Design of Smoke Extraction System in Commercial Kitchen

Fume from the catering industry floats in the air in the form of aerosols, which is one of the important sources of inhalable particulate pollution in the urban atmosphere, which can cause great harm to the atmospheric environment and human health. For the kitchen of the catering industry, whether the smoke exhaust is smooth is directly related to whether the kitchen can work normally. Oil fume purification is a mandatory function of air purification in the kitchen fume exhaust system. The reasonable matching of the exhaust system and the purifier is the key to ensure the purification efficiency of the oil fume purifier, and it is also a problem that the designer of the oil fume purifier is easy to ignore. If the design of the smoke exhaust system is unreasonable and the entire kitchen does not exhaust smoke smoothly, it will not be possible to install a fume purifier. Even if the oil fume purifier is installed, it cannot work normally. Grasping the characteristics of each part of the exhaust system and its impact on the overall exhaust of the kitchen is the prerequisite for the selection and installation of the fume purifier. And it is of great significance to ensure the normal operation of the fume purifier.

Based on the practical experience of design, manufacture, installation, and management of kitchen fume exhaust purification equipment, this article gives a preliminary introduction to the design and calculation principles of the oil fume exhaust system and its matching relationship with the fume purifier.

The composition of the kitchen exhaust system

The kitchen's smoke exhaust system is mainly composed of a baffle grease filter, a smoke exhaust pipe, a fume purifier, a smoke exhaust fan (including a silencer), and an overall kitchen air supply device, as shown in the figure below.

The composition diagram of the kitchen smoke exhaust system

The type and operation mode of the oil fume purifier can be selected according to the actual situation. Before installing the fume purifier, the actual operating parameters of each part of the fume exhaust system, including operating resistance and exhaust air volume, must be calculated. If it is a new project, the impact on the exhaust system after the installation of the fume purifier should be fully considered in the design. If it is a retrofit project, the exhaust system needs to be re-calculated when the oil fume purifier is installed. If necessary, the fume collection hood, exhaust ducts, and the overall air supply device of the kitchen should be transformed and the exhaust fan should be replaced.

Determination of exhaust air volume of kitchen fume

In order to ensure the purification effect, in the actual design, the exhaust air volume is calculated strictly according to the suction wind speed of the exhaust hood, and the suction speed of the hood is usually not less than 0.5m/s. The minimum exhaust air volume is usually calculated by the following formula: L = 1000PH

formula: 

L-Exhaust air volume of the exhaust hood, m3/h;

P-the perimeter length of the hood (not counting the side against the wall), m;

H-the distance from the hood to the stove surface, m.

After calculating the exhaust air volume with the above formula, calculate the suction speed of the hood according to the area of the hood to ensure that the suction speed of the hood is not less than 0.5m/s.

In actual engineering design, estimation methods are often used. According to the regulations on kitchen ventilation in "Technical Measures for Civil Building HVAC Design", kitchen ventilation can also be determined according to the following air changes:

Chinese food kitchen: L = 40 ~ 50 (times/h);

Western kitchen: L = 30 ~ 40 (times / h);

Staff canteen: L 2 25 ~ 35 (times / h)

Kitchen wind

Due to the different layout of the building where the kitchen is located and the location of the kitchen in the building, the conditional kitchen should be ordered by the building’s centralized air-conditioning system. But most of the situations encountered at present are that the building does not have an air supply system for the kitchen. Many buildings with restaurants, kitchens, etc. are only designed with centralized heating systems without centralized air conditioning systems. When the kitchen is in working condition, the exhaust system is put into operation, and the kitchen is under negative pressure. The sources of fresh air include outdoor fresh air infiltration and a large amount of air from the restaurant is supplied to the kitchen.

1. Disorderly air supply

The disorderly air supply will cause a harsh working environment in the kitchen. Especially in summer, although some local cabinet air conditioners are usually installed in the restaurant to reduce the indoor temperature of the restaurant, the supplement of indoor air into the kitchen cannot eliminate the heat load of the kitchen and reduce the kitchen temperature. Due to a large amount of oil fume in the kitchen, if a partial cabinet air conditioner is installed in the kitchen to reduce the temperature in the kitchen, the return air filter of the cabinet air conditioner should be cleaned regularly.

2. Orderly air supply

For buildings with centralized air-conditioning systems, the working environment of the kitchen can be guaranteed, and the air supply is mostly orderly and flexible. In the current kitchen ventilation system, there are mostly the following three ways to make up the air:

(1) The restaurant air conditioner is a fresh air direct current air conditioning system. The fresh air from the air conditioner is first sent into the dining room, then flows into the kitchen, and is discharged outside through the exhaust system. This method has less investment and good heating and ventilation effects in the restaurant. The disadvantage is that a large amount of air conditioning air will be directly discharged, and the fresh air consumes a lot of energy, which is not conducive to energy saving.

(2) Part of the fresh air from the combined air conditioning unit is directly supplied to the kitchen, and part is sent to the dining room and returned to the air conditioner through the return fan. This can partially solve the problem of large energy consumption of fresh air, and at the same time, the flexibility of use is also greater. A pressure measuring device can be installed in the kitchen. When the exhaust fan of the kitchen is fully working, the negative pressure increases, the return air motor valve is closed, the return fan stops, and a large amount of restaurant air is supplied to the kitchen. When a small part of the exhaust fan is turned on or the exhaust fan is not working, the negative pressure is not very large, the return air electric valve is opened, and the return fan starts to prevent all the air from the air conditioner from being discharged.

(3) Use two units to serve the restaurant and kitchen respectively. The kitchen is a fresh air direct current system, which directly supplements the exhaust air of the kitchen stove, while the restaurant is a fresh air system with return air. This scheme can ensure the air conditioning effect of the dining room and the kitchen, and the kitchen and dining room systems do not interfere with each other. The disadvantage is that the initial investment is relatively large.

Gas (smoke) hood

The gas (fume) hood is generally processed as a whole, including filtration, oil collection, water collection, and other equipment, mostly made by professional kitchen companies. Its appearance should be uniform, beautiful, and easy to clean. In the kitchen ventilation, a certain amount of fresh air needs to be added. On the one hand, it can balance the air pressure to avoid excessive negative pressure in the kitchen (generally not greater than 5Pa), on the other hand, it can reduce the temperature in the kitchen and improve the working environment. For the collection of cooking oil fume, there are mainly three types of gas collection (smoke) hoods:

(1) The simple air-gathering hood without baffle hood filters and air supply equipment is suitable for various flexible and maneuverable places, convenient for processing and installation, and low price. However, because there is no air supply equipment, the air collection effect is relatively poor, which easily causes the oil fume to swirl in front of the kitchen staff, and it is easy to cause oil droplets to scatter.

(2) The air collecting hood with the restaurant hood filters and the air supply equipment can better collect the oil fume generated during the cooking process, and the oil droplets will not be scattered. As long as the air supply is reasonable, the air quality of the kitchen operator's work surface is good. However, the installation requirements are relatively high and the investment is relatively large.

(3) Water transport and gas (fume) hood

The design principle of the water-carrying and collecting gas (fume) hood is to form a water curtain circulation and achieve the purpose of purifying the fume after colliding with the fume. The key to affecting the use effect of the water-carrying and collecting gas (fume) hood is to form a continuous water curtain, and the circulating water system needs to be cleaned regularly. In fact, due to poor management, most of the current water-carrying and gas-collecting (smoke) hoods cannot form a continuous water curtain, but only form intermittently flowing water droplets or small water columns, so the use effect is very poor.

The advantage of using the water-carrying gas (fume) hood is that it can remove part of the oil fume at the gas (fume) hood, reduce the processing load of the oil fume purification equipment, and improve the purification effect of the system. However, if the smoke-passing gap of the water-carrying air-collecting (smoke) hood is narrow, it is easy to cause the smoke exhaust system to be unsmooth. The design of the hood has an important influence on the smoke exhaust system. The unreasonable design of the air collecting hood can easily cause the entire smoke exhaust system to be unsmooth. The factors that cause the smoke exhaust system to be blocked are:

1) The gap between oil plates or the water-carrying and gas-collecting (fume) hood is too small, which is more common;

2) The total cross-sectional area of the opening for collecting oil fume in the hood is too small, causing the overall smoke exhaust to be unsmooth;

3) The distribution of openings is unreasonable or the openings do not increase with the increase of the distance from the main pipe, which causes local smoke exhaustion not to be smooth;

4) The cross-sectional area of the smoke collecting duct on the gas collecting (smoke) hood is too small, resulting in unsmooth discharge.

Smoke exhaust pipe

The horizontal section of the kitchen exhaust pipe should not be too long. Generally, the longest horizontal distance should not exceed 15m and the slope should be more than 2%. There is a live joint at the horizontal end to facilitate the cleaning of grease. According to the specification, the exhaust speed shall not be less than 10 m/s. To prevent the oil from adhering to the flue due to the low wind speed, the branch pipe connected to the exhaust hood should be provided with an air volume regulating valve.

The unreasonable design and installation of smoke exhaust ducts often result in unsmooth kitchen exhaust, which directly affects the use of oil fume purifiers. The main reasons are as follows:

(1) The ventilation area of the main exhaust duct is insufficient, and the gas flow rate is too large. According to engineering experience, the wind speed in the main smoke exhaust duct cannot exceed 15m/s. Otherwise, after a period of operation (such as one year later), it is easy to cause the smoke exhaust system to be unsmooth. In the operation practice of oil fume purifiers, most of the cases are caused by insufficient ventilation area of the exhaust pipes, and the flow velocity of the gas in the pipes is above 15m/s.

(2) Because the smoke exhaust pipe has to pass through the building beam, the ventilation cross-sectional area of the pipe becomes smaller, and the resistance on the cross-section increases, causing the airflow to be blocked.

 (3) When the smoke exhaust pipe passes through building beams or avoids other equipment pipes, a section of the smoke exhaust pipe will be lower than the plane of the overall exhaust pipe. After a period of use, because the oil-water mixture will accumulate in the lower section of the exhaust pipe, it will directly affect the smoothness of the exhaust.

(4) Kitchen oil fume on different floors is discharged through the same main pipe. The kitchen on the floor close to the outlet has a better smoke exhaust effect, while the kitchen on the lower floor is not smooth.

Smoke exhaust fan

Commonly used exhaust fans include axial fans, fan cases, and centrifugal fans.

(1) Axial fans have been used in many applications before, but due to their low total pressure, they are rarely used after the installation of oil fume purifiers.

(2) The full pressure of the air box is medium, the air volume can be adjusted, and the noise is low. It is often selected in the kitchen fume exhaust system where the distance between the fume collecting hood and the fume discharge port is medium.

(3) The full pressure and air volume of the centrifugal fan can be adjusted, but due to the large noise, noise reduction is often required. When the kitchen is at the bottom of the building, its exhaust fan should be placed on the roof. On the one hand, indoor noise pollution can be reduced; on the other hand, the exhaust air duct is in a negative pressure state, which can avoid odor overflow. When the fan is installed on the roof, a centrifugal fan is generally selected.

The reasonable design of the kitchen fume exhaust system is a prerequisite for the subsequent installation of oil fume purification devices. This paper analyzes the various components of the kitchen smoke exhaust system and proposes the calculation method of the kitchen theoretical smoke exhaust air volume and the design and selection principles of the air collecting hood, exhaust duct, and exhaust fan. It can be used to guide the selection of oil fume purifiers in the catering industry and improve the purification efficiency of oil fume purifiers.