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Discussion on the Ventilation and Air-Conditioning Transformation of the Tall Space Building

  • source:metmac;
  • Time:01/06/2021
Ventilation and air conditioning system transformation of large space buildings to cabin hospitals for COVlD-19
Abstract Presents the characteristics and construction requirements of the large space buildings transformed into cabin hospitals and their ventilation and air conditioning systems, and puts forward a simple and feasible transformation way for ventilation and air conditioning systems.  Analyses the ventilation rate in each area of the cabin hospital and its influence on the temperature of the ward. Suggests that some public buildings should reserve the reconstruction conditions and the related disaster preparedness materials in the construction in the future.
0 Preface
Since the outbreak of the new crown pneumonia in 2020, a number of square cabin hospitals have been built in a very short period of time, mainly treating patients with mild confirmed cases. Most of the square cabin hospitals built are transformed from existing tall space buildings (such as stadiums, exhibition halls, factories, etc.), but as a square cabin hospital as a field hospital in wartime, when used in the treatment of new coronary pneumonia, to ensure medical care The health of personnel, staff and patients, and how to prevent the virus from spreading to the surrounding environment, is a problem worthy of discussion.
In the transformation process of multiple shelter hospitals that the author participated in, the transformation of clean areas is simple and feasible, but how to reasonably carry out the transformation of contaminated and semi-contaminated areas is a more complicated problem. The following is a specific analysis and discussion.
 
 
1 Features and requirements of reconstructed buildings
1.1 Renovated building features
During the epidemic, Wuhan was transformed into an existing building of a shelter hospital
There are gymnasiums, exhibition halls, workshops, etc., all of which have common features:
1) The surrounding space of the building is spacious, the transportation is convenient, the distance from the residential area is far, and the facilities such as water, electricity and communication are complete.
2) The building space is tall, generally the indoor height is greater than 15m, and the workshop height is slightly lower, but the indoor height is also greater than 5m; there are many doors and windows, most of the roof is grid structure, and the air tightness is poor.
3) Centralized air-conditioning systems are installed in the exhibition halls and gymnasium buildings that have been confirmed to be transformed into square-cabin hospitals; the workshops that were transformed into square-cabin hospitals in the early stage of the epidemic were rarely equipped with centralized air-conditioning systems. For comfort, a factory building with a centralized air-conditioning system is determined as the basic requirement for the transformation of a shelter hospital.
4) Fire fighting facilities are basically complete.
 
 
1.2 Ventilation, air conditioning and smoke exhaust facilities
The air-conditioning terminal mode of the gymnasium and exhibition hall is a primary return air all-air air-conditioning system, which adopts the side air supply in the middle of the nozzle and the centralized return air at the lower part; some stadiums and exhibition halls are equipped with mechanical exhaust and mechanical smoke exhaust systems; the exhaust of the mechanical exhaust system The air volume is less than the air supply volume of the air conditioner. For workshops with centralized air-conditioning systems, the air-conditioning end method is a primary return air all-air air-conditioning system, and the air-conditioning air supply method is top air supply. The workshops are rarely equipped with mechanical exhaust and mechanical exhaust systems. The fresh air outlets of the all-air air-conditioning system are all arranged on the outer wall near the air-conditioning machine room; the air outlets with mechanical exhaust (smoke) system are all arranged on the outer wall or roof near the exhaust (smoke) machine room. When the air outlet is set on the roof, the height of the air outlet shall not be higher than the elevation of the roof by 2m. The horizontal distance between the fresh air outlet and the exhaust outlet is generally greater than 6m, and the horizontal distance varies greatly in different buildings.
 
 
1.3 Requirements and characteristics of ventilation and air conditioning transformation
According to the requirements of functional departments, users and related regulations, the transformation of the shelter hospital should achieve the following goals:
1) The shelter hospital after the transformation can ensure the health of medical staff, staff, and patients, and prevent viruses in contaminated and semi-contaminated areas. Spread to the surrounding environment;
2) Because the time for renovation is extremely short, the ventilation and air conditioning renovation needs to be based on the current situation, and the equipment and materials needed to be modified should be existing;
3) The square cabin hospital can quickly and simply restore the original Features.
Analysis of Ventilation and Air-Conditioning Transformation in 2 Square Cabin Hospital
The basic functions of the contaminated area and semi-contaminated area of the shelter hospital are shown in Figure 1. The following is an analysis of the ventilation and air conditioning transformation of each area.
 
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2.1 Ward
The main functions of the ward include hospital bed area, nurse station, rescue treatment room, activity room, meal preparation room, clothing storehouse, etc. Mild patients are divided into multiple areas according to their condition. The partition height is generally about 2m, and the average use area for patients is about 7~12m'. In a square cabin hospital with 1,000 people, medical staff are divided into 4 shifts every day, with approximately 60 people in each shift.
 
2.1.1 The layout of hospital beds
Many respiratory infectious diseases (including new coronary pneumonia) mostly break out in winter and spring. At this time, the outdoor temperature is low, heating is required indoors, and adjacent airflows are formed near the outer windows. The greater the temperature difference between indoor and outdoor, the worse the thermal insulation performance of windows, and the stronger the airflow against the wall. When people in this area communicate with each other, the airflow against the wall destroys the thermal plume on the surface of the human body and is easy to form cross-infection. Therefore, the hospital bed should be kept away This area can also avoid the impact of cold radiation on the patient. Combined with the actual use of the square cabin hospital, it is recommended that the channel width at the outer wall is more than 3m, and the distance between the beds is 1.5m. When the beds in the hospital bed area are densely arranged, medical air disinfection and purification devices can be installed to reduce the risk of infection of various diseases.
 
 
2.1.2 The influence of air supply and exhaust volume in tall and large space buildings on indoor negative pressure
Clean areas, semi-contaminated areas, and contaminated areas need to maintain an orderly pressure gradient. The pressure in the sick area is the lowest, but the air tightness of the tall space is poor. How to maintain a reasonable pressure difference to prevent indoor air spills is worth discussing. The following is the exhibition hall, The gymnasium and workshop were transformed into a square cabin hospital with 1,000 people as an example for analysis.
The standard of exhaust air volume in the ward is calculated based on not less than 150m3/h per person", the air volume is 150000m3/h. The architectural features of exhibition halls, gymnasiums, and factory buildings and different air supply and exhaust rates test the airflow effect See Table 1.
 
From Table 1, it can be concluded that the airtightness of the exhibition hall without external windows and grid structure roof is the best. When the air supply volume is less than 90% of the exhaust air volume, the interior of the square cabin hospital that is transformed into a stable Negative pressure can effectively prevent the virus from spreading to the surrounding environment and has good safety; however, actual research shows that it has certain shortcomings. Because there is no external window, the patient's living environment experience is poor and the pressure is high, which is not conducive to the recovery of mild patients health. Due to the large number of exterior windows and the grid structure roof, the indoor airtightness of the stadium is the worst. When the air supply volume is about 60% of the exhaust air volume, the indoor can form a stable negative pressure. The main reason is that the stadium has too much space and air leakage. The amount is large, and the exhaust system is arranged unevenly. Compared with exhibition halls and gymnasiums, the height of the factory building is relatively low. Although it has a larger area of external windows, when the same indoor negative pressure is maintained, the ratio of air supply to exhaust air volume is between the above two .
From the above analysis, it can be concluded that in order to avoid virus spillover, it is more reasonable to choose a factory or exhibition hall with a suitable height and fewer exterior windows. When the airtightness of the building is poor, the air supply volume should be reduced to maintain the negative pressure in the room. The air supply volume is recommended to be set at 60%~90% of the exhaust air volume. When the airtightness is poor, the lower value is used, and vice versa. .
 
 
2.1.3 Modification of ventilation and air conditioning
There are two schemes for retrofitting the terminal air conditioning system:
1) Fresh air operation mode;
2) Fresh air and return air mixed operation mode. The advantages and disadvantages of the two transformations are shown in Table 2.
It can be seen from Table 2 that the use of fresh air operation mode is more suitable for air conditioning renovation plans during the epidemic.
 
 
2.1.4 Analysis of indoor temperature in fresh air operation mode
The actual operation of Wuhan Fangcai Hospital shows that the temperature in the ward has a great influence on the mood of patients, and the new crown pneumonia epidemic broke out in winter and spring. This period belongs to the heating period. When the fresh air operation mode is adopted, the impact on indoor temperature The degree analysis is as follows.
Investigate the exhibition hall, gymnasium and factory building of Wuhan transformed into a shelter hospital. The ratio of the heating load to the cooling load of the central air conditioning is 0.4~0.6. At the same time, the author’s unit has designed many such projects in Hubei Province over the years, and the results show that The ratio of the heating load to the cooling load of the central air conditioner is 0.40~0.65, so the following calculation and analysis are carried out with the ratio of the air conditioning heat and cooling load of 0.4~0.6.
Take the exhibition hall transformed into a 1,000-person square cabin hospital in Wuhan as an example. The cooling capacity of the air-conditioning cold source is about 2000kW, and the heating capacity of the air-conditioning heat source is about 1200kW. The terminal air conditioner is selected based on the cooling load of the air conditioner. For tube selection, the total air supply volume of the air conditioner is at least 300,000 m3/h. When it is all converted to fresh air operation in winter, it is calculated according to formula (1):
 
                                                                                               456_--
 
Where Q is the heat load, kW; c is the specific heat capacity of the air, taking 1.01kJ/(kg.'C); M is the air mass flow, kg/h; Ot is the air temperature rise, C; 3600 is the conversion factor, s/ h.
Calculate △t=11C.
Calculations show that when all heating areas of the entire central heating system are operating, when the air conditioner (6 rows of tubes) sharing the cold and hot water coils is running with fresh air, the supply air temperature will increase by about 11' on the basis of the outdoor temperature. C, that is, when the outdoor temperature is -2'C, the air supply temperature of the air conditioner is only 9'C. When part of the heating area in the central heating system does not operate, only the modified square cabin hospital area operates, and the air conditioning heat source capacity meets the heat load of the square cabin hospital during fresh air operation, the heating capacity depends on the heat supply of the air conditioner itself Capacity and hot water flow. The hot water flow is taken according to the summer cold water flow. Similarly, it can be obtained by calculation: the supply/return water temperature is 60"C/45"C, and the 6-row air conditioner is running with fresh air. The heat provided by the unit air volume (1m3/h) is about 10~12W. If the value is 12W, the supply air temperature will increase by about 33.6 on the basis of the outdoor temperature. C. If you want to continue to increase the supply air temperature, you can take the following two measures:
1) Increase the flow of hot water, but it is limited by the existing air-conditioning pipe diameter and the head of the hot water circulating water pump, and the flow rate increase is small;
2 ) Increase the water supply temperature to 65. At C, the heating capacity is increased by about 12%, that is, the supply air temperature can be increased to 37.6. C. It can be seen that under normal circumstances, when the air conditioner in the return air state is changed to the fresh air operation mode, the supply air temperature will increase by 11.0~33.6'C on the basis of the outdoor temperature.
 
If the exhaust air volume of the ward is not less than 100m per person? /h Calculate the fresh air volume of the air conditioner according to 80% of the exhaust volume, and the average area per capita is 10m? When the outdoor temperature is -2"C, the supply/return water temperature is 60°C/45°C, and the air conditioner adopts 6 rows of pipes. In order to meet the indoor temperature reaching 16.C, according to the previous analysis, the air conditioner The maximum heat that the fresh air sent out can provide to the room is about 47W/m?, of which the heat offset by the penetrating wind is about 13W/m?, and the remaining heat 34W/m? can meet the thermal load of the enclosure structure, so the indoor temperature can be Reach 16. C, which can guarantee the use requirements of patients.
 
When the shelter hospital adopts the fresh air mode, the indoor temperature in winter is mainly determined by the following factors: the capacity of the air conditioning heat source, the exhaust air volume of the ward, the performance parameters of the air conditioner, and the hot water temperature. When the outdoor temperature is -2'C, the patient area will be exhausted according to the exhaust air volume of not less than 100m3/h per patient. The capacity of the air conditioning heat source can meet the air conditioning heat load of the fresh air operation in the patient area. The air conditioner adopts 6 rows Pipe, the supply/return water temperature is 60'C/45'C, and the indoor temperature can reach 16C. When the per capita exhaust air volume increases, the heat load of the air-conditioning in the shelter hospital increases, and the air-conditioning heat source cannot meet its demand, the indoor temperature will drop and it will not reach 16C. When the air conditioner adopts 4 rows of tubes, the air supply temperature is lower than that of a 6-row air conditioner, that is, the amount of heat provided to the room per unit of air volume is reduced; in order to maintain a 16"C room temperature, the per capita air supply needs to be increased Therefore, the heat load of the ward is greater. Unless the capacity of the air conditioning heat source and the heating capacity of the water system can meet the increase in heat load caused by the increase in air volume, the indoor temperature will become lower.
Other areas in hot summer and cold winter areas can refer to the above methods to ensure the indoor temperature in the winter ward; the indoor temperature in the ward in hot summer and warm winter and mild areas can easily meet the requirements for use. If a shelter hospital is renovated in a severe cold area, a building with a full-air air-conditioning system and an independent heating system (such as radiant floor heating) should be selected. When the full-air air-conditioning system is changed to a fresh air system, it should be reduced by the air conditioner frequency conversion method. The fresh air volume meets the requirements of per capita exhaust air volume and supply air temperature. The indoor temperature is mainly ensured by an independent heating system. In cold areas, if there is no independent heating facility in the shelter hospital, or although there are heating facilities, the amount of heat that can be provided is small. When the outdoor temperature is low, the indoor temperature cannot reach 16°C, and other auxiliary measures are required ( Such as adding a temporary heat source) to increase the indoor temperature to protect the health of the patient and stabilize the mood.
 
2.1.5 Analysis of per capita air volume
The value of the exhaust air volume of the ward is worth exploring, and its size needs to meet the following conditions:
1) meet the patient's fresh air volume demand;
2) establish an orderly pressure gradient;
3) ensure a suitable indoor temperature;
4) facilitate rapid renovation completion .
 
Since the Fangcang shelter hospital admits patients with mildly confirmed cases, the fresh air volume required by the patient should not be less than 40m'/h [2]. The tall and large space buildings are converted into square cabin hospitals. The airtightness is generally poor. It is difficult to achieve an absolute pressure of -5Pa indoors. It can only maintain a relative pressure difference with the outdoors to prevent the virus from spreading outdoors and reduce the risk of infection in the surrounding area. According to the previous analysis, for areas with hot summer and cold winter, when the exhaust air volume is not less than 100m3/h per person, it can meet indoor 16 requirements. The temperature requirement of C, it is more difficult to ensure the indoor temperature if the exhaust air volume is increased; the increase of the exhaust air volume means that the exhaust HEPA filter group will increase accordingly, the transformation difficulty increases, the construction time becomes longer, and the investment increases.
The minimum number of fresh air changes in the ward for respiratory infectious diseases is 6h-1 [3], if the ward is 10m per person? , Calculated with a net height of 2.6m, the exhaust air volume per person in the ward is 156m2/h. The amount of air breathed by the human body in a peaceful state is about 6~9L/min, that is, 0.36~0.54m3/h. Due to the long-term existence of patients in the shelter hospital, the amount of virus is very high. When the patient exists for 160 minutes, the virus production rate reaches 4680 quanta/h, as shown in Figure 2 [4], the use of air changes less than 10h in infectious disease wards cannot effectively reduce the risk of infection. When the per capita exhaust air volume of the square cabin hospital is calculated as not less than 150m/h per person, the indoor ventilation frequency of the gymnasium, exhibition hall and factory building is about 0.5~3.0h~l; at the same time, due to the large space, the ventilation efficiency is poor. The effective ventilation rate is lower, so increasing the exhaust air volume does not effectively reduce the risk of infection.
In order to meet the patient’s normal fresh air volume and room temperature requirements, maintain an orderly indoor pressure gradient and ease of transformation, while taking into account a certain safety margin, it is recommended that the exhaust air volume is not less than 100m3/h per person. For summer heat Warm winter and mild areas can appropriately increase the exhaust air volume standard
Based on the above analysis, combined with the air-tightness of the gymnasium, exhibition halls and workshops, air-conditioning and ventilation settings and architectural features, from the perspective of the effectiveness of ventilation and ensuring indoor temperature, the workshops and exhibition halls are more suitable for transformation into shelter hospitals and gymnasiums. difference.
 
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