Handbook of air conditioning and refrigeration / Shan K

Calculation Procedure
CLTD/SCL/CLF Method
TETD/TA Method
Weighting factors are transfer function coefficients presented in the form of z transform functions.
Weighting factors are so called because they are used to weight the importance of current and
historical values of heat gain and cooling load on currently calculated cooling loads.
The calculation of space cooling load using the transfer function method consists of two steps.
First, heat gains or heat loss from exterior walls, roofs, and floors is calculated using response
factors or conduction transfer function coefficients; and the solar and internal heat gains are calculated
directly for the scheduled hour. Second, room transfer function coefficients or room weighting
factors are used to convert the heat gains to cooling loads, or the heat losses to heating loads.
As described in Sec. 6.2, the sensible infiltration heat gain is the instantaneous sensible cooling
load. All latent heat gains are instantaneous latent cooling loads.
The TFM is limited because the cooling loads thus calculated depend on the value of transfer
function coefficients as well as the characteristics of the space and how they are varied from those
used to generate the transfer function coefficients. In addition, TFM assumed that the total cooling
load can be calculated by simply adding the individual components—the superposition principle.
However, this assumption can cause some errors.
The cooling load temperature difference (CLTD)/solar cooling load (SCL)/cooling load factor
(CLF) method first calculates the sensible cooling load based on the TFM. The result is divided by
the U value, shading coefficient, or sensible heat gain to generate the CLTD, SCL, or CLF. Thus, it
provides a direct, one-step space cooling load calculation instead of a heat gain–cooling load conversion,
a two-step calculation in TFM. Cooling load calculation using the CLTD/SCL/CLF
method can be either computer-aided or performed manually for a check or rough estimate. The
CLTD/SCL/CLF method is one of the members of the TFM family.
In the CLTD/SCL/CLF method, the CLTD is used to calculate the sensible cooling load for the
exterior wall and roofs. Recently, an SCL factor has been added which represents the product of the
solar heat gain at that hour and the fraction of heat storage effect due to various types of room
construction and floor coverings. CLF is used to calculate internal sensible cooling loads.
The limitations of the TFM are also carried through to the CLTD/SCL/CLF results. Furthermore,
the grouping of CLTD/SCL/CLF may cause additional errors.
In the total eqivalent temperature difference (TETD)/time-averaging (TA) method, heat gains of a
number of representative exterior wall and roof assemblies q w, Btu/h (W), are calculated as
q w � AU(TETD) (6.9b)
where A � area of wall or roof, ft 2 (m 2 )
U � overall heat-transfer coefficient of wall or roof, Btu/h�ft 2 �°F (W/m 2 �°C)
In Eq. (6.9b), TETD, in °F (°C), can be evaluated by:
LOAD CALCULATIONS 6.15
● Using the conduction transfer function as in TFM to determine q w. Then it is divided by the U
value to generate TETD values.
● Using the following relationship:
TETD � T sol, a � T r � �(T sol,� � T sol, a) (6.9c)