Afreeze stat is atemperature sensing device forHVAC that monitors aheat exchanger to prevent its coils fromfreezing. Freeze stats can be used on bothrefrigerant-to-air, andrefrigerant-to-liquid type heat exchangers and serve different purposes with similar goals for each.
The purpose of anair coil freeze stat is to keep the refrigerant-to-air heat exchanger (commonly calledair coils) from freezing. This kind of freeze stat is typically used for heating coils which are exposed to outside air and is usually installed on the supply air side of the coil.[1] To accomplish this, they typically shut down the flow of outside air to amixing box when the temperature reaches a predeterminedsetpoint. The setpoint for air coil freeze stats is typically about 12 °C which is approximately when thedew point temperature of the air starts to drop below freezing point.[2][3][4]
Thewater coil freeze stat performs a similar function to the air coil freeze stat, but is used on a different type of HVAC system. Its function is to keep the freon-to-liquid heat exchanger (commonly calledliquid coils) from freezing.
In practical situations, when an air coil freeze stat is not used, bypassed, or defeated, the air coil can freeze, and this causes a lack of air flow to the facility.[3] By contrast, when a water coil freeze stat is not used, the water coil can get so cold that it can freeze the cooling liquid in the exchanger and burst the exchanger.[5] A ruptured heat exchanger mixes contaminants such as freon andoil into the coolant. The intrusion of coolant into the delicately balancedrefrigeration system can cause expensive damage to components such as thefreon compressor, and thereversing valve.
Conceptually,mechanical freeze stats are constructed with adiaphragm, acapillary tube and bulb, and an electricswitch.[2][6] The capillary tube allows gas movement to and from the capillary bulb and the diaphragm with a fall or rise in temperature, respectively. When the temperature reaches a specific setpoint, the pressure in the diaphragm will trip a switch which typically shuts down the flow of outside air while the capillary bulb's temperature is at or below the setpoint.[6] Mechanical freeze stats can have more than one set ofcontacts, and the contacts can beNO orNC.[2]
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Digital freeze stats, also known aselectronic orsolid state freeze stats, use an electronic circuit,microprocessor, ormicrocontroller in place of the mechanical freeze stat's diaphragm and switch. They also use one or more electric or electronic sensors in place of the capillary tube and bulb for temperature sensing. Digital freeze stats utilizing microcontrollers or microprocessors can also perform more advanced functions than simply opening or closing switch contacts. Digital freeze stats can perform seemingly intelligent operations such as reversing the refrigerant flow and using some building heat to thaw a freon-to-liquid heat exchanger and then restoring the refrigerant flow to its normal direction once the danger of freezing the water coil has been averted.[7]
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