COMPONENTS OF COOLING LOAD IN HVAC
The total building cooling load consists of heat transferred through the building envelope (walls, roof, floor, windows, doors etc.) and heat generated by occupants, equipment, and lights. The load due to heat transfer through the envelope is called as external load, while all other loads are called as internal loads. The percentage of external versus internal load varies with building type, site climate, and building design. The total cooling load in HVAC on any building consists of both sensible as well as latent load components. The sensible load affects the dry bulb temperature, while the latent load affects the moisture content of the conditioned space.
Buildings may be classified as externally loaded and internally loaded. In externally loaded buildings the cooling load on the building is mainly due to heat transfer between the surroundings and the internal conditioned space. Since the surrounding conditions are highly variable in any given day, the cooling load of an externally loaded building varies widely. In internally loaded buildings the cooling load is mainly due to internal heat generating sources such as occupants, lights or appliances. In general the heat generation due to internal heat sources may remain fairly constant, and since the heat transfer from the variable surroundings is much less compared to the internal heat sources, the cooling load of an internally loaded building remains fairly constant. Obviously from energy efficiency and economics points of view, the system design strategy for an externally loaded building should be different from an internally loaded building. Hence, prior knowledge of whether the building is externally loaded or internally loaded is essential for effective system design.
COOLING LOAD CALCULATION METHOD
For a thorough calculation of the zones and whole-building loads, one of the following three methods should be employed:
- Transfer Function Method (TFM): This is the most complex of the methods proposed by ASHRAE and requires the use of a computer program or advanced spreadsheet.
- Cooling Load Temperature Differential/Cooling Load Factors (CLTD/CLF): This method is derived from the TFM method and uses tabulated data to simplify the calculation process. The method can be fairly easily transferred into simple spreadsheet programs but has some limitations due to the use of tabulated data.
- Total Equivalent Temperature Differential/Time-Averaging (TETD/TA): This was the preferred method for hand or simple spreadsheet calculation before the introduction of the CLTD/CLF method.
These three methods are well documented in ASHRAE Handbook Fundamentals, 2001.
ACCURACY AND RELIABILITY OF VARIOUS CALCULATION METHODS
For each cooling load calculation method, there are several benefits/limitations which feature each method. Simplicity and accuracy are two contradicting objectives to be fulfilled. If a method could be considered to be simple, its accuracy would be a matter of question, and vice versa.
While modern methods emphasize on improving the procedure of calculating solar and conduction heat gains, there are also other main sources coming from internal heat gains (people, lighting and equipment).
Handbooks include tables for the heat gain estimations from the internal sources. However, such tables are incomplete. For example, for equipment not mentioned in the tables, only limited information is indicated about them. Sometimes recommendations are mentioned about using 25% to 50% of the nameplate power consumption, where the final value is left to the interpretation of the designer. In other times it is the accurate predictability of the occurrence is also important, e.g. the frequency of using of equipment is very important to determine the heat gain. This example for internal heat gain shows that, when thinking about accuracy, it is not only the method (simple vs. complex) which is effective, but uncertainties in the input data are also important.
There are high degrees of uncertainty in input data required to determine cooling loads. Much of this is due to the unpredictability of occupancy, human behavior, outdoors weather variations, lack of and variation in heat gain data for modern equipments, and introduction of new building products and HVAC equipments with unknown characteristics. These generate uncertainties that far exceed the errors generated by simple methods compared to more complex methods. Therefore, the added time/effort required for the more complex calculation methods would not be productive in terms of better accuracy of the results if uncertainties in the input data are high. Otherwise, simplified methods would, likely, have a similar level of satisfactory accuracy.
For strictly manual cooling load calculation method, the most practical to use is the CLTD/SCL/CLF method as described in the 1997 ASHRAE Fundamentals. This method, although not optimum, will yield the most conservative results based on peak load values to be used in sizing equipment. It should be noted that the results obtained from using the CLTD/CLF method depend largely on the characteristics of the space being considered and how they vary from the model used to generate the CLTD/CLF data shown on the various tables. Engineering judgment is required in the interpretation of the custom tables and applying appropriate correction factors.