A Realistic Evaluation of Kitchen Ventilation Hood Designs
This post examines the performance of the most common hood designs in an effort to determine the best application and credibility of each design and gives results of field tests on two hood designs.
Determining the quantity of exhaust that will adequately ventilate the contaminants produced by a line of cooking equipment can be a difficult procedure. There are many factors to be considered, including the amount of heat, grease and smoke produced by the cooking equipment; code requirements; room drafts; HVAC system design: and hood design.
Several methods of determining the proper level of exhaust for cooking equipment ventilation hoods have been developed The three most common methods will be briefly discussed.
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The best known method for determining exhaust for ventilation hoods is set forth in NFPA 96 (NFPA 1980). The method presented in this standard bases its calculation on maintaining a given velocity of 100 to 150 fpm (0.508 to 0 762 m/sec) into the hood capture area The exhaust quantities predicted by this method prove to be far greater than what is actually necessary in almost every case.
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While using the exhaust quantities calculated by this procedure will ensure capture there are clear drawbacks to using this method. By exhausting large quantities of air from the kitchen, a great deal of conditioned room air is lost from the building. Therefore, large quantities of make-up air (MUA) must be brought back into the building. Depending on the geographical location of the building, the cost of tempering the MUA could be quite expensive. To help alleviate this cost. the short-circuit-type hood was developed. This type of hood delivers untempered MUA into the capture area, where it can be exhausted without affecting the kitchen environment.
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Other hood designs were developed with various MUA diffuser locations and designs to help reduce the amount of tempered MUA needed, each with its own benefits and drawbacks. The use of compensating hoods to meet exhaust quantities required by code and reduce the amount of tempered MUA required will be discussed in detail later in the paper.
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A more accurate method for determining the exhaust required to ventilate a particular line of cooking equipment is by estimating the amount of heated air produced by each piece of equipment. An example of this method is shown in a ventilation hood manufacturer's application guideline (GFC 1981). The air stream rising from the equipment can be approximated most closely by treating the process as natural convection from a hot body. The grease and smoke created during the cooking process are entrained in the rising heated air stream and are carried up to the hood by the natural buoyancy of the heated air (Hatch and Barron-Oronzco 1957). Therefore, if the heated air stream produced by the cooking equipment is captured, all the grease and smoke produced will also be exhausted.