1. When design a constant air volume (CAV) HVAC system for a commercial office building, the

following design conditions were considered:

The design ambient conditions are 92 F dry bulb and 78 F wet bulb. The building can be treated

as a single zone with a total zone load of 500,000 Btu/hr and a sensible heat ratio of 0.8. The

design occupancy is 80 people and the corresponding ventilation air flow rate is 1600 cfm, which

is based on 20 cfm of outdoor air per person. The zone temperature thermostat setting is 77 F.

which is within the summer comfort zone. The bypass factor for the coil is 0.175 and the chilled

water temperature is 50 F. Reheat is zero at design condition.

(1) Determine circulation and ventilation airflow rates, and thermodynamic states at design

conditions using iterative methods.

a. Guess the unknown values needed in the calculations shown in page 3. For example

guess 47% relative humidity in the zone and estimate the circulation air flow rate by

using the saturated air enthalpy at 50F for supply air with the following equation:

LT-ms (hz - hs)

b.

Follow the psychrometric processes (page 3) to calculate the thermodynamic states and

circulation air flow rate.

c.

Use the new zone state properties and circulation flow rate to repeat the calculations in b.

Until further the results are converged, i.e. the changes in two consecutive calculation

results are small. Take the results in the final iteration as the solutions. Record the results

in Table 1./nc. Use the new zone state properties and circulation flow rate to repeat the calculations in b.

Until further the results are converged, i.e. the changes in two consecutive calculation

results are small. Take the results in the final iteration as the solutions. Record the results

in Table 1.

(2) Calculate the zone relative humidity, coil load, and the circulation flow rate in cfm after

finding the converged solutions in (1). List the results in Table 2.

Iteration

Ambient (A)

Entering (E)

Coil exit (C)

Supply (S)

Water (w)

x-state (x)

Zone (Z)

Table 1 Properties of thermodynamics states of two iterations

Temperature

(F)

Humidity Ratio

(1bmw/1bma)

92

50

77

77

Relative

Humidity

1

1

2

Enthalpy (Btu/lbm)

1

2