Every commercial kitchen requires make-up air to compensate for the air ventilated from the kitchen space to remove heat, cooking odors and grease created by cooking operations. Ventilation is also essential to maintain a comfortable work environment. Make-up air can be taken from the building HVAC system of which the kitchen area is usually a part, as illustrated in Figure 29. While this is frequently done, it is an expensive and inefficient method of replacing exhausted air.
Quite apart from the obvious need to replace air removed from a kitchen, inadequate make-up air will prevent the kitchen operating the way it should. Air will be drawn in from other areas through doors and passageways.
In keeping with new requirements in kitchen ventilation, VM has introduced a number of devices for ventilating kitchens, which include kiosk ventilation units, air cleaning systems, water wash filter hoods with make-up air packages, dampers for adjustable volume control and fire dampers.
The easiest situation in which to provide make-up air is in a free-standing building of one level construction, such as found in food courts or restaurants.
The complexities of providing make-up air increase as the building in which the kitchen operates changes from the detached, single-purpose building to the multi-story tower structure (see the Design Section for further discussion of this subject).
Make-up air can come from the room HVAC system, either from outlets near to the hood or integrated into the hood. The use of cooled and heated make-up air is expensive. For example, utility bills can double and triple. (Every 200-400 cfm of air requires one ton of A-C system capacity, which is expensive.) Following is a summary of the devices and systems available to solve the
problem of providing make-up air.
Distributing air from the ceiling near the hood with low air velocity provides local cooling that is a necessity because of the strong radiant heat created by cooking appliances. Increased airflows did not raise metered indicators at the same ratio, which proves that good results in kitchen ventilation can be achieved with careful design and efficient equipment without excessive airflows.
The following are some basic principles of commercial kitchen ventilation:
1. Impurities and excess heat should be removed with efficient local exhaust.
2. Supply air should be brought to the working area in such a way that it first refreshes workers and then replaces convective flows.
3. Where workers are subjected to large heat radiation, supply air should be introduced directly to the working space (local cooling).
Down discharge supply air designs
The advantage of using integrated make-up air is that in large measure it reduces the burden on the building HVAC systems. Make-up air is supplied by a down discharge duct and blower close to the cooking surfaces. Three disadvantages, however, outweigh this advantage.
1. The air velocity from the hood supply air opening is greater than that of heated air rising from the cooking surface, which results in spillage of contaminated air held within the hood.
2. The lower temperature of the make-up air combined with the greater velocity causes the air to drop rapidly, pulling contaminant into the operator's face.
3. The direct down discharge can make it uncomfortable for kitchen staff, especially in cold climates. As a consequence, operators often shut off the make-up air system, thereby nullifying its usefulness.
Short cycle supply air
The short cycle method of supplying make-up air provides some
untempered outdoor air within the hood enclosure to reduce the amount of air the HVAC system must supply. This may seem to reduce the cost of using the HVAC system, but unless the velocity of the supply air is strictly controlled its velocity can exceed the velocity of the exhaust air and cause smoke emission from the hood cavity.
The difference between discharging make-up air into the kitchen space from the hood bottom front lip (Figure 30) and the short cycle method (Figure 31) lies in the air discharge exits of the two arrangements.
In addition to the chance of emitting smoke, safety problems can arise with short cycle hoods. In northern regions, for example, cold air can cause frost build-up on fire protection fusible links, which effectively prevents the links from detecting fire.
Front panel make-up air
The front panel type of make-up air (see Figure 32) is effective for virtually all applications and climates. A perforated stainless steel plate keeps the air flow velocity between 300 and 500 fpm. This permits the supply of a large volume of air at the hood, with little or no effect on the kitchen environment.
The low velocity of make-up air means that it is felt only 3 to 4 feet from the hood face.
During winter months, it may be necessary to heat the air of front panel type make-up air (50o - 60oF is recommended). The need to cool the air during the summer period is, however, negligible because the incoming air provides evaporative cooling to kitchen staff working under the effect of radiant heat generated by the cooking operation. Special consideration for some cooling may be required in hot, humid climates.
Exhaust and make-up air limitations
If one site provides ample access for duct shafts to exhaust to the roof and provide outdoor make-up air back to the kitchen, there are others with restricted access. This poses problems. With no obvious solution to providing the required air changes or to providing exhaust to the roof, and with space limitations that make the installation of equipment difficult, the designer has many problems to
overcome. A number of options are, however, available.
These are by no means the only options available for situations in which a
conventional solution to providing make-up air is difficult. The options shown in Figure 33 illustrate that even in demanding circumstances there can be more than one answer. The option chosen will of course depend on the particular circumstances of each project.
Why ventilation systems are essential
Cooking produces heat, odors, smoke, vapors, airborne grease and other pollutants, which is true of all cooking operations regardless of the type of food preparation being done. When a ventilation system breaks down, the kitchen atmosphere soon becomes stifling and impossible to work in. Hence, a constant supply of fresh, clean air is essential.
The ventilation system must exhaust the heat produced and remove the odors and pollutants. A kitchen which specializes in producing light snacks, sandwiches and salads generates less heat and odors than kitchens producing heavier fare: steaks, hamburgers, and French fries.
Venting cooking equipment
Gas, electric and steam cooking equipment must be allowed to breathe. Proper air flow is required for combustion, to exhaust fumes and odors, and to prevent moisture and heat build-up in control cabinets. Too much exhaust can suck the heat out of an oven, preventing it from properly baking or roasting the product;
too little exhaust can cause a control cabinet to overheat and burn out expensive equipment. Knowing what the cooking equipment requires for ventilation is critical to its performance and operating life. If the cooking equipment does not work properly, the kitchen will not remain in business long.
Maintaining negative pressure
The final reason why efficient ventilation is necessary is the need to maintain negative pressure in any kitchen area. Negative pressure means drawing air into the kitchen space atmosphere to contain the odors and pollutants that cooking operations generate.
Kitchens operating under positive pressure force cooking odors into spaces outside the kitchen envelope. This is unacceptable in most instances and certainly in places of shared space: food courts, hotels, office buildings, and even in large, stand-alone structures where the owner-operator wishes to maintain fresh air free from excessive cooking odors in large dining
areas.
Combination of equipment
The type and combination of kitchen equipment used affects the design and capacity of the ventilation system. Deep-fryers and open ranges produce more pollutants and continuous heat than closed ovens. When opened to remove roasts, pizzas and baked products, ovens release waves of intense heat with which the ventilation system must cope to maintain a stable atmosphere in the kitchen space. The type of equipment required is one factor; the use of kitchen space is another.
Regardless of the source of make-up air, it is necessary to calculate the make-up air required for a given kitchen operation. This will depend on the type of cooking being done, the equipment installed, and restrictions of the building configuration. Some considerations to take into account when calculating the make-up air and equipment required are:
C Keep make-up air velocities to a minimum.
C High velocity make-up air will disturb the capture of smoke by other equipment.
C Maintain a slight negative pressure in a kitchen to prevent odors and pollutants moving from the kitchen area to other parts of the building (a 20% negative pressure is recommended).
C Local regulations are an important consideration and no installation should be undertaken nor an existing operation changed without first checking local regulatory requirements.
CFM requirements of exhaust hoods
To calculate the required exhaust air for a hood, a number of factors must be known:
C The cooking equipment being used, to determine where the most smoke, grease and heat will be produced along the cooking bank.
C The type of cooking to be done: soup and sandwich preparation; burger and French fries cooking, fish and chip operation.
C The hood arrangement and wall locations around the cooking equipment, island operation, or a combination of island and wall locations.
C The kitchen layout, to determine if there is a combination of space restriction with a hot cooking operation.
A guide to use for estimating CFM requirements for wall-mounted hoods is:
C For light duty equipment such as steam and ovens, use 250 CFM per linear foot.
C For medium-duty equipment such as fryers, griddles and restaurant series ranges, use 300 CFM per linear foot.
C For heavy duty equipment, charbroilers and heavy-duty equipment, use 350 CFM per linear foot.
In the majority of instances, the CFM estimates given in the guide will provide ample air for the exhaust requirements. The guide serves for wall-mounted canopies only. It does not serve for island applications, single cooking bank arrangements or in cases where, say, the designer wants to know the absolute minimum requirements.
CFM calculations for hoods
The terms used to calculate CFM values are:
Minimum exhaust cfm = Capture area x capture velocity + equipment cfm correction factors.
Capture area = all open sides of the hood x the height above the cooking equipment (in feet).
Capture velocity = 50 fpm (minimum.)
Equipment cfm correction factors = see Table 5 in left hand column of this page.
A sample CFM calculation based on the elements discussed in this section is shown in Figure 34.
18' 3.5'
Using a rule of thumb for medium duty cooking of 300 cfm/linear foot of hood, the total cfm is
= 300 x 18 = 5400 cfm Note:
Figure 34 - Sample cfm calculation
Velocity readings on Cyclo Maze-type hoods
To measure the average velocity on intake slots for ventilators use one of the following formulas:
1. The velocity in feet per minute (FPM), Vol = FPM or CFM = FPM
OPN sq.ft.area where OPN is the total area of intake throat in sq, ft.
2. To find the volume in CFM, CFM = VEL x OPN
3. To calculate the velocity, divide the CFM by the total opening. The CFM or CFM/ft is information given on the U.L. label of the hood.
The standard Cyclo Wash opening is 3.5” x the length of the throat.
The total opening is 3.5” x the overall length of the intake throat.
The average throat velocity on most VM Cyclo-Maze Wash and Cyclo Maze Dry is calculated by multiplying the design air flow rating (shown on the ventilator nameplate) in CFM per linear ft. by 3.5. The result is the velocity in FPM.
Example
The specification is for 2250 CFM on a 9' ventilator.
OPN = 107” x 3.5” = 374.5 sq. in.
= 374.5 = 2.6 sq. ft.
144
VEL = 2250 = 865 FPM 2.6
For quick reference, use Table 7.
Measuring intake velocity
VM recommends the use of either an Alnor 6000 or Alnor Jr. to measure the air velocity through the slot of Cyclo Wash and Cyclo Maze ventilators.
To measure velocity at the throat using the Alnor 6000 (see Figure 35) position the tip of the velometer probe halfway between the inside rim and the face of the access panel at a plane perpendicular to the air stream. Do not put the probe too deeply into the intake throat or you will get erroneous readings.
Make sure that the hoses are of reversed polarity (+ to - and - to +) when measuring the exhaust. Take a minimum of three readings, evenly spaced, at each access door, then average the readings to calculate the exhaust rate.
The Alnor Jr. (see Figure 36) has a dual scale range to 1600 FPM and is another convenient way of measuring the intake velocity. Position the velometer as shown in the figure and take multiple readings at various points across the length of the intake throat. Because the instrument is calibrated for use in an upright position, expect a slight error (to 5%) on the high side. If accurate readings are needed, use an Alnor 6000 velometer.