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Commercial Kitchen Ventilation System Design

Views:165     Author:Site Editor     Publish Time: 2021-01-22      Origin:Site

In commercial kitchens, the design of the ventilation system is very difficult due to the numerous technological processes and the restrictions of the building layout. On the other hand, due to the lag of the kitchen process design, it is difficult to effectively coordinate with the ventilation design, resulting in poor kitchen ventilation, the indoor environment is very harsh, and the kitchen temperature in summer is often as high as 40℃. The hot oil fume during the cooking process fills the entire kitchen space, which not only endangers the health of the chef and reduces work efficiency, but also easily contaminates the dishes with sweat, which seriously affects the quality and image of the hotel. To this end, this article combines the actual engineering, a more detailed analysis of the kitchen ventilation system design and equipment selection.


Kitchen ventilation system

The kitchen ventilation system has five parts: local exhaust (oil smoke), comprehensive exhaust (room ventilation), supplementary air, accident ventilation, and fire exhaust smoke. Accident ventilation and fire-fighting smoke exhaust have been clearly stipulated in the relevant national regulations and will not be repeated here. This article mainly studies local exhaust (oil fume), comprehensive exhaust (room ventilation), and air supplement system.


  • Kitchen local exhaust system

The kitchen local exhaust system refers to the installation of a local exhaust hood in the hot processing cooking area (steaming, frying, frying, boiling, frying, roasting) that emits a lot of waste heat and humidity. Collecting the waste heat and humidity on the spot and purifying it to reach the standard then discharge, which is the most critical and core part of the kitchen ventilation system. The system usually includes a local exhaust hood (water fume hood), piping system, oil fume purification device, and oil fume exhaust fan.


  • Full kitchen exhaust system

The full kitchen exhaust system refers to the ventilation of the entire kitchen space. On the one hand, comprehensive ventilation can be used as a necessary supplement to the local exhaust system when the kitchen is running, and discharge the oil fume and hot and humid steam that overflow from the local exhaust hood to the outside. On the other hand, when the kitchen is stopped, the room can be ventilated to remove odors.


  • Kitchen air supply system

The kitchen air supply system is used to provide the air required for combustion and supplement the air taken away by the exhaust air. The air supply and exhaust volume should be basically balanced, and the kitchen should be in a negative pressure state to prevent the fume and odor from affecting other areas. The negative pressure cannot be too large, otherwise, the furnace will blowback. The data shows that the negative pressure in the kitchen should generally not be greater than 5Pa, and the supplemental air volume is 80% to 90% of the total exhaust air volume.


  • Post air supply system

The kitchen equipment generates a great amount of heat, and the air conditioning system should be used to cool down if necessary. Due to a large amount of oil fume and water vapor in the kitchen, it is easy to be adsorbed on the heat exchange coil of the air-conditioning equipment, blocking the fin and making it invalid. Therefore, the air-conditioning system of the hotel kitchen should adopt the direct-flow air-conditioning system, without considering the return air. 


Generally, some kitchen air supply after cooling can be used as post air supply, and other supplementary air will not be processed. This can not only ensure the basic comfort requirements of the personnel working area but also avoid the huge energy consumption caused by cooling all the supplementary air. The post air supply is directly sent to the position of the chef. The new air outlet should not be too close to the chef. The height of the air outlet from the top of the chef's head is generally 500 mm ~ 1000 mm as shown in Figure 1. The design chooses a diffuser guide vane air outlet with d=200mm, the diffusion angle of the air outlet θ=25°, and the air outlet is 2200mm ~ 2700mm from the kitchen floor.


 Figure 1

              Calculation of kitchen ventilation

1. Accurate calculation of exhaust air volume

The total exhaust volume of the kitchen mechanical ventilation system is the sum of the partial exhaust volume and the overall ventilation volume. According to the heat balance, it is determined by the following formula:


In the formula, L-ventilation rate (m3/h ); tp-indoor exhaust air calculation temperature, the following values can be used: summer 35℃, winter 15℃; ts-supply air temperature; Q-sensible heat in the kitchen, W.

Q =Q1+Q2+Q3+Q4

In the formula: Q1- the heat dissipation of the kitchen equipment, calculated according to the data provided by the process. If there is no information, please refer to the relevant literature. Q2- the heat dissipation of the operator, W; Q3-heat dissipation of lighting fixtures, W; Q4-cooling load of indoor enclosure structure, W.


2. Calculation of local exhaust air volume

The oil fume generated by the kitchen stove is collected by a fume hood installed on the stove, and the oil fume is drawn to the outdoors through a fan installed outdoors. The plane size of the exhaust hood should be 100mm larger than the size of the stove side. The distance between the bottom edge of the exhaust hood and the cooktop should not be greater than 1.0m, and the height of the exhaust hood should not be less than 600mm. The minimum exhaust volume of the exhaust hood is generally selected according to the following calculation:

A: L=1000× P×H

In the formula, L-the amount of smoke exhaust (m3/h); P-the perimeter length of the cover (not counting the side against the wall) (m); H-the distance between the cover and the stove surface ( m).

B:  L=3600x Sx V

In the formula, L—the amount of exhaust fumes (m3/h); A—the area of the hood (m2 ); V-the suction wind speed of the hood section (m/s), generally not less than 0.5m/s, the dishwasher exhaust hood can be 0.2m/s.


3. Calculation of total ventilation

The overall ventilation volume should be calculated and determined according to the difference between the total ventilation volume and the local exhaust volume, and the number of air changes should be checked not less than 6 times/h.


4. Estimation of ventilation

As the heat generation parameters of the kitchen are not easy to obtain, the designer can also estimate the number of air changes from 40 to 60 times/h. Taking this project as an example, the original design adopts the estimation of the number of air changes, the number of air changes is 50 times/h, the kitchen area is 250m2, the ventilation rate is G=250x50x3=37500m3/h. The local exhaust volume is 33000m3/h, and the overall exhaust volume is 4500m3/h. There are 19 exhaust hoods in the kitchen, and the air volume of each exhaust hood is about 1750m3/h, which basically meets the 1800 m3/h for air volume of each exhaust hood mentioned in the literature.


kitchen ventilation system


The influence of heat pressure on fan pressure and air volume

Thermal compression refers to the chimney suction effect that can not be ignored due to the different gas density of the smoke pipes or building air shafts of higher buildings. The suction force is related to the height and the difference in gas density. Its size is:


In the formula, g-acceleration of gravity, m/s; h-vertical distance between the inlet of the exhaust hood and the outlet of the air duct, m; p1-the air density at outdoor calculated temperature, kg/m3; p2-Density of hot air in the pipe, kg/ m3.


Next, calculate the hot air buoyancy of the kitchen. The air temperature rise is:


In the formula, △T- air temperature rise, ℃; q-average heat generated by a single stove, kJ/h; Cp- specific heat of air at constant pressure, kJ/kg, ℃; Cp=1.01 kJ/kg ℃; V- exhaust air volume, m3/h, V=1750 m3/h; p-air Density, kg/m3. When the air temperature is 94℃, p =0.9618kg/m3. When the air temperature is 70 ℃, p =1.029kg/m3.

It was found that the average heat generation per stove was 1.67×105kJ/h. Taking into account the average utilization rate of the kitchen stove and the heat dissipation along the air duct, q=1.67×105×0.7×0.85= 1.0×105kJ/h. Calculate the summer air temperature rise △T=1.0x 10^5/ (1.01 x 1750x0.959)≈59℃; summer air temperature rise △T=1.0x 10^{5} (1.01×1750x 1.029)≈55℃.


In summer, the kitchen room temperature is 35℃, and the exhaust gas is 94℃. At this time, the air density p2=0.9618kg/ m3; Summer ventilation outdoor temperature in Suzhou area 32 ℃, air density p1=1.157kg/m. Density difference p1-p2=0.1952 kg/ m3.


In winter, the kitchen room temperature is 15℃, and the flue gas is 70℃. At this time, the air density p2=1.029kg/m'; the winter ventilation outdoor temperature in Suzhou area is 32℃, and the air density p1=1.157kg/ m3. Density difference p1-p2=0.1952 kg/m3.


Therefore, the general vertical height of the floating pressure △H=gh (p1-p2)≈2.2Pa/m (the density difference is the average of winter and summer) .


The performance parameters of the fan are measured under the standard conditions of atmospheric pressure 760mmHg, temperature 20℃, and relative humidity 50%. Since the temperature of the exhaust gas from the kitchen is much higher than the room temperature in winter and summer, which seriously deviates from the rated working condition of the fan, the air pressure and air volume changes of the fan should be calculated and corrected. The influence of thermal pressure on air pressure and air volume should be considered when selecting the fan type so that the air volume in actual use can reach the designed air volume.

kitchen ventilation system-1

1. Impact on fan pressure

The influence on the fan pressure can be calculated by the following formula:


In the formula, p2- Actual pressure of the fan, Pa; p1-Pressure of the fan under standard conditions, Pa; t-Temperature of exhaust gas, ℃. From previous calculations, the exhaust gas temperature is 94℃ in summer and 70℃ in winter. The pressure change of the fan is p2Summer/p1=293/(273+94)=79.84%, p2Winter/p1=293/(273+70)=85.42%. Therefore, due to the influence of the exhaust gas temperature, the fan pressure in summer drops by 20.16%, and the fan pressure in winter drops by 14.58%.


2. Impact on fan air volume

Due to the change of air density, the mass flow of the fan has undergone a big change. That is, the ventilation volume of the actual kitchen has changed. The change in air volume can be calculated by the following formula:


In the formula, GT-Hot air mass flow rate, m3/h; pT-Hot air density, kg/ m3; GBStandard air mass flow rate, m3/h; pB-Standard air density, kg/m3.


The exhaust gas temperature in summer is 94℃, and its density is 0.9618kg/m3. The temperature of exhaust gas in winter is 70℃, and its density is 1.029kg/m3. The density of the fan under standard conditions is 1.2kg/m3, then the mass flow ratio is: GTsumer/GB=pT/pB=0.9618/ 1.2= 80.15 %; GTwinter/GB=pT/ pB=1.029/1.2=85.75%. Therefore, under the influence of exhaust temperature, the fan exhaust volume in summer drops by 19.85%, and the fan exhaust volume in winter drops by 14.25%.


Selection of kitchen exhaust fan


In the kitchen fume exhaust system, in most cases, the fan is downstream of the fume purifier. When the oil fume passes through the fan, its performance and functions have a greater impact. The adaptability of the fan to oil fume pollution has become an important factor in the selection. Generally, it is recommended to use the backward impeller fan, and the installation angle β of the backward impeller blade is less than 90°. Its characteristics are that the impeller blades are large and the number is small, and the dust accumulation has little effect on the impeller. The impeller flow channel is long and slowly expands, which is more in line with the flow direction of pollutants, so the impeller is less dusty. Moreover, the sparse structure of the fan blades also facilitates the removal of fouling.



Selection and arrangement of fume hood

The fume hood should be considered in accordance with the requirements of the kitchen technology. Generally, when the stoves are placed side by side, they are mainly integrated. The layout of the fume hood should be based on the kitchen process requirements, understand the overall dimensions of the heating and cooking equipment, and determine the size of the fume hood. The section of the hood opening should be similar to the horizontal projection of the oil smoke diffusion zone. Generally, it is made of an overall rectangular umbrella-shaped cover, with the oblique waist plate on both sides and the included angle a≤120°. The cover should be added with a certain height skirt, the height of the skirt is about 0.25m. The outer edge of the range fume hood should not exceed the edge of the cooktop. The bottom of the hood should be equipped with a 50mm square-shaped drain and water trough, and a drain pipe should be set at the lowest part of the trough to drain into the sewer. The cover material is mostly made of 3~5mm thick stainless steel plate. An baffle grease filter is installed in the hood and should be easy to replace and clean, so as to prevent oil fume from entering the exhaust fan after the long-term operation and affecting its normal use. Since the emission from the emission source spreads vertically upwards in a cone shape, the emission is generally concentrated within 0.8~1.0m from the emission source. Therefore, the umbrella cover should be as low as possible without affecting the operation.



The design of the kitchen exhaust system should also meet the following requirements:

(1) The air duct should be welded with 1.5mm thick steel plate or stainless steel plate, and its horizontal pipe section should be as short as possible;

(2) The air duct should be provided with a slope drainage point or exhaust hood with a slope of not less than 2%;

(3) The wind speed of the air duct should not be less than 8m/s, and should not be greater than 10m/s. The wind speed at the throat of the exhaust hood connecting duct should be 4~5m/s;

(4) The outdoor part of the exhaust duct should take heat preservation measures to prevent condensation;

(5) The setting of the exhaust fan should be considered to facilitate maintenance. And an external motor should be used.


In conclusion

The following points should be paid attention to in the ventilation design of star-rated hotel kitchens: ①Accurate calculation of air volume; ②Selection of fan and coordination of piping system and construction profession; ③Reasonable selection and arrangement of fume hoods and purification of oil fume. Reasonable design of kitchen ventilation system and air conditioning system, correct installation and use of exhaust hoods, strengthening of normal operation management, effective improvement of kitchen air quality, and keeping clean and sanitary are very important to ensure the safety of catering.


kitchen ventilation system-2











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