Li Jie

(Department of Power Engineering, Xi'an Aviation Technology College, Xi'an 710077, China)

Abstract: This paper discusses the importance of commercial air conditioning filter and heat exchanger cleaning, analyzes the actual measurement of air conditioning operation in a supermarket, and designs on this basis.

The corresponding measuring instruments are used to monitor the working state of the filter and the heat exchanger, so that the automatic control of the cleaning time of the equipment can be determined in time, and the automation technical support is provided for the operation management and energy saving measures of the commercial air conditioning system.

Key words: air conditioning; differential pressure; supply air temperature; monitoring

CLC number: TB57 Document code: A Article ID: 1008-9233 (2010) 03-0043-03

1 Introduction

During the operation of the air conditioning system, a large amount of dust will accumulate on the air filter, and the surface of the heat exchanger will also form a layer of dirt due to the accumulation of some fine dust passing through the air filter, and the thickness and density of the dirt will continue to be constant. increase. The dust accumulated on the filter and the dirt formed on the surface of the heat exchanger not only breed the bacteria, pollute the air, reduce the ventilation, but also affect the service life and heat exchange efficiency of the end equipment, and ultimately reduce the air conditioning effect. Therefore the air filter and heat exchanger must be cleaned regularly.

In air conditioning systems, monitoring instruments are one of the commonly used auxiliary devices. According to the monitoring data provided by the monitoring instrument, the air conditioning system can be effectively maintained in time by manual or automated judgment. Install a differential pressure switch at the filter, monitor the working state of the filter according to the differential pressure signal, and determine the cleaning time; install a temperature detecting instrument at the air outlet of the air conditioning unit on the air outlet, according to the supply air temperature signal The working state of the heat exchanger is monitored and the cleaning time is determined.

2 Determination of filter and heat exchanger cleaning time

In practical applications, if the air filter is cleaned too many times, it will cause more fine dust to pass through the filter, so that the dirt accumulated on the surface of the heat exchanger will increase, which will affect the heat exchange efficiency; otherwise, if the air filter is very Less cleaning, in a long period of time, the amount of dust collected at the filter will be a lot, so that the ventilation will not only decrease, but the ventilation quality will be greatly deteriorated. If the ventilation is increased in this case, it will definitely increase. The operating energy consumption of the system.

In view of this contradiction, through the experiment and analysis of the relationship between the differential pressure before and after the filter of the commercial air conditioning system, the air volume and the temperature of the air-conditioned room, we determined that the pressure difference between the two points before and after the filter is close to the initial value of 2 When used, it can be used as the cleaning time of the filter. The heat transfer effect of the heat exchanger is not only related to the amount of dust on the surface, but also related to the outdoor air quality and filter filtration effect. If the heat exchanger is not cleaned for a long time, the heat exchange efficiency will be reduced, thereby increasing the operating cost of the system. However, since the heat exchanger and the air conditioning water system are tightly connected, in order to prevent the leakage of the heating or cooling medium, the heat exchanger must be cleaned on site; and during the cleaning process, the air conditioning system must be stopped, which will affect the commercial building. The normal operation, therefore, the heat exchanger can not be cleaned frequently. In view of this contradiction, through the actual monitoring of the summer operating state of the commercial air conditioning system, the relationship between the air supply volume and the supply air temperature and the heat exchange efficiency is analyzed, and it is determined that when the increase value of the supply air temperature reaches 3 ° C, the indoor temperature is close to or When it reaches 28 °C, it can be used as the cleaning time of the heat exchanger. This reference data can also be appropriately adjusted according to the business operation. The filter and heat exchanger operating condition monitoring alarm point settings are shown in Figure 1.

3 On-site monitoring of the working condition of filters and heat exchangers

3.1 Monitoring of the working condition of the air filter

The air filter of commercial air conditioner is used to filter the mixed air of outdoor fresh air and air conditioner return air. The control principle is shown in Figure 2. The initial resistance of the filter is determined to be 90 Pa. When the dust resistance value of the filter reaches 180 Pa, the differential pressure switch automatically alarms, indicating that the filter needs to be cleaned. When the alarm in the system starts, a continuous alarm sounds until the alarm signal is acknowledged. At the air filter, see Figure 1 for the BESTON T series TP33B-30: 30Pa~300P differential pressure switch. On the one hand, when the pressure difference between the filter and the front and back exceeds the upper limit value of 180 Pa, the alarm can be sent in the field, and the output signal of the switch can be sent to the control center to realize the remote monitoring prompt. It should be noted that in order to ensure the accuracy of the action, the TP33B-30 differential pressure switch should be installed vertically, so as to ensure that the internal air film weight does not affect the product accuracy. The pressure connection position is marked as shown in Figure 2.




3.2 Monitoring of the working state of the heat exchanger

On the air outlet section of the air conditioning unit, we monitored the air supply temperature at the air outlet of the air-conditioned room after being processed by the air conditioning unit, thereby understanding the working state of the heat exchanger and the actual state after air treatment. According to the supply air temperature data, we can determine whether the air supply temperature of the air conditioning unit is suitable and determine the cleaning time of the heat exchanger. The automatic control principle is shown in Figure 3.



The electric contact bimetal thermometer uses the temperature change to drive the contact change. When it contacts or disconnects with the upper and lower limit contacts, the relay in the circuit acts to realize automatic control and alarm. In the system, the instrument for on-site monitoring and remote monitoring of the supply air temperature is selected, and the electric contact bimetal thermometer with the model WSSX-411W, the accuracy class of 1.5, and the temperature range of 0~50 °C is selected. The upper and lower limits of the electric contact bimetal thermometer are set to 12 ° C and 15 ° C, respectively. When the supply air temperature is equal to 12 °C, the green light in the control circuit is on. When the supply air temperature is greater than or equal to 15 °C, the red light in the control circuit is on, the relay contact is activated, and the live sound and light alarm sounds continuously until the alarm signal is confirm. After the system alarm, the WSSX-411W type electric contact bimetal thermometer signal and the relay contact signal in the circuit are simultaneously transmitted to the control center to realize remote monitoring of the supply air temperature and prompt to clean the heat exchanger.

4 Remote monitoring of the working state of filters and heat exchangers

The commercial air conditioning system adopts PLC automatic control system, and the PLC model is FX2N-48MR. The switch signal of the differential pressure switch TP33B-30 at the filter and the electric contact of the bimetal thermometer WSSX-411W at the air supply port are used as the input signals of the PLC; the PLC outputs the corresponding alarm signal to alarm in the control room, reminding the control room staff to be clear in time. The working state of the filter and heat exchanger. The input/out point of the corresponding PLC system is shown in Table 1, and the I/O terminal wiring is shown in Figure 4.





5 Conclusion

(1) Through on-site and remote monitoring and control of the working state of the filter, the system operation and maintenance personnel can timely and conveniently determine the cleaning time of the filter and the heat exchanger.

(2) Reasonable setting and selection of monitoring instruments is beneficial to the adjustment, testing, operation and maintenance of the entire air conditioning system, improving the work efficiency of the on-site management personnel and the management level of the enterprise.

references

[1] Geng Xinfeng, Du Fangli. Determination of Cleaning Time of Commercial Air Conditioning Filters[J].Journal of Xi'an Aerotechnical College,2009,24(5):22-23.

[2] Zhou Yuan. Application of Monitoring Instrument in the Design of Purifying Air Conditioning System[J]. Journal of Medical Engineering Design, 2003, 24(5): 37-39.

[3] Yan Xinfeng, Du Fangli. Cleaning of Commercial Air Conditioner Heat Exchanger[J]. Value Engineering, 2010(1): 134-135.