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Heat Recovery

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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 air before the heat recovery unit ( o F)

t 2 = air temperature of outside air after the 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 air before the heat recovery unit (Btu/lb)

h 2 = air enthalpy of outside air after the heat recovery unit (Btu/lb)

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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.


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