# Heat Recovery

A heat-recovery system captures building exhaust air and reuse some of the energy to precondition the make-up air before supplying it to the building.

Air-to-air heat-recovery systems for comfort applications can in general be categorized as

- sensible heat (dry bulb only) systems
- total heat (wet bulb - sensible heat plus latent heat) systems

### Sensible Heat-Recovery Systems

A sensible heat-recovery systems will transfer sensible (dry-bulb) heat from the exhaust air to the supply air entering the building. Rotary wheel heat exchangers and cross flow heat exchangers are some of the common designs used in typical comfort-to-comfort sensible heat-recovery applications.

The amount of heat transferred can be expressed as

H = 1.08 q (t_{ 2 }- t_{ 1 }) η(1)

where

H =heat transferred (Btu/h)

q =quantity of airflow (cfm)t_{ 1 }= air temperature of the exhaust airbeforethe heat recovery unit (^{ o }F)

t_{ 2 }= air temperature of outside airafterthe heat recovery unit (^{ o }F)

η= heat-recover efficiency

### Example - A Sensible Heat Recovery System

The outside air temperature is * 90ºF * and the exhaust air temperature is * 75ºF * . The system operates at * 40,000 cfm * at * 73 percent * efficiency.

The sensible heat-recovery can be expressed as

H =1.08 (40000 cfm) ((90^{ o }F) - (75^{ o }F)) 0.73

= 473040 (Btu/h)

### A Sensible Heat Recovery Calculator

The calculator below can be used to calculate the sensible heat recovery where the air flow, outside and inside temperature and recovery efficiency are known.

### Total Enthalpy Heat Recovery System

For total enthalpy heat-recovery systems the heat recovered can be expressed as

H = 4.5 q (h_{ 2 }- h_{ 1 }) η(2)

where

4.5 = (0.075 lb_{ m }/ft^{ 3 }) (60 min/h)

h_{ 1 }= air enthalpy of the exhaust airbeforethe heat recovery unit (Btu/lb)

h_{ 2 }= air enthalpy of outside airafterthe heat recovery unit (Btu/lb)

### Related Mobile Apps from The Engineering ToolBox

- free apps for offline use on mobile devices.

### Example - Total Enthalpy Heat Recovery System

The outside air wet-bulb temperature is * 70ºF * ( enthalpy * 34 Btu/lb * ) and the exhaust-air wet-bulb temperature is * 60ºF * ( enthalpy * 26.4 Btu/lb * ). The system operates at * 40,000 cfm * at * 73 percent * efficiency.

The total heat-recovery can be expressed like

H =4.5 (40000 cfm) ((34 Btu/lb) - (26.4 Btu/lb)) 0. 73

= 998640 (Btu/h)

### A Total Heat Recovery Calculator

The calculator below can be used to calculate the total (latent and sensible) heat recovery where the air flow, enthalpy of outside and inside air and the recovery efficiency are known.

## Related Topics

### • Ventilation

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

## Related Documents

### Air Heating Systems

Air heating buildings - heat supply vs. air flow and temperature.

### Heat Recovery Efficiency

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

### Humid Air - Heating

Enthalpy change and temperature rise when heating humid air without adding moisture.

### Moist Air - Enthalpy

Sensible and latent heat of moist air.

### Power Plants - Performance Efficiencies

Power plants heat rates, thermal efficiencies, capacity factors, load factors, economic efficiencies, operational efficiencies and energy efficiencies.