Handbook of air conditioning and refrigeration / Shan K

6.16 CHAPTER SIX
where T sol, a � daily average sol-air temperature, °F (°C)
T sol,� � sol-air temperature at time lag � h, °F (°C)
� � effective decrement factor �
The internal heat gains and conductive heat gain are calculated in the same manner as in the TFM.
The radiant fraction of each of the sensible heat gains is then allocated to a period including the current
and successive hours, a total of 1 to 3 h for light construction and 6 to 8 h for heavy construction.
The TETD/ TA method is also a member of the TFM family and is developed primarily for
manual calculation. TETD/TA is simpler in the conversion of heat gains to cooling loads. However,
the time-averaging calculation procedure is subjective—it is more an art than a rigorous scientific
method. Also the TETD/TA method inherits the limitations that a TFM possesses if the TFM is
used to calculate the TETD.
6.6 DETAILED CALCULATION PROCEDURES FOR TFM
Most of the widely adopted computer software programs for space load calculations are based on
the transfer function method, i.e., the weighting factors method. The following are the detailed
calculation procedures for TFM.
Conduction Heat Gain through Exterior Walls and Roofs
The sensible heat gain through an exterior wall or a roof using TFM can be calculated by the conduction
transfer function. It uses a sol-air temperature at time t, denoted by T sol,t, to represent the combined
temperature excitation of the outdoor temperature and solar heat on the exterior surface of an exterior
wall or roof, and a constant indoor temperature T r. Conduction-transfer function coefficients are calculated
based on an outdoor surface heat-transfer coefficient h o � 3.0 Btu/h�ft 2 �°F (17 W/m 2 �°C), and
an indoor surface heat-transfer coefficient h i � 1.46 Btu/h�ft 2 �°F (8.3 W/m 2 �°C).
The external heat gain through an exterior wall or roof at time t, denoted by q e,t, Btu/h (W), can
be calculated as
q e,t ���
n�0
bnTsol,t – n� � � dn qe,t – n�
A � Tr � cn� A
where t � time, h
��time interval, h
n � summation index of number of terms
T sol,t–n� � sol-air temperature at time t�n�, °F (°C)
q e,t – n� � conduction heat gain at time t�n�, Btu/h (W)
b n,c n,d n � conduction-transfer function coefficients; refer to ASHRAE Handbook for details
A � interior surface area of wall or roof, ft 2 (m 2 )
(6.10)
Harris and McQuiston (1988) provide the conduction-transfer function coefficients for 41 representive
wall assemblies and 42 roof assemblies with variations in components, insulation, and mass.
Heat Gain through Ceilings, Floors, and Interior Partition Walls
n�1
If the temperature of the adjacent space T ad, °F (°C), is constant, or the variation of T ad is small, the
sensible heat gain due to the heat transfer from the adjacent space through the ceiling, floor, or interior
partition wall at time t, denoted by q p,t, Btu/h (W), can be calculated as
n�0
q p,t � UA(T ad � T r) (6.11)