Several factors combine to make thermal recovery an important aspect of any HVAC system or retrofit: increased outside-air requirements and increased system efficiency and building efficiency standards, among other considerations. The outside-air requirements demand 20 percent or more fresh air depending on the occupancy (e.g., 15 ft3/min per person, typically) to be treated. Often the existing HVAC system doesn’t have the capacity to provide the additional cooling. This means that thermal energy must be recovered from exhausted air.
There are two types of energy-recovery units. The heat-pipe system will recover sensible heat, while the desiccant wheel recovers both sensible and latent energy. The latter is considerably more expensive, but is usually economical on large systems with a high rate of exhaust. The performance of a typical heat-pump unit is based on a heat- recovery factor that is the ratio of the supply- and exhaust-air temperatures:
RF = (Tsl – Tse)/(Tee – Tse)
RF = recovery factors
Tsl = supply air, leaving
Tse = supply air, entering
Tee = exhaust air, entering
The cost of this heat-transfer system is in the form of pressure drop across the coils: The exhaust fan must be capable of overcoming a higher static pressure, as must the supply fan side. This means extra energy will be spent to move the air. New systems can be easily sized to accommodate this change, while fans on existing systems can often be adjusted or provided with new sheaves.
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Excerpt from Energy Conservation Projects.