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Figure 4-3 40 30 20 10 0 -10 -20 -30 -40 -50 -600 10 20 30 40 50 60 70 % (by volume) Dowfrost HD in water freeze point temperature burst point temperature Systems designed with monobloc air-to-water heat pumps vary depending on the severity of the winter climate. In mild climates, where outdoor temperatures below freezing are rare, it is usually acceptable to install the heat pump with water in the piping circuit between the outdoor unit and indoor distribution system. Most heat pumps of this type have a controller that automatically turns on a circulator and an electric heating element when necessary to protect the unit from freezing. These components can add enough heat to maintain the water- filled portion of the heat pump above freezing, even when there is no load calling for heat pump operation. However, in situations where significant temperature setbacks or prolonged power outage are possible during subfreezing outdoor temperatures, and no backup generator capable of running the heat pump is available, it is advisable to install the unit as part of an antifreeze- protected hydronic circuit. Some manufacturers mandate that antifreeze solutions be used in all installations of their monobloc air-to-water heat pumps. FREEZE PROTECTION OPTIONS FOR MONOBLOC HEAT PUMPS All systems designed around monobloc air-to-water heat pump systems need to consider freeze protection. Even systems in southern states could experience a situation in which a prolonged power outage accompanied by subfreezing temperatures could allow water to freeze within the exterior piping and heat exchanger. Another possibility is when the heat pump is not operating due to a service issue while subfreezing temperatures occur. ANTIFREEZE-BASED FREEZE PROTECTION One of the simplest methods of preventing freezing is to fill the entire system with an antifreeze solution of adequate concentration to prevent damage to the system under the coldest expected temperatures. The amount of antifreeze needed depends on the expected function of the solution during cold weather. Is that function to allow flow through the system under the coldest expected temperature, or is it to prevent bursting of one or more components due to expansion of the solution when it freezes solid? This requires differentiation between the freeze point and burst point temperatures. The freeze-point temperature of an antifreeze solution is the minimum temperature at which the solution remains flowable. Small ice crystals are just beginning to form in the fluid when it drops to its freeze-point temperature. This temperature is well below the normal operating temperature of a typical air-to-water heat pump. However, if it is possible that the heat pump may have to start after being off for several hours in very cold ambient conditions, the antifreeze solution used should have a freeze-point temperature as low as the minimum ambient air temperature at which the cold start could occur. The burst-point temperature of an antifreeze solution is the lowest temperature at which the piping and piping components that contain the solution will not be subject to expansion forces that could rupture them. The antifreeze solution will be mostly ice crystals when it drops to burst point temperature, and thus, not flowable. Figure 4-3 compares the freeze protection temperature and burst protection temperature for a range of volumetric concentrations of an inhibited propylene glycol antifreeze. Based on Figure 4-3, a 35% solution of propylene glycol antifreeze remains flowable down to 5oF and protects the heat pump and piping against bursting down approximately -30oF. A 50% solution of the same antifreeze remains flowable down to -20oF and provides burst protection to temperatures below -60oF. The objective is to select a concentration that adequately protects the system but doesn’t use excessive amounts of antifreeze. The higher the antifreeze concentration, the higher the viscosity of the fluid, and the greater the flow resistance of the hydronic circuit. Adding antifreeze to water also decreases the heat transfer capacity of the solution. A 50% solution of propylene glycol antifreeze lowers the specific heat of the solution to approximately 90% that of water. This can be compensated for by using higher flow rates, but that can significantly increase circulator power requirements. 29 Freeze Point & Burst Protection temp (oF)PDF Image | Heat Pump Systems 2020
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