Heat Recovery Efficiency

Classification of heat recovery efficiencies - temperature efficiency, moisture efficiency and enthalpy efficiency - online heat exchanger efficiency calculator

Common Energy Recovery Principles

Heat recovery units used in ventilation and air condition systems are based on some common principles:

  • Return air
  • Rotating heat exchangers
  • Air-Fluid-Air exchangers
  • Cross flow exchangers
  • Heat pumps

Cross flow and rotating heat exchangers are illustrated below:

heat recovery efficiency

Return Air Recovery Units

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.

Rotating Heat Exchangers

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.

Air-Fluid-Air Exchangers

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.

Cross Flow Heat Exchangers

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.

Heat Pumps

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.

Temperature Transfer Efficiency

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 outside air before the heat exchanger (oC)

t2 = temperature outside air after the heat exchanger (oC)

t3 = temperature inside air before the heat exchanger (oC)

Moisture Transfer Efficiency

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 outside air before the heat exchanger (kg/kg)

x2 = moisture outside air after the heat exchanger (kg/kg)

x3 = moisture inside air before the heat exchanger (kg/kg)

Enthalpy Transfer efficiency

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 outside air before the heat exchanger (kg/kg)

h2 = enthalpy outside air after the heat exchanger (kg/kg)

h3 = enthalpy inside air before the heat exchanger (kg/kg)

Related Mobile Apps from The Engineering ToolBox Engineering Toolbox Apps

- free apps for offline use on mobile devices.

Heat Exchanger Efficiency Calculator

The calculator below can be used to calculate the temperature, moisture or enthalpy efficiency of an heat exchanger.

1 - outside air before the heat exchanger (temperature, moisture or enthalpy)

2 - outside air after the heat exchanger (temperature, moisture or enthalpy)

3 - inside air before the heat exchanger (temperature, moisture or enthalpy)

air flow through heat exchanger (optional) (m3/h - same flow on both sides)

Related Topics

  • Ventilation - Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more

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