Cooling load temperature difference calculation method
CLTD/CLF/SCL cooling load calculation method
The CLTD/CLF/SCL (cooling load temperature difference/cooling load factor/solar cooling load factor) cooling load calculation method was first introduced in the 1979 ASHRAE Cooling and Heating Load Manual (GRP-158) [1] The CLTD/CLF/SCL Method is regarded as a reasonably accurate approximation of the total heat gains through a building envelope for the purposes of sizing HVAC equipment. This method was developed as a simpler calculation alternative to difficult and unwieldy calculation methods such as the transfer function method and the Sol-air temperature method.[2] Error when using the CLTD/CLF/SCL method tends to be less than twenty percent over and less than ten percent under.[1]
History
After its introduction in the 1979 [[ASHRby Spitler, McQuiston, and Lindsey.[1]
Application
Explanation of variables
The first of the cooling load factors used in this method is the CLTD, or the Cooling Load Temperature Difference. This factor is used to represent the temperature difference between indoor and outdoor air with the inclusion of the heating effects of solar radiation.[1][3]
The second factor is the CLF, or the cooling load factor. This coefficient accounts for the time lag between the outdoor and indoor temperature peaks. Depending on the properties of the building envelope, a delay is present when observing the amount of heat being transferred inside from the outdoors. The CLF is the cooling load at a given time compared to the heat gain from earlier in the day.[1][3]
The SC, or shading coefficient, is used widely in the evaluation of heat gain through glass and windows.[1][3]
Finally, the SCL, or solar cooling load factor, accounts for the variables associated with solar heat load. These include the global coordinates of the site and the size of the structure.[1][3]
Equations
The equations for the use of the data retrieved from these tables are very simple.
Q= heat gain, usually heat gain per unit time
A= surface area
U= Overall heat transfer coefficient
CLTD= cooling load temperature difference
SCL= solar cooling load factor
CLF= cooling load factor
For heat gain through walls, doors, roofs, and windows (only window conduction)
For heat gains due to people, equipment (hooded and unhooded), and lighting
For solar heat gains through windows and glazed surfaces
Data tables
In addition to tables published by ASHRAE for select latitudes, a computer program called CLTDTAB, available since 1993, can be used to generate custom CLTD/CLF/SCL tables for a specific zone type for any latitude and month. This allows the use of this method, without interpolation, for any area in the world.[1]
If the program CLTDTAB is used, the results obtained using this method will tend to be very close to the more rigorous TFM Method mentioned earlier.[1]
References
- ^ a b c d e f g h i j k l Spitler, J.D., F.C. McQuiston, K. Lindsey. 1993. The CLTD/SCL/CLF Cooling Load Calculation Method, ASHRAE Transactions. 99(1): 183–192.
- ^ McQuiston, F.C., and J.D. Spitler. 1992. Cooling and heating load calculation manual. Atlanta: ASHRAE
- ^ a b c d e f g McQuiston, Faye C., Parker, Jerald D., Spitler, Jeffrey D. Heating, Ventilation, and Air Conditioning: Analysis and Design, p216-278. 2005, John Wiley and Sons, Inc.