Study on the Detection Method of Defrosting Start Time by Measuring Static Pressure Difference in Air Source Heat Pump System

Abstract

This study shows the feasibility and effectiveness of the detection method for determining the defrosting start time by measuring the air static pressure differences between the atmospheric pressure and the pressure measured before the outdoor unit fan in air source heat pump system (ASHP).

Generally, the frosting occurs under cold and humid environmental air condition, which means outdoor air temperature around the heat exchanger is lower than both dew point temperature of air and freezing temperature of water.

As the frost accumulation increases, the air pressure differences occur between the atmospheric pressure and the pressure measured inside the outdoor unit because of becoming narrow the area of airflows. It is possible to predict the frost accumulation of the heat pump evaporator indirectly measuring these changes of the pressure differences.

The pressure of the air is consist of the static pressure and dynamic pressure. The static pressure is used for obtaining the consistent and reliable data for the airflows. If the dynamic pressure is used, the experimental data will be influenced by the position of the differential pressure sensors and be possible of being blocked by the dust or moisture. On the contrary, if we use the static pressure, the negative pressure occurs at the position of measuring the pressure inside the outdoor unit. When the outdoor fan operates, the pressure inside the outdoor unit becomes lower than the external pressure and the negative pressure is generated. This shows that the possibility of going the dust or moisture inside sensors is minimized and the static pressure is less influenced by the change of the air velocity, unlike the dynamic pressure.

The pressure measurement position inside the outdoor unit is an important factor in this study. The heat exchanger is generally divided into three parts such as high, middle and low. As the pressure is measured at a position close to the fan, the value of the static pressure increases. On the contrary, it gets the lower static pressure value being farther away from the fan.

As the frost accumulation increases, the air pressure differences between the atmospheric pressure and the pressure inside the outdoor unit also increases.

It shows that the position of measuring the pressure is getting lower, the pressure differences become bigger.

Measuring the pressure at the same height, the distance between the measuring point and the heat exchanger is an important factor to consider.

The pressure is measured at five locations: center, front, rear, right, and left to find the optimal position at the same height. The distance between each sensor is 8 cm and the right position is about 3 cm away from the outdoor heat exchanger.

The differential pressure value increases as the pressure measurement position is closer to the heat exchanger due to the influence of the airflows.

The performance of various operating conditions such as pressure measurement position, outdoor air temperature, relative humidity and compressor speed is tested to identify the relation of pressure drop with the heating capacity of the system. The pressure difference control method is suggested by providing an appropriate threshold value and comparing with the result with other estimation methods.

As a result, the suggested method is more reliable than conventional time control method for deciding the defrosting start time.

Conclusively, the defrosting start time can be controlled by measuring the air static pressure difference between the atmospheric pressure and the pressure measured before the outdoor unit fan in the air source heat pump system.