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Design Of Air-Conditioning Fresh Air System For Market In Changsha

  • source:metmac;
  • Time:12/16/2020

Abstract: Air quality and passenger flow characteristic in Changsha were analyzed to get air conditioning loadcharacteristic and air-flow organization principle of market in Changsha city. The research focused on the determinationof fresh air load, principle and methods of control of fresh air volume, advantages and methods of heat recovery inmarket. The results showed that: BIN method can be used to calculate passenger flow related fresh air load of market, thedesign of fresh air system should depend on market location, passenger flow volume, floor etc; variable frequency orvariable speed control of fresh air fan is more suitable, and the control of fresh air volume should base on passenger flowvolume to decrease fresh air load and to reduce air conditioning energy consumption; the large passenger flow volumemarket can develop heat recovery to achieve air-conditioning energy savingand to improve energy efficiency.

Keyword: market, air conditioner, fresh air system, cooling load, heat recovery

 

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Shopping malls are public buildings with dense population and high mobility. According to the information, the air quality in the Changsha city shopping malls, when the fresh air system is not operating normally, the CO concentration is more than 0.2%, and the dust concentration is 0.5mg/m, both of which exceed the national standard by more than one time. The survey found that shopping mall customers generally report poor comfort. At present, relying on fresh air to dilute the concentration of indoor pollutants is the main way to improve the air quality of air-conditioned rooms and to meet people's hygiene requirements. At the same time, the air-conditioning system of commercial buildings accounts for a large proportion of the fresh air energy consumption, coupled with the shortage of domestic energy, therefore, the energy-saving design of the fresh air system has practical significance.

 

1 Store air conditioning cooling load and air distribution

1.1 Characteristics of cooling load of air conditioning in mall

In the cooling load of air conditioning in shopping malls, human body heat dissipation and fresh air cooling load account for a large proportion, which is an important part of the cooling load of air conditioning. According to the actual air-conditioning load statistics calculation of Changsha shopping malls, the composition of air-conditioning cooling load is shown in Table 1. It can be seen from Table 1 that the fresh air cooling load and the heat dissipation of the human body are the main components of the cooling load of the air conditioning in Changsha shopping malls, followed by the heat dissipation of the lighting, the heat dissipation of the equipment and the heat transfer of the enclosure structure.

ystem1

 

Analyzing the components of the cooling load of the air conditioner shows that the heat dissipation of the lighting is basically unchanged during the business hours of the mall, and the heat dissipation of the equipment changes very little. It is mainly affected by the power change of the inverter air conditioner. The heat transfer of the envelope structure is affected by the temperature difference between the outside and the inside. It accounts for a small proportion of the air-conditioning load of the entire shopping mall, and it has little impact on the changes in the air-conditioning load of the shopping mall. The amount of heat emitted by the human body in a shopping mall a day depends on the number of people in the shopping mall. The fresh air cooling load is the load generated to meet the air quality of the indoor personnel. Its main size is affected by the amount of polluted gas generated by the personnel, that is, the fresh air load is only affected by the people in the shopping mall. The influence of quantity, the heat dissipation of human body and the load of fresh air are also related to the location, floor, and types of goods sold in the city where the mall is located. In a shopping mall in a specific location, the heat dissipation of the human body and the fresh air load only depend on the flow of people in the shopping mall. Figure 1 is a graph of the flow of people in a shopping mall in Changsha obtained through statistics. It can be seen from Figure 1 that the flow of people in shopping malls fluctuates significantly during a day's operating hours, with peaks at 10:00, 15:30, and 19:00, respectively. It can be seen that the human body heat dissipation and the fresh air volume in the air conditioning load of the mall fluctuate greatly during the business hours of the mall. Combining the air conditioning energy consumption distribution in Table 1 and the above analysis, it can be seen that the change in the flow of people is the main factor affecting the air conditioning load change in the mall.

ystem2

 

 

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1.2 Characteristics of air distribution in the mall

1) Shopping malls are highly mobile, customers stay in the mall for a short time, and staff work indoors for a long time. The distribution of fresh air should reflect the satisfaction of the environment and the actual needs of people. That is, in crowded places and areas where customers stay for a long time, more fresh air will be allocated accordingly.

2) The air volume of the air conditioning in the mall is large, and the speed is large, and the air stays in the mall for a long time. In order to improve the utilization efficiency of fresh air, the positions of the air supply outlet, return air outlet and exhaust outlet should be arranged reasonably to prevent the short residence time of fresh air.

 

2 Determination and adjustment of fresh air volume

 

2.1 Determination of fresh air load

The elements to determine the fresh air load 14-5 include: ①The fresh air volume standard that meets people's hygiene requirements. ②The number of people present during peak hours of shopping malls. The pollution sources of shopping malls are mainly CO discharged by people's activities, dust and other harmful gases. Practice has proved that the use of fresh air to dilute indoor pollutants, when the concentration of CO and indoor hygiene standards, the concentration of other pollutants can generally meet the hygiene requirements. Therefore, the concentration of CO in the air in a shopping mall is an important indicator to measure indoor air quality. The national standard "Indoor Air Quality Standard" (GB/T18883-2002) stipulates the minimum allowable limit of pollutants in indoor air: CO, with a concentration of 0.1%, and inhalable particles of 0.15mg/m’. The characteristics of the number of people in the mall is that the flow of people is dense; the time of staying indoors is short and changes greatly with time. Therefore, to determine the energy-saving and comfortable fresh air supply, the variable air volume that changes with the number of people should be used to supply air, and the calculation of fresh air load should consider the fluctuation of the number of passengers. Therefore, the BIN method can be used for calculation to determine the optimal fresh air volume.

 

2.1.1 BIN method

The BIN method is a simplified energy consumption calculation method proposed by ASHRAE. According to the meteorological parameters of a certain place, the outdoor dry bulb temperature is segmented at a certain interval, and the temperature in each temperature section is obtained during the whole year or a certain period of time. Number, that is, the time frequency of temperature. Take the midpoint temperature of each temperature band as the representative temperature, and take the ratio of the sum of the moisture content corresponding to all the dry bulb temperatures in this band to the temperature frequency as the representative moisture content. Then calculate the building energy consumption in different temperature frequency bands, and multiply the calculation results by the number of hours in each frequency band, and add them to get the imaginary air-conditioning load for the whole year or season.

 

2.1.2 Calculation method of fresh air load in shopping mall

The outdoor temperature parameters change throughout the year, and the number of people in the mall also changes all the time, so the fresh air volume should be calculated by time period. Because the number of people gathered in the mall from 8:00 to 21:00 can represent the characteristics of the change in the number of people in a day, the time period is divided into 14 hours, and the fresh air load per minute for each time period is calculated, and the fresh air volume at that time is calculated from this. . Then the fresh air volume of all time periods is accumulated, and finally the annual fresh air volume is obtained.

The fresh air load calculation formula in the BIN method is as follows:

Qn=0.34xV,x1x(T-T)/ A, (1)

Q,mn =0.83xVxix(d-d)/A, (2)

V;=vXn; (3)

Where: Q is the sensible heat load of the fresh air; Q, the new latent is the latent heat load of the fresh air; i is the i-th moment of the day; V; is the fresh air volume at the i moment, m'; n; is the j-th BIN temperature The total number of people in the shopping mall in the i-th time period; v is the minimum fresh air volume, m/h, calculated as CO, with a concentration of 0.1%; t is the BIN time interval; dd is the difference between indoor and outdoor humidity; A is the building Area of air conditioning, m? ; T is the outdoor temperature (jth BIN temperature), "C; T is the indoor temperature, C.

 

2.1.3 Outdoor parameter selection

The summer BIN parameters of Changsha area are shown in Figure 2, and the winter BIN parameters are shown in Figure 3.

ystem3

 

2.1.4 Simulation calculation

The annual load is calculated in units of days, and each day is divided into 14 hours (calculated from 8:00 to 21:00), each minute of each time period is expressed as i; the dry bulb temperature is expressed as T.

1) Algorithm idea

First set and initialize all parameters; call outdoor meteorological parameters and various calculation parameters, and judge the nature of the working day; then use formulas (1) ~ (2) to calculate the fresh air load under the outdoor temperature, and judge T during calculation; , So as to decide whether to calculate the cooling load or the heat load; then perform unit conversion to obtain the fresh air volume; enter the next moment to calculate the fresh air load; perform another day's calculation; so after repeating the cumulative calculation, when the time exceeds 366 days, the output calculation new The result of the load is the annual fresh air load.

2) Algorithm flow

①Parameter setting and data initialization: The data includes time M=0, H=0, D=0 (M is month, D is day, H is hour), and the store's fresh air cooling load X. =0, heat load Xn=0;

 

2.1.4 Simulation calculation

The annual load is calculated in units of days, and each day is divided into 14 hours (calculated from 8:00 to 21:00), each minute of each time period is expressed as i; the dry bulb temperature is expressed as T.

 

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1) Algorithm idea

First set and initialize all parameters; call outdoor meteorological parameters and various calculation parameters, and judge the nature of the working day; then use formulas (1) ~ (2) to calculate the fresh air load under the outdoor temperature, and judge T during calculation; , So as to decide whether to calculate the cooling load or the heat load; then perform unit conversion to obtain the fresh air volume; enter the next moment to calculate the fresh air load; perform another day's calculation; so after repeating the cumulative calculation, when the time exceeds 366 days, the output calculation new The result of the load is the annual fresh air load.

2) Algorithm flow

1. Parameter setting and data initialization: The data includes time M=0, H=0, D=0 (M is month, D is day, H is hour), and the store's fresh air cooling load X. =0, heat load Xn=0;

2. Call the outdoor meteorological parameters: outdoor temperature T, the representative temperature Tic of the lowest temperature section where cooling starts, the representative temperature T of the temperature section where heating starts, and the outdoor enthalpy h; Determine the number of people n gathered by judging the nature of the working day, calculate the fresh air volume at that moment by formula (3), and then calculate the fresh air load according to formula (1) and formula (2);

3. Perform the calculation at the next moment;

4. If the annual fresh air load calculation has been completed, add the fresh air cooling load and the fresh air heat load and output.

The calculation process is realized by computer, using PowerBuilder9.0 and SQL Server2005 database to complete. As shown in Figure 4.

Fresh-Air-System4

2.2 Adjustment of fresh air volume

As mentioned earlier, the number of people in the mall is constantly changing within a day, that is to say, the amount of fresh air in the mall can change as the number of people changes. The adjustment of the fresh air volume has a significant effect on reducing the energy consumption of the air conditioner. The adjustment method of the fresh air volume is related to the form of the air conditioning system.

2.2.1 Fresh air volume control method

There are three main problems in the control of fresh air volume: the distribution of fresh air, the determination of the total amount of fresh air, and the uniformity of fresh air. At present, the commonly used fresh air volume control methods are: ① Fan tracking method. Its control principle is: the fresh air volume is equal to the difference between the air volume of the supply fan and the air volume of the return fan. Therefore, no matter how the air volume changes during the operation of the system, the input air volume and the return air volume are measured at the same time to indirectly control the fresh air volume. ② Direct measurement method of fresh air volume. This method is currently the simplest method to control the fresh air volume of ventilation and air conditioning systems.

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