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Classification of heat recovery efficiencies - temperature efficiency, moisture efficiency and enthalpy efficiency
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Heat recovery units used in ventilation and air condition systems are based on some common principles:
With a return air recovery unit - used air is mixed in to the make up or supply air. Energy in outlet air is supplied directly in to the make up air. Both sensible and latent (moisture) heat is transferred.
With a rotating heat exchanger outlet air heats (or cools) the exchanger when the wheel passes through the outlet air flow. The energy is transferred to the make up air when the wheel passes through the make up air.
Both sensible and latent heat may be transferred. Latent heat when moisture in the outlet air condensates on the wheel. Moisture may be transferred with heat exchangers using hygroscopic wheels. For exchangers without hygroscopic wheels, the condensate is drained out.
With an air-fluid-air heat recovery unit heat is transferred in an heat exchanger from the outlet air to a circulating fluid. The fluid is circulated to the an heat exchanger in the make up air where the heat is transferred to the supply air.
Both sensible and latent heat may be transferred. Latent heat when moisture in the outlet air condensates on the heat exchanger. Moisture is not transferred.
In a cross flow heat exchanger heat is transferred directly from the outlet air to the make up air through the separating walls of the heat exchanger.
Both sensible and latent heat may be transferred. Latent heat when moisture in the outlet air condensates on the heat exchanger. Moisture is not transferred.
A heat pump makes it possible with some additional energy to move more outlet air energy to the make up air than any other system. The energy consumption is approximately 1/3 to 1/5 of the recovered energy.
Both sensible and latent heat may be transferred. Latent heat when moisture in the outlet air condensates on the heat exchanger. Moisture is not transferred.
The temperature transfer efficiency of an heat recovery unit can be expressed as:
μt = (t2 - t1) / (t3 - t1) (1)
where
μt = temperature transfer efficiency
t1 = temperature in outside air to the exchanger (oC)
t2 = temperature in outside air from the exchanger (oC)
t3 = temperature in inside air to the exchanger (oC)
The moisture transfer efficiency of an heat recovery unit can be expressed as:
μm = (x2 - x1) / (x3 - x1) (2)
where
μm = moisture transfer efficiency
x1 = moisture in outside air to the exchanger (kg/kg)
x2 = moisture in outside air from the exchanger (kg/kg)
x3 = moisture inside in air to the exchanger (kg/kg)
The enthalpy transfer efficiency of an heat recovery unit can be expressed as:
μe = (h2 - h1) / (h3 - h1) (3)
where
μe = enthalpy transfer efficiency
h1 = enthalpy in outside air to the exchanger (kg/kg)
h2 = enthalpy in outside air from the exchanger (kg/kg)
h3 = enthalpy in inside air to the exchanger (kg/kg)
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