A simple air-handling unit (AHU) contains dampers that allow mixing of outdoor air (ODA) and
return air (RA), a fan, and a cooling coil (CC). This piece of equipment, shown in Figure 1, is
commonly used to provide cooling to buildings; a heating coil may also be present, depending on
the system design. Figure 1 Air handling unit used for cooling of buildings This AHU has the
following characteristics and operating conditions for this problem.
•600 CFM of ODA at 80°F, 70°F WBT
•1200 CFM of RA at 65°F, 30% RH
•Supply is at 50°F,45°F WBT
•The fan has a small heat output, which we will neglect for this homework.
Find the following, using the psychrometric chart:
a. Mass flow rate, relative humidity, enthalpy, and humidity ratio of ODA.
b. Mass flow rate, wet bulb temperature, enthalpy, and humidity ratio of RA.
c. Mass flow rate, wet bulb temperature, enthalpy, and humidity ratio of MA (mixed air).
d. Mass flow rate, humidity ratio, enthalpy, and relative humidity of Supply air.
e. Sensible heat transfer rate of cooling coil I.e. sensible cooling provided, in Btu/hr.
f. Latent heat transfer rate of cooling coil. I.e. latent cooling provided, in Btu/hr. Mass
flow rate (lb/hr) of condensate.
g. Provide a clearly labeled psychrometric chart; label the points ODA, RA, MA, DA (for
discharge air); show the process lines for mixing and the cooling coil.
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To solve this problem, we will use the psychrometric chart to find the required properties.
a. For ODA:
Using the provided values of ODA:
- Dry bulb temperature: 80°F
- Wet bulb temperature: 70°F WBT
Using the psychrometric chart, we can find:
- Mass flow rate of ODA.
b. For RA:
Using the provided values of RA:
- Dry bulb temperature: 65°F
- Relative humidity: 30% RH
Using the psychrometric chart, we can find:
- Mass flow rate of RA.
- Wet bulb temperature of RA.
c. For MA:
Since the ODA and RA are being mixed in the AHU, we need to find the properties of the mixed air (MA).
- Mass flow rate of MA.
- Wet bulb temperature of MA.
d. For Supply air:
Using the provided values of Supply air:
- Dry bulb temperature: 50°F
- Wet bulb temperature: 45°F WBT
Using the psychrometric chart, we can find:
- Mass flow rate of Supply air.
- Humidity ratio of Supply air.
- Enthalpy of Supply air.
- Relative humidity of Supply air.
e. For Sensible heat transfer rate:
Using the properties of ODA, RA, and MA:
- Enthalpy change between MA and Supply air.
f. For Latent heat transfer rate and Condensate mass flow rate:
Using the properties of ODA, RA, MA, and Supply air:
- Enthalpy change between MA and Supply air.
- Latent heat transfer rate.
- Condensate mass flow rate.
g. To create the psychrometric chart, you can use software or websites that generate them based on the provided data points. Label the points ODA, RA, MA, and DA. Show the process lines for mixing and the cooling coil.
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