Understanding Commercial Kitchen Exhaust Hoods
You would think that commercial kitchen hoods would be a fairly simple thing to design and install. However, once the decision to install one is made, many installation complications occur due to code requirements. The first step in any kitchen hood application is to understand what type of hood is required. The hood type needed is based on the usage. Thus, kitchen hood specifications depend on the type of cooking equipment that will be installed under the hood.
Commercial kitchen exhaust hoods serve very different purposes. For example, one that removes moist warm air from a dishwasher or steamtable does not need to be capable of removing grease from the exhausted air. A hood over a kitchen grill or deep fat fryer does need to have grease removing capabilities. Thus, the first rating that needs to be established for hood selection is the 2015 International Mechanical Code (IMC) “duty type”. For Kitchen hoods there are two duty types:
- Type I exhaust hoods are designed to remove cooking-related heat, odors, smoke, grease, and other cooking residue that becomes airborne. This residue is commonly called effluent (IMC requires a fire suppression system, and air velocity in exhaust ducts of 500 FPM, or more). Fire protection requirements will not be covered in this bulletin. Suffice it to say, fire protection systems are required for all Type I kitchen hood applications. HVACR contractors often need to coordinate with the fire suppression system subcontractor to make sure the final project will pass inspection. This is due to the electrical interlocks in the exhaust and makeup air systems required, openings in the hood for conduits, and possible fire damper requirements for makeup air pathways.
- Type II exhaust hoods, often called condensate hoods, are designed to collect fumes, odors, and steam. Thus, no fire requirements apply.
The IMC moves kitchen hood ratings a step further by including kitchen hood temperature operating ranges and types of cooking equipment as follows:
- Light: Air exhaust temperatures of 400OF or less. Gas and electric ovens, smoker, rotisserie, pizza ovens, pasta cookers, steamers, and non-cooking appliances.
- Medium: Air exhaust temperatures of 400OF or less. Combination Ovens, Gas and electric fryers and griddles, tilting skillets or brazing pans, grills, salamanders (or small broilers).
- Heavy: Air exhaust temperatures of 600OF or less. Upright broiler, electric char-broiler
- Extra Heavy Duty: Air exhaust temperatures of 700OF or less. Solid fuel burning. Chain Broiler, Gas char-broiler, mesquite, infrared broiler, lava rock char-broiler, wok.
Note: Exhaust temperatures above came from the 2015 ASHRAE HVAC Applications Handbook (Chapter 33 Section 1.5 Table 1).
The IMC also requires minimum airflows based on the hood’s design style. Six common design styles are:
- Back Shelf
- Wall Mounted Canopy
- Double Island Canopy
- Single Island canopy
For each type, and style there is a CFM and temperature range. Plus there are further requirements on construction materials and methods, safety controls, and fire suppression systems. Figure I shows the CFM requirement based on style of Type I kitchen hood design, and its duty type. Additionally, the hoods locational relationship, and required overhangs must be in compliance with IMC code requirements.
Further complicating kitchen hood designs is the need for large quantities of makeup air to replace the conditioned air exhausted through them. This air may be 100% heated and cooled, or simply filtered and brought in untreated as 100% outside air through a dedicated makeup air fan. In commercial kitchens, the makeup air will generally be supplied through a combination of air that has been treated or simply filtered and otherwise left untreated. Thus, once the exhaust and makeup air systems are installed, airflow balancing will be required in order for the systems to work correctly.
For more kitchen exhaust hood basics and a list of applicable codes and references, see ACCA’s latest Technical Bulletin 2017-2 Commercial Kitchen Exhaust Hood Basics listed in the Technical Bulletin Section on the downloads page at: https://www.acca.org/members/downloads,
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