Household Low Temperature Air Source Heat Pump (Water Chilling) Packages
As the water heating system using the air source heat pump is introduced above, the following is the introduction to the air source heat pump unit, which is particular for the household applications here.
According to the outdoor ambient temperature in heating mode, household air source heat pump (water chilling) packages can be classified into two categories.
One is the conventional type, which is normally used in mild climate region. This kind of heat pump is designed based on a rated heating condition of outdoor air dry bulb(wet bulb) temperature of 7°C(6°C), as specified in GB/T 18430.2-2016.'
The other one is the low temperature type, which is applicable for cold climate region. This kind of heat pump is designed based on a rated heating condition of -12°C(-14°C) according to GB/T 25127.2-2010 and can operate stably when the outdoor ambient temperature drops even to -20°C.
As for the conventional type, usually a single-stage compression heat pump is enough, while for the low temperature type, a two-stage compression heat pump is necessary and some special designs must be done.
In the low temperature air source heat pump (water chilling) package, the refrigerant circuit is relatively short, therefore the two-stage compression two- step throttling interstage incomplete cooling cycle is adopted.
Figure 7.3 shows the system schematic diagram of low temperature air source heat pump (water chilling) package using a triple-cylinder two-stage rolling piston compressor with variable volume ratio (referred to as two-stage compressor with variable volume ratio), which is in the smallest broken frame.
The heat pump system is mainly composed of the compressor 1, the oil separator 3, the four-way valve 4, the water-refrigerant heat exchanger 14, the throttling devices 6 and 8, the flash tank 7, the air-refrigerant heat exchanger 5 and others.
In heating mode, after leaving the compressor 1, refrigerant of high pressure and temperature state enters the water-refrigerant heat exchanger 14, releasing heat to water and being cooled down to liquid. Then it’s throttled by the throttling device 8 and flashes in the flash tank 7, where the refrigerant is separated into the liquid refrigerant and the gas refrigerant. The liquid refrigerant is throttled for the second time and then enters the air-refrigerant heat exchanger 5, where the refrigerant is vaporized and overheated by the outdoor air. Later it’s sucked into the low pressure stage cylinder and compressed. The discharge gas enters the interstage mixing chamber, mixing with the gas refrigerant from the flash tank and being sucked into the high pressure stage cylinder to complete an entire cycle.
In Figure 7.3, the solenoid valves 10 and 11 are used for switching the cylinder operation mode. To realize this function, one end of each valve is connected to the switch device, which is arranged on the low-pressure stage cylinder with variable volume. Another end of valve 10 is connected to the suction pipe while that of valve 11 is connected to the discharge pipe. When valve 10 is off and valve 11 on, the compressor operates in the triple-cylinder operation mode; conversely, the two-cylinder operation mode is on instead.
System schematic diagram of low temperature air source heat pump (water chilling) package
1. Compressor; 2. Muffler; 3. Oil separator; 4. Four-way valve; 5. Air-refrigerant heat exchanger; 6. Throttling device; 7. Flash tank; 8. Throttling device; 9. Vapor injection solenoid valve; 10. Solenoid valve; 11. Solenoid valve; 12. Oil-return solenoid valve; 13. Gas-liquid separator; 14. Water-refrigerant heat exchanger; 15. Water pump
Outlet water temperature
Most household low temperature air source heat pump (water chilling) packages are designed for dual functions: cooling in summer and heating in winter.
Generally speaking, in cooling mode, the outlet water temperature of the unit (approximately equal to the supply water temperature for the terminals) ranging from about 7 to 18°C can meet the requirements of the terminals such as fan coils; while in heating mode, a temperature ranging from about 30 to 55°C is enough to cover the need for almost all kinds of terminals, i.e., radiant floor, fan coil, radiator, etc.
According to the outlet water temperature range, low temperature air source heat pump (water chilling) packages can be classified into two types: the dedicated type and the general type. The former is designed for specific terminals which have special need for supply water temperature, while the latter is designed for most terminals and its outlet water temperature can be adjusted according to different on-site terminals.
So the outlet water temperature of the general type must be designed with a much wider range than that of the dedicated one. So far, most units are general type, which can match with different types of terminals flexibly to form different heating (or cooling) systems.