Handbook of air conditioning and refrigeration / Shan K

6.34 CHAPTER SIX
thickness of the duct insulation layer, air volume flow rate, and the size and shape of the duct.
Detailed calculations are presented in Secs. 17.4 and 20.16. A rough estimate of the temperature increase
of the supply air for an insulated duct is as follows:
Temperature of Plenum Air and Ventilation Load
Supply air velocity Air temperature rise
� 2000 fpm (10 m/s) 1°F/100 ft (0.6°C/30 m) duct length
� 2000 fpm (10 m/s) 0.75°F/100 ft (0.45°C/30 m) duct length
For a ceiling plenum using a return plenum, the plenum air temperature can be calculated from
Eq. (6.22). The temperature increase of the plenum air, caused by the heat released from the electric
lights (Tp � Tr), is affected by their power input, type of lighting fixture, return air volume flow
rate, and construction of the ceiling plenum. The temperature increase of plenum air Tp � Tr is usually
between 1 and 3°F (0.6 and 1.7°C).
From Eqs. (6.26) and (6.27), the sensible and latent loads Qo, s and Qo, l, Btu/h (W), which are
attributable to the outdoor air intake, can be similarly calculated, except in Eqs. (6.26) and
(6.27) should be replaced by the volume flow rate of outdoor air , cfm (m 3 /min).
System heat gains are mainly due to convective heat transfer from the surfaces. For simplification,
they are considered instantaneous cooling coil loads.
6.9 COOLING LOAD CALCULATION BY FINITE
DIFFERENCE METHOD
Finite Difference Method
When both heat and moisture transfer from the surface of the walls, ceiling and carpet or floors
should be considered in the space cooling load calculation during the cool-down period in summer
in a location where the outdoor climate is hot and humid, the finite difference method might be the
best choice.
Because of the rapid increase in the use of microcomputers in the HVAC&R calculations, it is now
possible to use a finite difference method, a numerical approach, to solve transient simultaneous
heat- and moisture-transfer problems in heating and cooling load calculations and energy
estimations. The finite difference method divides the building structures into a number of sections.
A fictitious node i is located at the center of each section or on the surface, as shown in Fig. 6.8. An
energy balance or a mass balance at each node at selected time intervals results in a set of algebraic
equations that can be employed to determine the temperature and moisture for each node in terms
of neighboring nodal temperatures or moisture contents, nodal geometry, and the thermal and moisture
properties of the building structure. The stored heat energy and moisture are expressed as an
increase of internal energy and moisture content at the nodes.
Heat conduction can be approximated by using the finite difference form of the Fourier law, as
q i�1:i � kA i(T t i�1 � T t 1)
�x
where k � thermal conductivity, Btu/ h�ft�°F (W/m�°C)
A i � area of building structure perpendicular to direction of heat flow, ft 2 (m 2 )
V˙ o
V˙ inf
(6.50)