Handbook of air conditioning and refrigeration / Shan K

Load Profile
Peak Load and Block Load
LOAD CALCULATIONS 6.9
A load profile shows the variation of space load within a certain time period, such as a 24-h operating
cycle or an annual operating cycle, as shown in Fig. 6.5. In a load profile, the space load is
always plotted against time. For a space cooling load, the magnitude of the curve is positive; for a
space heating load, the magnitude is negative. A load profile may be used to illustrate the load variation
of an air conditioned space—a room, a zone, a floor, a building, or a project. The shape of a
load profile depends on the outdoor climate and, therefore, the latitude, orientation, and structure of
the building. It is also affected by the operating characteristics and the variation of the internal
loads.
The load duration curve is the plot of number of hours versus the load ratio. The load ratio is
defined as the ratio of cooling or heating load to the design full load, both in Btu/h, over a certain
period. The period may be a day, a week, a month, or a year. The load duration curve is helpful in
many operating and energy consumption analyses.
The zone peak load is the maximum space cooling load in a load profile of a control zone of the
same orientation and similar internal load characteristics calculated according to the daily outdoor
dry-bulb and coincident wet-bulb temperature curves containing summer or winter outdoor design
conditions. For a zone cooling load with several components, such as solar load through window
glass, heat transfer through roofs, or internal load from electric lights, the zone peak load is always
FIGURE 6.5 Load profile, peak load, and block load.