Commercial Kitchen Ventilation (CKV) Exhaust Calculation

Commercial kitchen ventilation (CKV) is a standalone ventilation system. It is different from other ventilation and exhaust systems. Engineers and kitchen specialists often use the rule of thumb (cfm per linear foot) to estimate the exhaust requirement. But, is there a way to calculate it?

In ASHRAE Standard 154, an alternative method to calculate CKV exhaust rate is provided and it is based on the Association of German Engineers, Verein Deutscher Ingenieure (VDI) 2052 Manual.

The VDI 2052 method calculate the thermal plume of each kitchen appliance. Then, the kitchen hood exhaust requirement is the total thermal plume under the hood times the flushout factor and divided by the hood effectiveness.

The calculation is more than just multiple 300 cfm by the hood length. It factors in different kitchen appliances, kitchen hood type, position and makeup air design. In the following, I’ll breakdown the calculation in steps for you to have a better understanding.

Kitchen Appliance Convective Heat

Every appliance has a power rating. It is often stated on the nameplate. So, it is also known as the nameplate power. With power, the appliance emits heat that float upward where the kitchen hood must exhaust out. This heat is known as convective heat.

However, not 100% of the nameplate power will be converted into convective heat.

In the VDI 2052 manual, we can find the percentage of power will be converted into the convective heat that the kitchen hood must exhaust out. It is expressed in W/kW (watt of convective heat per kilowatt of appliance nameplate power).

Qk = P x Er x Qs

where,
Qk = Convective heat, W
P = Nameplate power of appliance, kW
Er = Fraction of nameplate power converted into input power
Qs = Fraction of input power converted into convective heat, W/kW

In worst case scenario, we assume the nameplate power is the input power. So, Er is 1.0. However, VDI 2052 assumed Er is 0.5. Meaning, VDI 2052 expects only 50% of the nameplate power is converted into the input power used for the convective heat calculation. In other words, VDI 2052 assumes the appliance doesn’t operate at full power. Nonetheless, use Er = 1.0 for peak load design.

Convective Heat Calculation Example

Given a small commercial kitchen as shown in the below photo. The meal serves per day is expected to be less than 100 proportions. The nameplate power and size of the appliances are as follows:

By referring to the heat fraction (Qs) provided in VDI 2052, the nameplate power of each appliance can be converted into convective heat, assuming Er is 1.0 as follows:

Thermal Plume

Thermal plume is the rising airflow of the appliance’s convective heat. Since the heat rises from the appliance, the size of the appliance matters. In addition, the heat is drawn by the kitchen hood at a certain distance above the appliance. So, the height (z) of the exhaust point (kitchen hood) above the heat source (appliance) must be considered.

Vth = (Qk^1/3) x (1.7Dh + z)^5/3 x k r s

where,
Vth = Thermal Plume, m³/hr
Qk = Convective Heat, W
Dh = Hydraulic Diameter of Appliance, m
z = Appliance to Hood Distance, m
k = Empirically Determined Constant, 18 m^4/3 W^-1/3 h^-1
r = Reduction Factor
s = Simultaneous Factor

Simultaneous Factor is the factor where all appliances are operating together. This value is determined by the kitchen type. Smaller kitchens have a greater chance to operate all appliances at once while larger kitchens are less likely. It is sort of like a diversity factor, ranging from 0.6 to 1.0.

Reduction Factor can be viewed as the effectiveness of the kitchen hood exhaust based on the location of the appliance/hood. The appliance is either place against the wall or free/island which means not against the wall. As a matter of fact, appliances that place against the wall have better exhaust efficiency (or less reduction) because the wall helps concentrate the heat (prevent it from spreading out).

So, if an appliance is placed against the wall, the reduction factor is 0.63 which means less exhaust airflow is needed. Otherwise, the reduction factor is 1.0 which means no external help.

Hydraulic diameter is mainly used for calculations involving turbulent flow. So, the appliance length and width need to be converted into hydraulic diameter using the below formula:

Dh = (2LW) / (L+W)

where,
Dh = Hydraulic Diameter, m
L = Appliance Length, m
W = Appliance Width, m

The appliance to hood distance (z) is the difference between the height of the kitchen hood above the floor and the appliance height. So, if the kitchen hood is mounted 2 meters above the floor and the kitchen appliance is 850 mm tall, z is 1.15 meters.

Thermal Plume Calculation Example

Given the above kitchen type, the simultaneous factor is 1.0 based on VDI 2052. Since the appliance/hood is not against the wall, the reduction factor is 1.0. Hence, the thermal plume can be calculate as follows:

Ventilation Exhaust

The amount of exhaust airflow needed to extract the thermal plume is affected by the effectiveness of the kitchen hood and the design of the makeup air system.

Ve = Vth x a x (1/Eh)

where,
Ve = Ventilation Exhaust, m³/hr
Vth = Thermal Plume, m³/hr
a = Flushout Factor
Eh = Hood Effectiveness

Many studies including those by VDI 2052 and ASHRAE show that different ways of introducing the makeup air have an impact on how well the kitchen hood perform. The effect of makeup air on the exhaust performance is known as flushout factor.

Ideally, the makeup air should not disturb the thermal plume airflow into the kitchen hood. In best case scenario, the flushout factor is 1.0 which means the makeup air doesn’t disturb the thermal plume airflow at all.

However, based on VDI 2052, the flushout factor ranges from 1.10 to 1.35. A well-designed makeup air system will have a smaller flushout factor which doesn’t increase the exhaust airflow requirement by a significant amount. For example, makeup air through ceiling diffusers has a flushout factor of 1.20 while makeup air through floor grilles has a flushout factor of 1.15.

Lastly, the effectiveness of the kitchen hood is determined by the type of kitchen hood. Canopy hood has a hood effectiveness of 0.7 while backshelf hood is 0.5. The kitchen hood type shown in the above photo is known as double-island canopy (Eh = 0.7).

CKV Exhaust Calculation Example

With the above data, the ventilation exhaust airflow for the example kitchen can be finalized as follows (assume flushout factor is 1.20):

In conclusion, the ventilation exhaust requirement is 14451 m³/hr (8500 cfm) for the given kitchen specifications. Given the kitchen hood linear length is 11 ft x 2 hoods = 22 ft, the exhaust requirement came at 8500 cfm ÷ 22 ft = 386 cfm/ft which is higher than the International Mechanical Code (IMC) minimum exhaust flow for unlisted hoods of 300 cfm/ft for medium-duty equipment (eg: gas stove, fryer, tilting pan, etc.)

Kitchen Hood Overhang Requirement

Take a closer look at the kitchen hood size. It is observed that the overhang on the front and end is 290 mm and 275 mm respectively. However, according to ASHRAE Standard 154, the minimum overhang requirement for double-island canopy is 305 mm for both front and end.

To comply ASHRAE Standard 154, the kitchen hood must be extended to 12 ft (3660 mm). As a result, the exhaust requirement becomes 8500 cfm ÷ 24 ft = 354 cfm/ft which is still above the IMC minimum requirement.

Remember we use Er = 1.0 for peak load design? If we use Er = 0.5 as per VDI 2052, the exhaust requirement becomes 6950 cfm ÷ 24 ft = 290 cfm/ft which is below the IMC minimum requirement. But, if the kitchen hood is 11 ft, then we get 316 cfm/ft.

Final Thought

Although ASHRAE doesn’t list the VDI 2052 calculation as part of their official Standard 154, understanding the VDI 2052 approach to CKV exhaust requirement is insightful as we learn the different types of kitchen, hood, location and makeup air design affect the final exhaust requirement. With that, we can also make suggestion to the client and kitchen designer to reduce the exhaust airflow and thus, the operating cost of the kitchen.