Outdated methods of ventilation design categorized appliances into different classes of light, medium, heavy duty and extra heavy duty (solid fuel). Average exhaust rates were established for each category based on rules of thumb, cfm per foot of hood. With the advent of U.L. 710 test, this morphed into minimum exhaust per surface temperature (400° F (204°C), 600°F (315°C) and 700°F (371°C) (solid fuel). The test was established for fire safety reasons for the removal of visible cooking vapors. The intent was not to establish design airflow rates, but exhaust minimums for a specific cooking duty. This information falls short of determine the actual “load” generated by the cooking equipment.
Load can be defined as the energy released by cooking equipment. It comes to the space in several forms. There is the radiated load (think of the sun’s rays), it is a form of electromagnetic radiation and cannot be exhausted. The placement supply air can offset the impact of the radiated load. Energy consumed by the appliance heating elements (gas, electric or sold fuel), energy into the food product being cooked and energy release to the air along with grease and smoke.
Knowing the cooking equipment make and model will allow for an accurate calculation of the exhaust required to remove the convective load (heat transferred to the air) by determining the connected power of the appliance (kW, mbh), type of appliance, distance of appliance cooking surface to the hood and the efficiency of the exhaust hood. This is known as heat load based design and provides a very accurate determination of the exhaust required to remove the generated load by the appliance. In addition, provide the engineer with a calculation on radiated load to the space.