Cooling Load Calculations and Principles in HVAC – Part 1

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The Cooling Load

The cooling load on refrigerating equipment seldom results from any one single source of heat. Rather, it is the summation of the heat usually evolves from several different sources.

Cooling load calculations may be used to accomplish one or more of the following objectives:

  • Provide information for equipment selection, system sizing and system design.
  • Provide data for evaluating the optimum possibilities for load reduction.
  • Permit analysis of partial loads as required for system design, operation and control.
Terminology

Commonly used terms relative to heat transmission and load calculations are defined below in accordance with ASHRAE Standard 12-75, Refrigeration Terms and Definitions.

Space – is either a volume or a site without a partition or a partitioned room or group of rooms.

Room – is an enclosed or partitioned space that is usually treated as single load.

Zone – is a space or group of spaces within a building with heating and/or cooling requirements sufficiently similar so that comfort conditions can be maintained throughout by a single controlling device.

British thermal unit (Btu) – is the approximate heat required to raise 1 lb. of water 1 deg Fahrenheit, from 59°F to 60°F. Air conditioners are rated by the number of British Thermal Units (Btu) of heat they can remove per hour. Another common rating term for air conditioning size is the “ton,” which is 12,000 Btu per hour and Watts. Some countries utilize one unit, more than the others and therefore it is good if you can remember the relationship between BTU/hr, Ton, and Watts.

  • ƒ 1 ton is equivalent to 12,000 BTU/hr. and
  • ƒ 12,000 BTU/hr is equivalent to 3,516 Watts – or 3.516 kW (kilo-Watts).

Cooling Load Temperature Difference (CLTD) – an equivalent temperature difference used for calculating the instantaneous external cooling load across a wall or roof.

Sensible Heat Gain – is the energy added to the space by conduction, convection and/or radiation.

Latent Heat Gain – is the energy added to the space when moisture is added to the space by means of vapor emitted by the occupants, generated by a process or through air infiltration from outside or adjacent areas.

Radiant Heat Gain – the rate at which heat absorbed is by the surfaces enclosing the space and the objects within the space.

Space Heat Gain – is the rate at which heat enters into and/or is generated within the conditioned space during a given time interval.

Space Cooling Load – is the rate at which energy must be removed from a space to maintain a constant space air temperature.

Space Heat Extraction Rate – the rate at which heat is removed from the conditioned space and is equal to the space cooling load if the room temperature remains constant.

Dry Bulb Temperature – is the temperature of air indicated by a regular thermometer.

Wet Bulb Temperature  is the temperature measured by a thermometer that has a bulb wrapped in wet cloth. The evaporation of water from the thermometer has a cooling effect, so the temperature indicated by the wet bulb thermometer is less than the temperature indicated by a dry-bulb (normal, unmodified) thermometer. The rate of evaporation from the wet-bulb thermometer depends on the humidity of the air. Evaporation is slower when the air is already full of water vapor. For this reason, the difference in the temperatures indicated by ordinary dry bulb and wet bulb thermometers gives a measure of atmospheric humidity.

Dew point Temperature – is the temperature to which air must be cooled in order to reach saturation or at which the condensation of water vapor in a space begins for a given state of humidity and pressure.

Relative humidity – describes how far the air is from saturation. It is a useful term for expressing the amount of water vapor when discussing the amount and rate of evaporation. One way to approach saturation, a relative humidity of 100%, is to cool the air. It is therefore useful to know how much the air needs to be cooled to reach saturation.

Thermal Transmittance or Heat Transfer Coefficient (U-factor) – is the rate of heat flow through a unit area of building envelope material or assembly, including its boundary films, per unit of temperature difference between the inside and outside air. The U-factor is expressed in Btu/ (hr °F ft²).

Thermal Resistance(R) – is the reciprocal of a heat transfer coefficient and is expressed in (hr °F ft²)/Btu. For example, a wall with a U-value of 0.25 would have a resistance value of R = 1/U = 1/0.25=4.0. The value of R is also used to represent Thermal Resistivity, the reciprocal of the thermal conductivity.

Sizing Of Air-Conditioning System

Concepts and fundamentals of air conditioner sizing is based on heat gain, and/or losses in a building. It is obvious that you will need to remove the amount of heat gain – if it is hot outside. Similarly, you’ll need to add in the heat loss from your space – if outside temperature is cold. In short, heat gain and loss, must be equally balanced by heat removal, and addition, to get the desired room comfort that we want.

The heat gain or heat loss through a building depends on:

  1. The temperature difference between outside temperature and our desired temperature.
  2. The type of construction and the amount of insulation is in your ceiling and walls. Let’s say, that you have two identical buildings, one is build out of glass, and the other out of brick. Of course the one built with glass would require much more heat addition, or removal, compared to the other – given a same day. This is because the glass has a high thermal conductivity (U-value) as compared to the brick and also because it is transparent, it allows direct transmission of solar heat.
  3. How much shade is on your building’s windows, walls, and roof? Two identical buildings with different orientation with respect to the direction of sun rise and fall will also influence the air conditioner sizing.
  4. How large is your room? The surface area of the walls. The larger the surface area – the more heat can lose, or gain through it.
  5. How much air leaks into indoor space from the outside? Infiltration plays a part in determining our air conditioner sizing. Door gaps, cracked windows, chimneys – are the “doorways” for air to enter from outside, into your living space.
  6. The occupants. It takes a lot to cool a town hall full of people.
  7. Activities and other equipment within a building. Cooking? Hot bath? Gymnasium?
  8. Amount of lighting in the room. High efficiency lighting fixtures generate less heat.
  9. How much heat the appliances generate. Number of power equipments such as oven, washing machine, computers, TV inside the space; all contribute to heat.

The air conditioner’s efficiency, performance, durability, and cost depend on matching its size to the above factors. Many designers use a simple square foot method for sizing the air-conditioners. The most common rule of thumb is to use “1 ton for every 500 square feet of floor area”. Such a method is useful in preliminary estimation of the equipment size. The main drawback of rules-of-thumb methods is the presumption that the building design will not make any difference. Thus the rules for a badly designed building are typically the same as for a good design.

It is important to use the correct procedure for estimating heat gain or heat loss. Two groups—the Air Conditioning Contractors of America (ACCA) and the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE)—publish calculation procedures for sizing central air conditioners.

Reputable air conditioning contractors will use one of these procedures, often performed with the aid of a computer, to size your new central air conditioner.

Heating Load V/s Cooling Load Calculations

As the name implies, heating load calculations are carried out to estimate the heat loss from the building in winter so as to arrive at required heating capacities. Normally during winter months the peak heating load occurs before sunrise and the outdoor conditions do not vary significantly throughout the winter season. In addition, internal heat sources such as occupants or appliances are beneficial as they compensate some of the heat losses. As a result, normally, the heat load calculations are carried out assuming steady state conditions (no solar radiation and steady outdoor conditions) and neglecting internal heat sources. This is a simple but conservative approach that leads to slight over estimation of the heating capacity. For more accurate estimation of heating loads, one has to take into account the thermal capacity of the walls and internal heat sources, which makes the problem more complicated.

For estimating cooling loads, one has to consider the unsteady state processes, as the peak cooling load occurs during the day time and the outside conditions also vary significantly throughout the day due to solar radiation. In addition, all internal sources add on to the cooling loads and neglecting them would lead to under estimation of the required cooling capacity and the possibility of not being able to maintain the required indoor conditions. Thus cooling load calculations are inherently more complicated.

In determining the heating load, credit for solar heat gain or internal heat gains is usually NOT included and the thermal storage effects of building structure are generally ignored. Whereas in cooling load calculations, the thermal storage characteristics of the building play a vital role because the time at which the space may realize the heat gain as a cooling load will be considerably offset from the time the heat started to flow.

Heat Flow Rates

In air-conditioning design, four related heat flow rates, each of which varies with time, must be differentiated:

  1. Space heat gain —————-How much heat (energy) is entering the space?
  2. Space cooling load ————-How much energy must be removed from the space to keep temperature and relative humidity constant?
  3. Space heat extraction———–How much energy is the HVAC removing from the space?
  4. Cooling load (coil)—————How much energy is removed by the cooling coil serving various spaces plus any loads external to the spaces such as duct heat gain, duct leakage, fan heat and outdoor makeup air?
Space Heat Gain

This instantaneous rate of heat gain is the rate at which heat enters into and/or is generated within a space at a given instant. Heat gain is classified by:

The manner in which it enters the space –

  1. Solar radiation through transparent surfaces such as windows
  2. Heat conduction through exterior walls and roofs
  3. Heat conduction through interior partitions, ceilings and floors
  4. Heat generated within the space by occupants, lights, appliances, equipment and processes
  5. Loads as a result of ventilation and infiltration of outdoor air
  6. Other miscellaneous heat gain

Whether it is a sensible or latent gain –

Sensible heat – Heat which a substance absorbs, and while its temperature goes up, the substance does not change state. Sensible heat gain is directly added to the conditioned space by conduction, convection, and/or radiation. Note that the sensible heat gain entering the conditioned space does not equal the sensible cooling load during the same time interval because of the stored heat in the building envelope. Only the convective heat becomes cooling load instantaneously. Sensible heat load is total of

  1. Heat transmitted thru floors, ceilings, walls
  2. Occupant’s body heat
  3. Appliance & Light heat
  4. Solar Heat gain thru glass
  5. Infiltration of outside air
  6. Air introduced by Ventilation

Latent Heat Loads – Latent heat gain occurs when moisture is added to the space either from internal sources (e.g. vapor emitted by occupants and equipment) or from outdoor air as a result of infiltration or ventilation to maintain proper indoor air quality. Latent heat load is total of

  1. Moisture-laden outside air form Infiltration& Ventilation
  2. Occupant Respiration & Activities
  3. Moisture from Equipment & Appliances

To maintain a constant humidity ratio, water vapor must condense on cooling apparatus at a rate equal to its rate of addition into the space. This process is called dehumidification and is very energy intensive, for instance, removing1 kg of humidity requires approximately 0.7 kWh of energy.

Space Heat Gain V/s Cooling Load (Heat Storage Effect)

Space Heat Gain is ≠ to Space Cooling Load

The heat received from the heat sources (conduction, convection, solar radiation, lightning, people, equipment, etc…) does not go immediately to heating the room air. Only some portion of it is absorbed by the air in the conditioned space instantaneously leading to a minute change in its temperature. Most of the radiation heat especially from sun, lighting, people is first absorbed by the internal surfaces, which include ceiling, floor, internal walls, furniture etc. Due to the large but finite thermal capacity of the roof, floor, walls etc., their temperature increases slowly due to absorption of radiant heat. The radiant portion introduces a time lag and also a decrement factor depending upon the dynamic characteristics of the surfaces. Due to the time lag, the effect of radiation will be felt even when the source of radiation, in this case the sun is removed.

 

Cooling Load Calculations
                               Differences between Space Heat Gain and Space Cooling Load

 

Differences between instantaneous heat gain and cooling load is due to heat storage affect.

The relation between heat gain and cooling load and the effect of the mass of the structure (light, medium & heavy) is shown below. From the figure it is evident that, there is a delay in the peak heat, especially for heavy construction.

 

Cooling Load Calculations
Actual cooling load and solar heat gain for light, medium and heavy Construction
Space Cooling V/s Cooling Load (Coil)

Space cooling is the rate at which heat must be removed from the spaces to maintain air temperature at a constant value. Cooling load, on the other hand, is the rate at which energy is removed at the cooling coil that serves one or more conditioned spaces in any central air conditioning system. It is equal to the instantaneous sum of the space cooling loads for all spaces served by the system plus any additional load imposed on the system external to the conditioned spaces items such as fan energy, fan location, duct heat gain, duct leakage, heat extraction lighting systems and type of return air systems all affect component sizing.

Reference
  • Principles of Refrigeration, Second edition by Roy J. Dossat
  • Cooling load calculations and principles by A. Bhatia

Part 2 of Cooling load calculations

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