CLTD/SCL/CLF Method

Ruday and Duran (1975) compared the TETD/TA and TFM. As part of this work, data obtained by using the TFM on a group of applications considered representative were then used to generate cooling load temperature differential (CLTD) data, for direct one-step calculation of cooling load from conduction heat gain through sunlit walls and roofs and conduction through glass exposure. Cooling load factors (CLF) for similar one-step calculation of solar through glass and for loads from internal sources were also developed. More recent research (McQuiston 1992) developed an improved factor for solar load through glass, the solar cooling load (SCL) factor, which allows additional influencing parameters to be considered for greater accuracy. CLTDs, SLFs, and CLFs all include the effect of (1) time lag in conductive heat gain through opaque exterior surfaces and (2) time delay by thermal storage in converting radiant heat gain to cooling load. This simplification allows cooling loads to be calculated manually; thus, when data are available and are appropriately used, the results are consistent with those from the TFM, thus making the method popular for instruction.

Application Experience

The CLTD and CLF tables published in previous editions of the Fundamental volume and in the original Cooling and Heating Load Calculation Manual (ASHRAE 1979) are normalized data, based on applications of the original TFM data presented in the 1972 Fundamentals volume. Subsequent studies investigating the effects of 1981 to 1985 RTF data indicated results generally less conservative than those computed with the 1972 data. More recent research, however, suggests otherwise (McQuiston 1992), and the revised values for 1993, including the new SCLs, are currently considered more realistic for design load purposes.

CLTD Data

The originally developed CLTD data were so voluminous that they were first limited to 13 representative flat roof assemblies (with and without ceilings, for 26 total cases) and 7 wall groups (into which 41 different wall assemblies can be categorized). Twenty-four hourly CLTD values were tabulated for each of the 26 roof cases and each of the 7 wall groups, broken down for walls into 8 primary orientations. Adjustments were then required for specific north latitude and month of calculation. Reliability of adjustments was reasonably consistent during summer months but became much less realistic for early and late hours during traditionally non-cooling load months.

Solar Heat Gain Data

Solar heat gain through glass required similar data compression to present a corresponding range of conditions. Tables of maximum solar heat gain factors (SHGF) were listed for every 4o of north latitude between 0o and 64o, for each month and by 16 compass directions and horizontal. Cooling load factors (CLF), decimal multipliers for SHGF data, were tabulated for unshaded glass in spaces having carpeted or uncarpeted floors and for inside-shaded glass with any room construction. Unshaded CLFs were presented for each of 24 hours by 8 compass directions plus horizontal, further categorized by light, medium, or heavy room construction. Inside-shaded CLFs disregarded construction mass but included 16 orientations plus horizontal. The product of the selected CLTD and CLF values represented cooling load per unit area as a single process. CLF values published in the Handbook were derived for the period May through September as normally the hottest months for load calculation purposes. As with CLTDs, the reliability of CLF data deteriorated rapidly for applications during early and late hours of months considered "noncooling load" periods.