system to dining room with fresh air provision to ensure + pressurisation between dinign and kitchen,airflow from dining to kitchen) are interlocked

b) Fresh air to dining room should be based on local code requirements + additional amount to ensure pressurisation of dining room with acceptable limits. gas fired make up air should be sized for a fraction of kitchen exhaust system, so that outside air from dining room + gas fired makeup air in combination is exhausted via kitchen hood.

These two requirements overcome any problems associated flue gass spills and general comfort of staff in kitchen.

 

[carlosgw]

OK, here is what we did (and it was really the only choice due to lack of mechanical space, actually it may be hard to actually fit this in) - The dining room is served by a indirect gas fired makeup air unit (modulating control) with two parallel DX cooling coils. The ventilation air is a little more than 1/3 the total dining room air and also turns out to be about 1/3 of the kitchen exhaust.
The kitchen hood has a perforated make-up plenum that right at the front of the hood that is served by a direct gas fired unit (the remaining 2/3 of the exhaust volume). The kitchen is wide open to the dining room by the way. The kitchen hood, exhaust fan, and direct makeup air unit are all supplied by the kitchen equipment supplier in this case.

 

[lilliput1]

Did the kitchen hood direct fired makeup air unit have a filter? Did it have a heating coil? How is its discharge air temperature controlled? Is it interlocked to shut down while the hood exhaust remain on in case of fire?

 

[lilliput1]

Also what type heating coil does each unit have? Can the dining area unit reheat to control humidity?

 

[carlosgw]

I think so.
Are you asking what type of coil the direct fired furnace has?
Duct temperature control.
Yes.
No reheat (having two parallel DX coils will help) (northern NY).