HEAT RECOVERY FROM KITCHEN HOOD EXHAUST

One of the main requirements of a successful restaurant design is a healthy ventilation system. The biggest part of the restaurant ventilation load comes from the exhaust of kitchen hoods. In the winter months, heating the make up air brings a very serious investment and, more importantly, serious operating cost. Because of the high temperature values ​​of the hood exhaust air, heat recovery systems always appear as an attractive alternative. However, due to the high investment costs of heat recovery systems, there are important points to consider. Because the exhaust air from the kitchen hood is highly contaminated, the system should be best protected by an electrostatic precipitator with self cleaning automatic washing system.

Compared to other commercial enterprises, the energy consumption per m2 of the restaurants is higher. In this consumption, the costs of ventilation system take a big place. Operating costs and investment pay back potential of the heat recovery system depend on factors other than system design. The investment pay back period and operating costs of kitchen ventilation systems depend on different parameters such as climate, working hours, equipment efficiency and comfort level.

In ASHRAE Standard 90.1 kitchen ventilation systems with a total exhaust flow rate of more than 8,400 m3 / h are required to apply one of the following options:

• 50% of the kitchen supply air must be the transfer air to be exhausted otherwise.

• A 50% efficiency heat recovery system.

• Variable speed system, which can control at least 75% of the exhaust flow rate. This demand controlled system should be able to reduce the exhaust air and fresh air supply by 50%.

Heat recovery systems are more useful in areas with low outdoor temperatures. It was observed that the exhaust hood capture was sufficient at 40 ° C exhaust air temperature values. Exhaust air at this temperature seems attractive for heat recovery. However, oil and soot in exhaust air rapidly cover and obstructs completely heat recovery surfaces. Therefore, the necessary measures should be planned before.

On-demand control of the exhaust system is intended to reduce exhaust flow at partial loads when nothing is cooked. At partial loads, an effective suction from the hood should be achieved while at the same time the fresh air flow rate must be reduced proportionally to maintain pressure differences and air currents from room to room.

Reducing the exhaust flow rate and in-duct air velocities has no negative effect on oil and soot accumulation on the duct surfaces. The NFPA 96gave a minimum air velocity of 2.54 m / s  in the duct. Another factor that affects the accumulation of oil and soot on the channel is the temperature difference between the exhaust air temperature and the duct surface. When the temperature difference increases, the accumulation of oil and soot also increases.

Evaporative cooling can be used because the hourly air changes in the restaurant kitchens are very high.

In our applications, due to both installation space problems and the possibility of using the recovered heat in different regions, it was observed that coil to coil run-around type heat recovery systems are more suitable for ease of installation and maintenance. The coil surface section is the same as the cross-sectional area of ​​the electrostatic filter. The device is reduced to the smallest possible dimensions. A 2.5 mm fin spacing is the best choice for heat recovery efficiency, cleaning and air side pressure drop. The efficiency of each coil was compared in different water side pressure drops, different water velocities in the coil, air velocities in partial loads, different internal and external air temperatures, finaly the coil selection was made in the best way under all conditions.

The automatic control panel gives the proportional control signal to make up air fan corresponding to exhaust flow rate. Each fan and duct system reacts differently at different air flows. For this reason, driving the exhaust fan and the fresh air fan with the same frequency value or in direct proportion creates imbalances. The automatic control panel we have developed allows for a different number of fresh air fan cycles in response to the different speeds of the exhaust fan and the values ​​are transferred proportionally. The exhaust fan is assigned different frequency values ​​for the fresh air fan at 5 different frequency values, and the fresh air fan at each frequency of the exhaust fan provides the right amount of fresh air. Kitchen workers only adjust the exhaust flow rate, fresh air is adjusted to an appropriate amount  by automatic control panel. The electrostatic precipitator with self cleaning automatic washsystem is always in operation.