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How is ventilation calculated?

views: 41  time: 2022-07-20


How is ventilation calculated?

1 Kitchen ventilation

1.1 Exhaust air volume

The total exhaust air volume of the kitchen should be determined according to the larger value calculated by removing the heat from the kitchen and calculated by the exhaust vent (or estimated by the number of air changes).

1 Exhaust air volume of the exhaust hood: Calculated according to the area of the cooker and the suction speed of not less than 0.5m/s, and the following formula, whichever is larger in the result.

L=1000 P·H

L:The exhaust air volume of the exhaust vent (m3/h);

P:The perimeter length of the exhaust vent (the side length against the wall is not counted) (m);

H:The distance between the air vent and the stove surface (m).

2 The number of air changes: usually the number of air changes is 20 times/h

The ventilation frequency of the room with stove required 30~40 times/h (cook Western food), 40~60 times/h (cook Chinese food);

1.2 Air volume and air volume balance of air supply and exhaust equipment

1) Exhaust system settings

(1) Other rooms such as thermal room and catering should be equipped with exhaust equipment respectively;

(2) The thermal room should be equipped with comprehensive exhaust facilities according to 5 air changes per hour, but it can be used in different time as the stove exhaust;

(3) Each cooking appliance should be equipped with an exhaust system separately.

2) Make-up air system settings

(1) When conditions permit, other rooms such as the thermal/bed room and the catering room should be equipped with air supply equipment respectively;

(2) When the overall exhaust air and the stove exhaust air are not used at the same time, the total air supply volume does not consider the overall exhaust air volume.

(3) The supplemental air volume of the supplementary air unit should adapt to the change of the exhaust air volume at any time to ensure the design negative pressure value and air volume balance of the kitchen.

(4) When using the fresh air of the catering as the supplementary air of the kitchen, the balance of the air volume between the supplementary air and the exhaust air of the catering should be considered.

1.3 Control

1) Air volume balance control

① The supplementary air equipment corresponding to the exhaust air equipment should be started and stopped in a chain;

② When each exhaust system shares the supplementary air equipment, the supplementary fan should change the air volume according to the number of open air exhaust equipment and the air volume.

2) Cooling and heating control

Since the calorific value of the kitchen equipment changes, the supply air temperature should also change accordingly, so as to avoid the waste of energy during ventilation in summer, such as the equipment calorific value is too small and the room temperature is too low, or the room temperature cannot be guaranteed or the room temperature is too high in winter. It is located in a suitable position indoors to control the heating and cooling capacity (water valve).

1.4 Heating and cooling capacity of public kitchen fresh air treatment units

1 Winter heating quantity Qr

Qr≈0.337Vx(ts-tw)/1000 (kW)

Where Vx——fresh air volume (m3/h);

ts——supply air temperature, determined according to the room temperature when heating on duty (℃),

According to the energy-saving standards for public buildings in Beijing, ts = 10°C for the thermal processing room, ts = 16°C for the production and catering rooms, and higher values for combined use;

tw——calculated temperature for outdoor winter heating (℃).

Note: The heat load of the enclosure structure is borne by the radiator, etc., and the ventilation heating capacity is the most unfavorable situation without considering the heat generation of the cooker.

2 Summer cooling capacity QL

1) Analysis and assumptions: When there is no data on the heat dissipation and moisture dissipation of kitchen equipment, the following assumptions can be made to estimate:

(1) The kitchen has a certain amount of moisture dissipation, but the heat generation is very large, so the heat and humidity are relatively large, which is assumed to be ε≈10000kj/kg;

(2) The exhaust air volume Vp has been determined. It is assumed that when the air supply is in the dew point state of the machine corresponding to the indoor state, the heat generated by the kitchen equipment can be eliminated, so that the air state of the room can reach the design parameters.

(3) In outdoor areas with hot and humid summer climate, it is recommended to use a lower dry bulb temperature (tn=30°C in the thermal processing room, tn=26°C in other rooms such as meal preparation), and a higher relative humidity (ψn=65%) ) as the temperature and humidity of the assumed indoor state point N.

(4) In outdoor areas with hot and dry summer climate, control the air supply to the same humidity level as the outdoor.

state, the above-mentioned assumption of indoor dry bulb temperature is still recommended, then the relative humidity ψn<65%.

(5) If the estimated calorific value is smaller than the actual maximum calorific value, the indoor state point will move along the ε line, the room temperature will increase and the relative humidity will decrease; if the actual calorific value is small, room temperature control cooling can be adopted amount, so that the room temperature will not drop too low to waste energy.

The changing process of ventilation air in the kitchen in summer

2) Calculation formula

(1) When the moisture content dw of the outdoor air state point (W) is greater than or equal to the moisture content ds of the air supply state point (S) corresponding to the assumed indoor state point N, it is calculated as follows:

QL= 1.2Vx(Iw-Is)/3600 (kW)

where Iw is the enthalpy value of outdoor air in summer (kJ/kg);

Is——The enthalpy value (kJ/kg) of the air supply state point S, the S point is the enthalpy value passing through the intersection point (machine dew point) of the heat-moisture ratio ε=10000 and ψ=90% of the N point.

(2) When the moisture content dW' of the outdoor air state point (W') is less than the moisture content ds of the assumed air supply state point (S), it is calculated as follows:

QL= 1.2Vx(Iw’-Is’)/3600 (kW)

In the formula, Iw' indicate tothe enthalpy value of outdoor air in summer (kJ/kg);

Is’ indicate toThe enthalpy value of the point S' in the supply air state (kJ/kg), the point S' is the intersection of the isotherm at the point S and the isotherm at the point W' (kJ/kg).

References:N- HVAC- A



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