Cooling load temperature difference calculation method: Difference between revisions

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==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) <ref name=SML>Spitler, J.D., F.C. McQuiston, K. Lindsey. 1993. The CLTD/SCL/CLF Cooling Load

Calculation Method, ASHRAE Transactions. 99(1): 183-192.</ref> 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. <ref name=MS>McQuiston, F.C., and J.D. Spitler. 1992. Cooling and heating load calculation manual. Atlanta: ASHRAE</ref> Error when using the CLTD/CLF/SCL method tends to be less than twenty percent over and less than ten percent under. <ref name=SML/>

==History==

After its introduction in the 1979 [[ASHRAE]] handbook, research continued on increasing the accuracy of the CLTD/CLF Method. Research completed in 1984 revealed some factors which were not accounted for in the original publication of the method; these findings were a result of the [[ASHRAE]] Research project 359. In 1988 [[ASHRAE]] Research Project 472 worked to correct these oversights with the introduction of a classification system for walls, roofs, and zones. Additionally, a weighting factor database was generated to help correct for previous inaccuracies. Additional research in [[Thermal radiation]] and appliance heat gain with respect to CLTD data was also completed shortly after the original publication of the method. The advancements in each of these areas inspired a revision/compilation effort, and in 1993 the CLTD/CLF/SCL Method was succinctly compiled by Spitler, McQuiston, and Lindsey. <ref name=SML/>

==Application==

The CLTD/CLF/SCL Method utilizes pre-determined set of data to expedite and simplify the process of cooling/heating load approximation. The data is divided into many different sections based on many different variables. These variables include, building material of the envelope, thicknesses of the building materials, day of the year, time of day, orientation of the surface (e.g. wall or roof, 90 degrees or 180), and wall face orientation (cardinal directions, i.e. N, NW, S, SE, etc), to name a few. In order to determine which set of CLTD/CLF/SCL data to look at, all the requisite variables must be defined. <ref name=SML/>

The respective tables of data were generally developed by using the more complex [[Transfer Function Method]] to determine the various cooling loads for different types of heating. <ref name=MS/> <ref name=Matalas> Matalas, Gintas P. 1972. Transfer Function Method of Calculating Cooling Loads, Heat Extraction And Space Temperature, ASHRAE Journal. Vol. 14, No. 12: 54-56.</ref> The results gained by doing so are then normalized for each type of heat gain used for the tables, CLTD, CLF, and SCL. <ref name=Lindsey> Lindsey, K. 1991. Revision of the CLTD/CLF Cooling Load Calculation Method. M.S. thesis, Oklahoma State University.</ref>

==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]].<ref name=SML/><ref name=Textbook> 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.</ref> <ref name=SML/>

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. <ref name=TextbookSML/> <ref name=SMLTextbook/>

The SC, or [[Shading coefficient]], is used widely in the evaluation of heat gain through glass and windows. <ref name=TextbookSML/> <ref name=SMLTextbook/>

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. <ref name=TextbookSML/> <ref name=SMLTextbook/>

==Equations==

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SC= [[shading coefficient]]

===For heat gain through walls, doors, roofs, and windows (only window conduction):===

Q = U*A*CLTD <ref name=TextbookSML/> <ref name=SMLTextbook/>

===For heat gains due to people, equipment (hooded and unhooded), and lighting:===

Q = Q*CLF <ref name=TextbookSML/> <ref name=SMLTextbook/>

===For solar heat gains through windows and glazed surfaces:===

Q = A*SC*SCL <ref name=TextbookSML/> <ref name=SMLTextbook/>

===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. <ref name=SML/>

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. <ref name=SML/>

==References==

{{reflist}}

[[Category:Heating, ventilating, and air conditioning]]