In operation, the cooling or heating loads are only a part of the design loads. HVAC system needs to respond to this lower demand, and some of the strategies are:
• CAV-RH (Constant Air Volume)
• VAV-RH (Variable Air Volume)
• Face and bypass coil
• Variable T for the heat exchangers
Analyze these processes by the same methods as design conditions.
Problems for cooling systems: thermostat controls TDb, and therefore humidity correct only at design (in general). As a result, space humidity varies with loads, and may need to iterate to space conditions. This is not a problem for heating conditions.
• same flow rate and W as design conditions, variable TDb
• good control at part loads
• lower flow rate in proportion to the sensible loads
• same coil dewpoint temperature => less dehumidification
Face and bypass coil
• bypass fraction is proportional to sensible loads
• no dehumidification of bypassed air => supply humidity is proportional to bypass fract.
Face and Bypass Coil
• used in spring or fall
• supplies outdoor air without operating a cooling coil; potential humidity problems
• limit is 100% outdoor air; control humidity rise with reheat
C: RA Economizer is off => lost cooling opportunity D: RA Economizer is on => energy penalty results
No reliable, durable and inexpensive enthalpy sensor for the enthalpy economizer.