Trane OABE OADE OAKE OANE (Water Source Heat Pump) Katalog
OAU-PRC003A-EN
17
Application Considerations
Establishing Capacity Requirements
Determining the OAU capacity requirements requires careful thought. Light Commercial
equipment is typically selected based on design sensible conditions. Since latent loads drive the
need for the OAU, base the selection on design latent conditions.
equipment is typically selected based on design sensible conditions. Since latent loads drive the
need for the OAU, base the selection on design latent conditions.
Cooling & Dehumidification Selection Criteria
Evaporator Design Entering Conditions
For many climates the peak outdoor air enthalpy occurs at a time when the outdoor dry-bulb
temperature is not the highest. Refer to the chapter on climatic design information in the ASHRAE
Handbook of Fundamentals. The cooling and dehumidification design condition data is provided
three ways:
temperature is not the highest. Refer to the chapter on climatic design information in the ASHRAE
Handbook of Fundamentals. The cooling and dehumidification design condition data is provided
three ways:
1.
Design dry-bulb temperature with mean coincident wet bulb temperature
2. Design wet-bulb temperature with mean coincident dry-bulb temperature
3. Design dew point temperature with mean coincident dry-bulb temperature
The design wet-bulb condition typically represents a significantly higher outdoor air enthalpy than
the design dry-bulb condition. Use the condition that represents the highest enthalpy as the
entering evaporator selection condition.
the design dry-bulb condition. Use the condition that represents the highest enthalpy as the
entering evaporator selection condition.
Evaporator Design Leaving Conditions
Due to the uncertainty of the local terminal unit’s latent capacity at part load, it is usually most
straightforward to size the OAU to handle the entire latent load on the system, both indoor and
outdoor. With this design approach, the terminal units may do some latent cooling
(dehumidification) during periods of higher sensible load. At these times, the space will run slightly
drier than the design RH limit. This is why it makes sense to select the OAU to limit the space RH
to a maximum allowable level for those conditions when the terminal units are providing no space
latent cooling, such as 60 percent RH. Using lower humidity limits may result in an unnecessary
increase in system operating energy use.
straightforward to size the OAU to handle the entire latent load on the system, both indoor and
outdoor. With this design approach, the terminal units may do some latent cooling
(dehumidification) during periods of higher sensible load. At these times, the space will run slightly
drier than the design RH limit. This is why it makes sense to select the OAU to limit the space RH
to a maximum allowable level for those conditions when the terminal units are providing no space
latent cooling, such as 60 percent RH. Using lower humidity limits may result in an unnecessary
increase in system operating energy use.
, and
to identify the appropriate supply air dew point
for specific design conditions. For a more detailed discussion on determining the selection criteria
of a OAU, refer to SYS-APG001-EN (Application Guide: Designing Dedicated Outdoor-Air Systems)
or SYS-APM004-EN (Applications Engineering Manual: Dehumidification in HVAC Systems).
of a OAU, refer to SYS-APG001-EN (Application Guide: Designing Dedicated Outdoor-Air Systems)
or SYS-APM004-EN (Applications Engineering Manual: Dehumidification in HVAC Systems).
Figure 9.
Psychrometric chart with cooling and heating only
Dry Bulb Temperature
Dewpoint T
emperature
W
et Bulb T
emperature
t3
t4
heating
t1
t2
cooling