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Better Computers Paved The Way For Today's Calculation Methods.
HONOLULU, HI - In commercial, residential, industrial, and institutional HVAC design, it is critical for design contractors and engineers to perform a load calculation. Not only is it important for occupant comfort, it is also needed in order to avoid indoor air quality (IAQ) problems, which can lead to costly, even ruinous litigation.
The importance of running a load calculation is not a new concept - but the load calculations are. And even the calculations are not entirely new. They are based on older load calculation methods, which we can now compute much more quickly than when they were first devised.
Lynn Bellenger, PE. (Pathfinder Engineers, Pittsford, NY), opened the ASHRAE seminar "Review of Load Calculations Procedure" with an overview: "How Did We Get Here! It is probably the best way to understand and appreciate the current load calculation methods.
FROM THERE TO HERE
Bellenger described the evolution of the current load calculation methodology as "a continuing process" that started in 1967.
It began with Total Equivalent Temperature Difference/Time Average (TETD/TA) methods, which Bellenger said proceeded in logical steps and were easy to understand. The equations were "time consuming but clear," she said. And the results validated field results.
Their disadvantages: They were "repetitive, time-consuming equations," Bellenger said. And although their mathematical results were validated in the field, they were still "really an approximation."
Then came the Transfer Function Method (TFM), which factors together heat gain by conduction through exterior walls and roofs; conversion of the cooling load from heat gain; and use of room transfer functions. The method had the benefit of being flexible for load variations, Bellenger said. However, "the calculations were formidable," and the computers available in 1972 were too costly for most engineering firms to purchase and maintain.
A few years later, the Cooling Load Temperature Difference/ Cooling Load Factor (CLTD/CLF) method was devised, offering greater simplicity due to its singlestep calculation, tabular data, and manual method.
"It was simple to use and easy to teach," Bellenger says, but it didn't give engineers a feel for the cooling load process. Moreover, it had a limited range of application. In 1984, caveats were announced that stated limitations to the manual method...