Handbook of air conditioning and refrigeration / Shan K

17.12 CHAPTER SEVENTEEN
where Tr, set � room set point temperature, °F (°C)
Ts,d � design supply air temperature for the zone in which the thermostat is located, °F(°C)
�pfil, �pHR � total pressure loss of the filters and of the heat recovery coils, in WC (Pa)
pr, f � name plate relief fan motor power, hp (kW)
� volume flow rate of relief fan at cooling design operation, cfm (L/s)
Consider a VAV system with a design supply volume flow rate V˙ s,d � 20,000 cfm (9440 L/s). If a
rooftop packaged system is used with a combined fan, drive, and motor efficiency �f,d�d,d�m,d �
0.45, from Eq. (17.25c), the allowable system pressure loss for this rooftop package system (including
supply and return fans) is
If the design system pressure loss �psy � 4.3 in. WC (1070 Pa), the HVAC&R system designer is
recommended either to use a relief fan instead of a return fan for relief fan credit, or to take into account
the pressure credit if filter’s total pressure loss is greater than 1 in. WC (250 Pa) or if there is
heat recovery coil, increase the supply temperature differential �Ts � (Tr, set � Ts) to a value greater
than 20°F (11.1°C) if they are cost effective to meet the fan power limitation in ASHRAE/IESNA
Standard 90.1–1999.
For a VAV reheat central system of V˙
s,d � 20,000 cfm (9440 L/s), if combined efficiency is 55
percent, from Eq. (17.25c), the allowable system pressure loss is
If the design system total pressure loss � 5.2 in. WC, the same as for the rooftop packaged system,
means of relief fan credit, pressure credit due to filters and heat recovery coils, and the increase of
the supply temperature differential greater than 20°F (11.1°C) should be considered. Refer to
ASHRAE Standard 90.1-1999 for details.
17.3 DUCT CONSTRUCTION
Maximum Pressure Difference
Material
V˙ r, f
�p sy � � f,d� d,d� m,d (9.5) � 0.45 � 9.5 � 4.3 in. WC (1070 Pa)
�p sy � � f,d� d,d� m,d (9.5) � 0.55 � 9.5 � 5.2 in. WC (1300 Pa)
Duct systems can be classified according to the maximum pressure difference between the air inside
the duct and the ambient air (also called the static pressure differential) as � 0.5 in. WC
(� 125 Pa), � 1 in. WC (� 250 Pa), � 2 in. WC (� 500 Pa), � 3 in. WC (� 750 Pa), � 4 in. WC
(� 1000 Pa), � 6 in. WC (� 1500 Pa), and � 10 in. WC (� 2500 Pa). In actual practice, the maximum
pressure difference of the supply or return duct system in commercial buildings is usually less
than � 3 in. WC (� 750 Pa).
In commercial buildings, a low-pressure duct system has a static pressure differential of 2 in.
WC (500 Pa) or less, and the maximum air velocity inside the air duct is usually 2400 fpm
(12 m/s). A medium-pressure duct system has a static pressure differential of 2 to 6 in. WC (500 to
1500 Pa) with a maximum air velocity of about 3500 fpm (17.5 m/s). In industrial duct systems, including
mechanical ventilation, mechanical exhaust, and industrial air pollution control systems,
the pressure difference is often higher. In residential buildings, the static pressure differential of the
duct systems is classified as � 0.5 in. WC (� 125 Pa) or � 1 in. WC (� 250 Pa).
Underwriters’ Laboratory (UL) classifies duct systems according to the flame spread and smoke developed
of the duct material during fire as follows: