Heat Storage, Diversity and Stratification in HVAC - Better understanding of these factors in cooling load estimation

Heat Storage, Diversity and Stratification in HVAC

 The normal load estimating procedure has been to evaluate the instantaneous heat gain to a space and to assume that the equipment will remove the heat at this rate. Generally, it was found that the equipment selected on this basis was oversized and therefore capable of maintaining much lower room conditions than the original design. Extensive analysis, research and testing have shown that the reasons for this are:

1. Storage of heat in the building structure.

2. Non-simultaneous occurrence of the peak of individual loads.

3. Stratification of heat, in some cases.

The actual cooling load is generally considerably below the peak total instantaneous heat gain, thus requiring smaller equipment to perform a specific job. In addition, the air quantities and or water quantities are reduced, resulting in a smaller overall system. Also, if the equipment is operated somewhat longer during the peak load periods, and/or the temperature in the space is allowed to rise a few degrees at the peak periods during cooling operation, a further reduction in required capacity results. The smaller system operating for longer periods at times of peak load will produce a lower first cost to the customer with commensurate lower demand charges and lower operating costs. It is a well-known fact that the equipment sized to more nearly, meet the requirements result in a more efficient, better operating system. Also, if a smaller system is selected, and is based on extended periods of operation at the peak load, it results in a more economical and efficient system at a partially loaded condition.

        With multi-story, multi-room application, Generally, it is recommended that the full reduction from storage and diversity be taken on the overall refrigeration or building load, with some degree of conservatism on the air side or room loads. This degree should be determined by the engineer from project requirements and customer desires. A system so designed, full reduction on refrigeration load and less than full reduction on air side or room load, meets all of the flexibility requirements, except at time of peak load. In addition, such a system has a low owning and operating cost.

Storage of Heat in Building Structures:

                The instantaneous heat gain in a typical comfort application consists of sun, lights, people, transmission through walls, roof and glass, infiltration and ventilation air, and in some cases, machinery, appliances, electric calculating machines, etc. A large portion of this instantaneous heat gain is radiant heat which does not become an instantaneous load on the equipment, because it must strike a solid surface and be absorbed by this surface before becoming a load on the equipment. The breakdown on the various instantaneous heat gains into radiant heat and convected heat is approximately as shown in table below.

Diversity of Cooling Loads:

                Diversity of cooling load results from the probable non-occurrence of part of the cooling load on a design day. Diversity factors are applied to the refrigeration capacity in large air conditioning systems. These factors vary with location, type and size of the application, and are based entirely on the judgement of the engineer.

            Generally, diversity factors can be applied to people and light loads in large multi-story office, hotel or apartment buildings. The possibility of having all of the people present in the building and all of the lights operating at the time of peak load are slight. Normally, in large office buildings, some people will be away from the office on other business. Also, the lighting arrangement will frequently be such that the lights in the vacant offices will not be on. The size of the diversity factor depends on the size of the building and the engineer's judgement of the circumstances involved. The table shown below lists some typical diversity factors for Large Buildings, based on judgements and experience.

Stratification of Heat:

                    There are generally two situations where heat is stratified and will reduce the cooling load on the air conditioning equipment. They are:

1. Heat may be stratified in rooms with high ceilings where air is exhausted through the roof or ceiling.

2. Heat may be contained above suspended ceilings with recessed lighting or ceiling plenum return systems.

The first situation generally applies to industrial applications, churches, auditoriums, and the like. The second situation applies to applications such as office buildings, hotels and apartments. With both cases, the basic fact that hot air tends to rise makes it possible to stratify loads such as convection from the roof, convection from lights, and convection from the upper part of the walls. The convective portion of the roof load is about 25% (the rest is radiation), the light load is about 50% with fluorescent (20% with incandescent), and the wall transmission load is about 40%.

 In any room with a high ceiling, a large part of the convection load being released above the supply air stream will stratify at the ceiling or roof level. Some will be induced into the supply air stream. Normally, about 80% stratified and 20% induced in the supply air. If air is exhausted through the ceiling or roof, this convection load released above the supply air may be subtracted from the air conditioning load. This results in a large reduction in load if the air is to be exhausted. It is not normally practical to exhaust more air than necessary, as it must be made up by bringing outdoor air through the apparatus. This usually results in a larger increase in load than the reduction realized by exhausting air. Hot air stratifies at the ceiling even with no exhaust but rapidly builds up in temperature, and no reduction in load should be taken where air is not exhausted through the ceiling or roof.

Bypass air and Bypass Factor:

                The Air passes through cooling coil and gets cooled to designed temperature. But, some percentage of this air does not get cooled while passing through cooling coil and is simply supplied to the conditioned space. This partly air is known as Bypass Air and percentage is called as Bypass Factor. Bypass Factor of a coil depends on coil construction, tubes arrangement, fins per inch and coil's face velocity. 

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