Handbook of air conditioning and refrigeration / Shan K

To save energy, most AHU and PU manufacturers also offer air economizers for single-zone
constant-volume systems when the supply volume flow rate of the air system is 2000 cfm (1000
L/s) and greater. Most single-zone constant-volume systems employ zone temperature control,
night setback, diagnostics, emergency stop, and safety controls. Air economizer and air system controls
are discussed in the chapters that follow.
Energy per Unit Volume Flow
As specified in ASHRAE/IESNA Standard 90.1-1999 and discussed in Sec. 17.2, the fan power
input to the fan motors per unit volume flow Psy / V˙ sd in hp/cfm (W·s/L), for constant-volume air
systems as calculated by Eq. (17.24a):
(20.29)
For variable-air-volume (VAV) systems, the fan power input per unit volume flow can be calculated
by Eq. (17.24b) as:
20.11 AIR CONDITIONING PROCESSES
Sensible Heat Ratio
P sy
V˙ sd
P sy
V˙ sd
P sy
V˙ sd
P sy
V˙ sd
AIR SYSTEMS: BASICS AND CONSTANT-VOLUME SYSTEMS 20.41
� 0.0012 hp / cfm (0.0019 W�s / L) when V˙ sd � 20,000 cfm (9440 L / s)
� 0.0011 hp / cfm (0.00174 W�s / L) when V˙ sd � 20,000 cfm
� 0.0017 hp / cfm (0.0027 W�s / L) when V˙ sd � 20,000 cfm
� 0.0015 hp / cfm (0.0024 W�s / L) when V˙ sd � 20,000 cfm
An air conditioning process determines the change in thermodynamic properties of moist air between
the initial and final states of conditioning and also the corresponding energy and mass transfer
between the moist air and a medium, such as water, refrigerant, or moist air itself during this
change. The energy balance and the conservation of mass in nonnuclear processes are the two principles
most often used in the analysis and calculation of the change of thermodynamic properties in
air conditioning processes.
In general, for a single air conditioning process, heat transfer or mass transfer is positive. However,
for calculations that involve several air conditioning processes, the heat supplied to the moist
air is taken to be positive, and the heat rejected from the moist air is taken to be negative.
The sensible heat ratio (SHR) of an air conditioning process is defined as the ratio of the absolute
value of sensible heat to the absolute value of total heat, or
SHR �
In Eq. (20.30), total heat, in Btu/h (W), is given as
� qsen �
� qtotal � �
� qsen �
� qsen � � � ql �
q total � � q sen � � � q l �
(20.30)