Heat Load (& Cooling Load) for Air-Conditioning Calculator
For a quick estimation, Heat Load of a room can be estimated by Room Area.
The Room Area forms the base value of Heat Load (btu/h) that the cooling system (or Heating system) needs to work on, added with other forms of heat loads such as human heat load, solar heat load, equipment heat load, process heat load etc.
For more accurate results, heat loads may be simulated using a variety of Heat Load software, but a general rule of thumb estimation of this kind may be helpful for quick initial estimations and equipment selections. Total Room Heat Loads can be computed by summing up estimated heat loads arising from sun, humans, and other equipment or processes.
For even more simplified calculation, Heat Load of a room can be estimated by Room Volume.
Human Heat Load depends mainly on Room Temperature and Types of Activities. As a rule of thumb, one person can be estimated to contribute a heat load of 500 btu/h, unless moderate to vigorous works are involved.
A general guide can be referred from the tabulation below. Increasing Room Temperature has slight effect in reduced sensible heat and increased latent heat but has little to no effect on the overall or total heat load.
The Sun contributes significantly to the Heat Load of an area, more so if the room has large area of windows. The amount of solar heat load depends on climate and time, zone, direction, area, shading and material of windows etc. But again, a general rule of thumb may define a heat load of 80 btu/h per unit area of window (square-foot) facing the North or South, and 100 btu/h per square-foot of window facing the East or West.
Heat gain from lights can be directly converted from the Wattage of lights, where 1 Watt = 3.412 btu/h. A factor of 25% is added for fluorescent light to include heat in ballast.
Heat Load from Equipment varies from one application to another. Heat-producing appliances such as dryers and burners may contribute up to nearly all of its power rating as heat. With proper exhaust hood, half of the heat load may be discounted. For moving equipment such as motors, for a rule of thumb, it may be estimated that 100% – Efficiency (%) of the power rating of Motors contribute to the heat load.
Motor Heat Load = (100% - Efficiency (%)) x Motor Power Rating (kW)
In the Industry, the Heat Load per unit Area (btu/h/sqft) is a key parameter used as to determine a required cooling or heating capacity. It may be a usual practice for designers to consider other forms of Heat Loads by directly applying a higher Heat Load per unit Area (btu/h/sqft). For example, for a common area designed for 55 btu/h/sqft, 80 btu/h/sqft may be applied for rooms with full-height glass windows to account for the additional solar heat load through the glass windows.
Heat load can be classified into latent heat and sensible heat. Sensible heat is the heat that arises from change of temperature only, without any changes of phase. Electrical, electronic and lighting contribute mostly to sensible heat load. Latent heat is the heat arising from changes of phase such as from solid to liquid, liquid to gas or vice versa. Boiling water, human activity, industrial processes may contribute a large amount of latent heat.
HVAC Equipment Cooling Capacity
The equipment selected shall have optimal latent and sensible cooling load to cover the respective types of heat loads arising from the surrounding. Most of the HVAC systems drive hot and humid outdoor air down in the South-West direction (Green Arrow below) of the psychrometric chart to cooler and drier air as shown below. In other words, most common HVAC systems perform Cooling and Dehumidification treatment to unconditioned air within a building.
The Cooling Load estimation for a selected HVAC equipment can be done via the computation of a few parameters below, which largely depends on the HVAC equipment selection including coil temperature or density, fan speed etc. which can be summed up as the total cooling capacity of the equipment.
The value of Enthalpy can be derived from Dry Bulb temperature and Relative Humidity with a limited degree of accuracy, through the mathematical models in Enthalpy Calculator.
The Mixed Air component (MA) can be replaced with 100% Return Air component (RA) if there is no fresh air introduced to the HVAC system. The Mixed Air component considers the enthalpy or temperature based on proportion of Fresh Air (which can either be directly drawn in untreated, or pre-treated with a Pre-cooler unit).
An Excel version of this Heat Load /Cooling Load 2-sheet Calculator can be downloaded via the link attached below for better reference.
AC Heat / Cooling Load Calculator (Excel)