In general, building ventilation systems must bring in enough fresh air for occupant comfort, while also controlling indoor temperature, humidity, and air quality to ensure personnel and building safety. All of this must be accomplished at reasonable energy costs. Food service kitchen ventilation systems have the additional burden of removing the grease vapors, odors, moisture, smoke and heat generated by cooking. This is essential to a safe, comfortable, and productive kitchen environment. Fire protection is also an essential element in food service ventilation design.
The design of kitchen and other food service ventilation systems requires a professional, and it should not be attempted by unqualified persons using the guidelines and general comments offered here. The intent of this information is only to familiarize you with ventilation design considerations and typical equipment solutions.
The creation of grease vapors, odors, moisture, products of combustion, and smoke is generally focused in specific kitchen areas and varies based on the equipment used. The common solution is to vent, capture, and possibly eliminate contaminants through carefully designed hoods with adequate venting flow rates and fresh air makeup. Simply oversizing the vent system is not a proper approach and can be very costly in energy bills. A successful and economical design requires careful consideration of cooking equipment selections, cooking equipment arrangement and coordination in the kitchen, and appropriate vent-hood design.
Heating, ventilating, and air conditioning (HVAC) costs are significant in most food service businesses. Furthermore, failure of the HVAC system in the kitchen is reflected in worker productivity and possibly in worker turnover. HVAC system design requirements in the dining areas are simpler, but must consider both customer comfort and economical building operation.
In addition, the HVAC design must consider the way space is operated and maintained. If the space is a commercial kitchen in a much larger building, like a hospital cafeteria, energy systems are probably maintained by a professional staff. In these cases, system options can be reasonably sophisticated. On the other hand, if a food service establishment is an isolated small building, like a fast food restaurant, the staff are probably only going to operate systems by pressing simple start and stop buttons, so the HVAC systems should not be difficult to maintain or operate. These are probably not situations where more elaborate and complicated systems should be considered.
Types of Ventilation Systems
There are a wide range of food service ventilation systems designed to meet different ventilation requirements. The most common types are described below.
A back-shelf ventilator is the best alternative in kitchens where low ceiling height or a lack of space prevents use of an overhead canopy. The unit is installed at the rear of the cooking equipment, closer to the actual cooking surface than an overhead canopy. Back-shelf units are not intended for heavy production usage, nor for use with high-exhaust-surge cooking equipment like char-broilers.
The typical minimum clearance (distance between the cooking surface and filters) is 18 to 25 inches. This distance prevents overheating that can cause accumulated deposits to bake on the vent filters. Excessive temperatures also tend to vaporize grease, which allows it to pass through the filter and deposit on internal system components. This increases cleaning and maintenance costs.
The hood of the back-shelf ventilator should extend from the wall a distance of at least 24 inches, but be set back enough from the front to allow adequate head clearance for cooks. Cooking equipment should extend no more than 36 inches.
Canopy hoods are installed either against a wall or above cooking equipment (called island canopies). The length and width of the hood face should equal the total dimensions of the cooking appliance plus an appropriate overhang on each side. This overhang amount depends on the hood style and the kitchen appliance used.
A wall canopy with side curtains is possibly the most efficient design for capturing contaminated air. An island canopy hood is the largest type but is quite susceptible to cross-drafts and air spillage. Side air curtains prevent cross-drafts. Also, back paneling or tempered glass may be installed to produce a rear wall effect.
Eyebrow style hoods are mounted directly to ovens and dishwashers to catch effluents. This hood type can be designed to operate only when appliance doors are opened or at certain points in the cycle.
The pass-over hood configuration is used over counter-height equipment where a pass-over capability is required. That is, prepared food is passed over from the cooking surfaces to the serving side.
Introducing make-up air into a kitchen to produce a completely comfortable environment is very difficult. Achieving uniform comfortable temperatures, odor control, gentle air circulation, and minimal aggravating updrafts requires careful design and placement of wall registers, ceiling diffusers, and slotted ceiling panels.
Kitchen exhausts should be located away from the HVAC fresh air intake. If an existing HVAC system draws in odor-saturated exhaust, a baffle or barriers should be erected between the roof exhaust and the fresh air intake. These are just some of the reasons why kitchen design should be reserved for qualified professionals.
Wall registers are installed close to the ceiling, projecting return air across the ceiling in a straight line. The make-up air mixes with current air, circulating into the occupied zone. Problems often arise with wall registers because their high velocity operation may create additional updrafts.
Ceiling diffusers are normally flush mounted in the ceiling panels. These discharge supply air in a circular motion, outward along the ceiling. Where wall canopies are used, ceiling diffusers operate exceptionally well, if located a "sufficient distance" from all appliances and hoods ("sufficient distance" is defined as the equivalent of the maximum throw distance listed for the diffuser). When an island hood is used, it is difficult to apply a ceiling diffuser in a manner that effectively avoids updrafts.
Slotted ceiling panels provide a gentle uniform distribution of make-up air. Ideally, discharge air from ceiling slots should penetrate to face level at the rate of 20 to 25 feet-per-minute. In a properly designed system, return make-up air should barely affect the overall ventilation process.
Pollution Control Devices
Many local environmental ordinances require installation of pollution control devices with kitchen exhaust systems, especially where char-broilers or other smoke generating cooking equipment is used. Two devices commonly implemented for "pollution control" are electrostatic precipitators and fume afterburners. Both are installed within the exhaust system and are essential in reducing air pollution.
Heat Recovery Devices
Heat recovery devices are becoming more common. These devices are usually designed to recover and recycle energy for space heating and water heating. Without recovery units, this energy would be wasted.
All energy recovery systems operate on the same principle. Energy is recovered from outgoing air exhaust using a wheel, coil, pipe, or other device. The recovered energy is transferred to incoming air or water. Air-to-air heat recovery systems rely on the fact that air leaving the kitchen is hotter than incoming fresh air for most of the year. In this design situation, incoming air is warmed. Where the air leaving the kitchen is colder than outside air, it is cooled. This reduces the load on the primary heating and cooling system, reducing energy costs.
Another common energy recovery system captures waste heat from on-site refrigeration units or kitchen exhausts to produce hot water. Such a system is called a heat pump system and is available either as an option in the refrigeration system or as an add-on spot cooling system. Spot cooling systems are commonly specified for kitchens with inadequate cooling and are sold on the basis that they provide economical cooling and "free" hot water.
There are also many other types of heat recovery devices on the market including rotary or "heat wheel" regenerators, air-to-air plate-type exchangers, heat pipes, and liquid "run-around" coils. Predicting the cost performance of these systems and properly implementing them into a kitchen HVAC design is work for a professional and should not be based on advertising or equipment supplier claims. All of these designs have proven cost effective in certain situations when properly incorporated into the overall kitchen design. However, all of these designs have also failed when not properly integrated into the kitchen design.
Hoods: Operation & Specification Issues
Most people have experience with ventilating hoods in their home kitchen and recognize the need for the hood to control smoke and fumes that might otherwise create discomfort. The hood needs to be designed for three key factors:
- the location of the food preparation device emitting the smoke and fumes to be vented
- the thermal updraft that forms above the surface of operating cooking equipment
- the resultant inrush of air to replaces this rising air flow
The location of cooking equipment relative to the ventilation hood greatly impacts the system's effectiveness. Cooking equipment is commonly installed so the rear faces the wall. Since the heated surface is only minimally exposed to open air, the inrush created by the heat vacuum is greater at the front edge of the appliance, and modern hoods are designed to use this added updraft.
The hood must have enough capacity to capture and exhaust contaminated air from the kitchen. Surges of contaminated air in excess of this capability may cause "spill" out of the hood and can create unpleasant, uncomfortable, and even unsafe conditions. Also, the use of gas-fired cooking equipment may need additional allowances for the exhaust of combustion products and combustion air. Therefore, proper hood size should be specified to reflect the actual cooking equipment and the cooking duty required. These hood design criteria include an overhang requirement and a minimum exhaust flow rate, expressed in cfm (cubic feet per minute) per linear foot of hood.
Kitchen hoods are designed for specific cooking situations and are broken into two broad categories: Type I and Type II. Type I hoods are used for the collection and removal of grease and smoke. Type II hoods are general duty hoods for the collection and removal of steam and water vapors, heat, and odors where grease is not present.
Ventilation Maintenance Issues
HVAC systems are a significant part of energy costs for most food service operations. The single most cost effective way to reduce these costs is to ensure that energy systems are operating properly. However, building systems are often unintentionally defeated when occupants complain about being hot or cold. For example, complaints about being cold during the hottest days of the year may cause adjustments to be made in setpoints for the system. When the weather later changes, the setpoints are probably not reset to a more economical setting. Furthermore, checking for a system problem that caused the discomfort in the first place is rarely done.
When breakdowns occur, there is a natural tendency to quickly patch the system back into operation instead of looking at improper system operation as part of the cause for the system failure. For example, an important routine maintenance item is changing filters and cleaning external components. Most buildings are operated such that these tasks are done only when their neglect causes a problem. However, a clogged filter puts unnecessary strain on the HVAC system fans and heating and cooling system, and consequently raises energy costs. It also usually creates occupant discomfort.
The volume and type of food cooked in the kitchen determines how frequently an exhaust system needs cleaning and maintenance. Select an appropriate schedule for HVAC maintenance by monitoring the rate of build-up in the exhaust filters and duct access doors for a least a month. This should provide enough information for specialists to suggest an ideal schedule for system cleaning and repair. Many professional building operators also put a pressure drop indicator on filters to indicate when they need maintenance.
Another key way of reducing system operating and maintenance costs is to educate site personnel in proper HVAC operation. Turning systems off at appropriate times, setting thermostats, and even simple things like closing doors can create significant annual savings and increased equipment life. However, major equipment maintenance tasks should be left to professional maintenance contractors.
Commercial kitchen heating, ventilating, and air conditioning systems are similar to standard building designs except for make-up air systems and hoods. Make-up air systems include wall registers, ceiling diffusers, and slotted ceiling panels. Kitchen hoods are designed for specific cooking situations and are divided into two broad categories: Type I and Type II.
Type I hoods are used for the collection and removal of grease and smoke. They always include filters or baffles for grease removal and are normally required over fryers, ranges, griddles, broilers, ovens, and steam jacketed kettles.
Type II hoods are general-duty hoods for the collection and removal of steam and water vapors, heat, and odors where grease is not present. Therefore, these units may not have grease filters or baffles. They are typically used over dishwashers, steam tables, and similar equipment. However, they may also be specified for use over other equipment when allowed by local codes and authorities. Always check with a design professional for these rulings.
For more information about the benefits of ventilation systems, please contact us for a copy of an EPRI performance or ventilation report.