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HVAC Fundamentals: Psychrometrics
Figure 4.8
Two types of heat recovery units.
CHAPTER 4 PROBLEMS
4.1 A sling psychrometer measures the air temperatures
to be 85°F dry bulb and 72°F wet bulb. Find:
relative humidity, dew-point temperature, humidity
ratio (in lbmv/lbma and grains), specific volume,
and enthalpy. Show results on a chart and verify
with the program PsychProcess.xls (on the accompanying
CD). Assume sea level elevation.
4.2 Air flowing at 4000 cfm is heated from 70°F
(RH = 40%) at rate of 95,000 Btu/h. Find the outlet
air conditions (db, RH, wb, υ). Sketch the process
on a psychrometric chart.
4.3 Outside air (100°F/75°F) flowing at 1000 cfm is
mixed with return air (75°F/63°F) at 5000 cfm.
Find the mixed air conditions (db, RH, wb, υ, and
h). Sketch on the psychrometric chart.
4.4 A gas furnace produces 60,000 Btu/h with an airflow
of 1400 cfm heated air with an inlet condition
of 65°F (RH = 45%). Find the outlet air conditions
(db, RH, wb, υ). Sketch the process on a psychrometric
chart.
4.5 Outside air (95°F/75°F) flowing at 2500 cfm is
mixed with return air (75°F/63°F) at 7500 cfm.
Find the mixed air conditions (db, RH, wb, υ, and
h). Sketch on the psychrometric chart.
4.6 A quantity of 1600 cfm of air at 80°F/67°F enters
an evaporator coil with a 0.12 bypass factor and a
45°F apparatus dew point. Find the outlet air conditions
(db, wb, RH, h), the sensible cooling capacity,
the latent cooling capacity, total cooling
capacity, and the SHR of the coil. Sketch on the
psychrometric chart.
4.7 A 500 cfm outdoor air heat recovery unit (HRU)
has a total effectiveness of 75% (both sensible and
latent are equal). If the exhaust and makeup airflow
rates are equal, find the conditions of the air (db,
wb, h) leaving the HRU and entering the room
when outdoor conditions are 94°F/77°F and the
Figure 4.9 EPACT efficiency for 1 to 100 hp, fourpole
motors (NEMA 1997).
room air entering the HRU is 75°F/63°F. What is
the capacity of this unit?
4.8 A sensible heat recovery unit (HRU) with 80% efficiency
draws in 1000 cfm of outside air at –10°F
and exhausts an equal amount of room air at 70°F.
Calculate the air temperature leaving the HRU and
entering the room. What is the capacity of this unit?
What is the capacity for 40°F outside air? Calculate
the EER (= capacity in Btu/h ÷ power input in W)
for both conditions if two fans that draw 700 W
each are used.
4.9 A quantity of 2500 cfm of air at 82°F/70°F enters
an evaporator coil with a 0.08 bypass factor and a
45°F apparatus dew point. Find the outlet air conditions
(db, wb, RH, h), the sensible cooling capacity,
the latent cooling capacity, total cooling
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