The purpose of most building ventilation systems is to bring in adequate fresh air for occupant comfort while controlling temperature, humidity, and indoor air quality for personnel and building safety at reasonable energy costs. Food service kitchens have the added task of removing grease vapors, odors, moisture, smoke and heat generated by the cooking process. This is critical to a safe, comfortable, and productive kitchen environment. Fire protection is also an essential element in the design. Needless to say, the design of the ventilation system is the domain of a professional and should not be undertaken by unqualified persons using the guidelines and general comments offered in this section. The intent of this interactive Knowledge System is only to familiarize you with ventilation design considerations and typical equipment solutions.
The generation of grease vapors, odors, moisture, products of combustion, and smoke is highly localized and dependent on the equipment used in a kitchen. The typical solution is to vent, capture, and possibly destroy these contaminants through the careful design of hoods with adequate venting flow rates and fresh air makeup. Merely oversizing the vent system is not necessarily correct and could be very costly in annual energy bills. The careful selection of cooking equipment, its arrangement and coordination in the kitchen, with appropriate vent-hood design is critical to a successful and economic design.
Heating, ventilating, and air conditioning (HVAC) costs are significant in most food service businesses. In addition, failure of the HVAC system in the kitchen often shows in worker productivity and possibly even in worker turnover. The HVAC system design requirements in the dining areas are obviously simpler, but must also consider customer comfort as well as economic building operation. Finally, the design should consider the way the space is operated and maintained. If this space is a commercial kitchen in a much larger building, like a hospital cafeteria, the energy systems are probably maintained by a professional staff. In these cases, the system options can be reasonably sophisticated. However, where the food service establishment is an isolated small building, like a fast food restaurant, and the staff are probably only going to operate systems by pressing simple start and stop buttons, the systems should be designed to reflect those attributes.
Just remember, if the cash register no longer has numbers, just buttons with pictures of "burgers and fries," these are probably not the situations where you should consider the more elaborate and complicated systems such as desiccant dehumidification. However, simple and reliable methods of dehumidification such as heat pipes may be highly cost effective.
Heating, ventilating and air conditioning systems are a significant part of most food service operations energy costs. And the single most cost effective way of reducing these costs is to be sure the energy systems are operating properly. However, most building systems are partially or even completely defeated when occupants complain about being hot or cold. For example, an occupant who complains about being cold during the hottest days of the year will often cause set points in the system to be adjusted to eliminate the complaint. When the weather changes, it's likely that the setpoints are not changed back to a more economic setting. And checking the system to see if there was a problem that caused the discomfort in the first place is rarely done.
When breakdowns do occur, there is a natural tendency to quickly patch the system back into operation and not to look at whether the system operation itself was partly to blame for causing the system to fail. For example, one of the most important routine maintenance items is changing filters and routinely cleaning the external components. Most buildings are operated in such a manner as to only change or clean them when they cause a problem. Obviously, a clogged filter puts unnecessary strain on the HVAC system fans, heating and cooling system, and consequently raises energy costs. All this, while usually creating occupant discomfort.
The volume and type of food cooked in the kitchen determines how frequently an exhaust system requires cleaning and maintenance. You can select an appropriate schedule for HVAC maintenance by monitoring the rate of effluent build-up in the exhaust filters and duct access doors for a least a month. This should provide sufficient information for specialists to suggest an ideal schedule for system cleaning and repair. Most professional building operators 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 equipment operation. Turning systems off at appropriate times, setting thermostats, and even simple things like closing doors can all add up to significant annual savings and increased equipment life. Leave major equipment maintenance tasks for the professional maintenance contractors.
Hoods: Operation Specification
Most of us have experience with ventilating hoods from our home kitchen. We see the obvious need for the hood to control smoke and fumes that might otherwise create discomfort and even stain our home environment. The hood needs to be designed for three key factors:
- the location of the emitting food preparation device
- the thermal updraft forming above the surface of operating cooking equipment
- the consequent inrush of air that replaces this rising air flow
The location of cooking equipment relative to the ventilation hood has considerable impact on the system's overall effectiveness. Cooking equipment is commonly installed so that the rear of the appliance faces the wall. Since the heated surface is minimally exposed to open air, the inrush created by the heat vacuum has a greater velocity at the front edge of the appliance, and modern hoods are designed to use this added updraft force.
The hood must have sufficient 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 operating conditions. The use of gas-fired cooking equipment may require an additional allowance for the exhaust of inherent combustion products and combustion air. Therefore, proper hood size is specified to reflect the actual cooking equipment and the cooking duty required. These hood design criteria include a minimum exhaust flow rate (expressed in cfm-per-lineal-foot of hood) and an overhang requirement.
Commercial kitchen heating, ventilating, and air conditioning systems are similar to standard building designs with the exception of hoods and make-up air systems. Kitchen hoods are designed for specific cooking situations and are broken into two broad categories: Type I and Type II. Make-up air systems include wall registers, ceiling diffusers, and slotted ceiling panels.
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 where allowed by local codes and authorities. Always check with a design professional for these rulings.
- Back-Shelf Ventilators
A back-shelf ventilator is the best alternative in kitchens where a low ceiling height or a lack of space precludes the installation of an overhead canopy. It's installed at the rear of the cooking application, 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, such as char-broilers.
The typical minimum clearance, or distance between the cooking surface and filters, is 18 to 25 inches to prevent overheating that can cause accumulated deposits to bake on the vent filters. Excessive temperatures also tend to vaporize grease effluents allowing them to pass through the filter and precipitate on the 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 from the front to allow adequate head clearance for the cooks. Actual cooking equipment should extend no more than 36 inches.
- Canopy Hoods
Canopy hoods are installed either against a wall or above the cooking equipment (these are called island canopies). The length and width of the hood face should equal the total dimensions of the appliance plus an appropriate overhang on each side. This overhang is based pon the hood style and the kitchen application.
A wall canopy with side curtains is possibly the most efficient design for capturing contaminated air. The island canopy hood is the largest hood type but has a high susceptibility to cross-drafts and air spillage. Cross-drafts can be prevented using side air curtains. Back paneling or tempered glass may be installed to simulate a rear wall effect.
- Eyebrow Hoods
Eyebrow style hoods are mounted directly to ovens and dishwashers to catch effluents. They can be designed to operate only when appliance doors are opened or at certain points in the cycle.
- Pass-Over Hoods
The pass-over hood configuration is used over counter-height equipment where a pass-over capability is required. That is, the prepared food is passed over from the cooking surfaces to the serving side.
Introducing make-up air into a kitchen to provide 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 should be erected, or barrier, between the roof exhaust and the fresh air intake. These are just some of the reasons why kitchen design is the domain of 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 the ambient air, subsequently circulating into the occupied zone. Problems often arise with wall registers due to their high velocity operation, which may create additional updraft disturbances.
Ceiling diffusers are normally flush mounted within the ceiling panels. They 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 updraft disturbances.
Slotted ceiling panels provide a gentle uniform distribution of make-up air. Optimally, the discharge air from the ceiling slots should penetrate to face level, at the rate of 20 to 25 feet-per-minute. With a properly designed system, the motion of return make-up air should barely affect the overall ventilation process.
Pollution Control Devices
Many local environmental ordinances require the installation of pollution control devices with kitchen exhaust systems, especially where char-broilers or other smoke generating cooking equipment is operated. Two devices commonly implemented for "pollution control" are electrostatic precipitators and fume afterburners. Both are installed within the exhaust system and play integral roles in reducing overt air pollution.
Heat Recovery Devices
Heat recovery devices are becoming much more common in contemporary kitchens. These devices are usually designed to recover and recycle energy for space and domestic water heating. Without recovery units, this energy would be wasted as exhaust.
All of these 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 then 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, the incoming air is warmed. Where the air leaving the kitchen is colder than outside air, it is cooled. This relieves some of the burden on the primary heating and cooling system, reducing energy costs.
Another common energy recovery system captures heat rejected from on-site refrigeration or kitchen exhausts to produce hot water for domestic water heating. These are referred to as heat pump systems and are available either as a design alternative in the refrigeration system or as an add-on spot cooling system. The latter are commonly specified for kitchens that have inadequate cooling and are sold on the basis that they provide economic cooling and essentially "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 economic performance of these systems and properly implementing them into a kitchen HVAC design is the domain of a professional and should not be based on advertising hype or equipment supplier claims. All of these designs have been proven cost effective in certain situations and when properly incorporated into the overall kitchen design. Then again, all of these have failed when not properly integrated into the kitchen design.