Air Conditioning Load Estimate - Detailed article on how to estimate HVAC load

Air Conditioning Load Estimate

The cooling load is assessed to give the premise to choose the molding gear. It should consider the hotness coming into the space from outside on a plan day, as well as the hotness being created inside the space.

A Design day is defined as, a day on which the dry and wet bulb temperatures are peaking simultaneously, a day when there is little or no haze in the air to reduce the solar heat. A day on which all of the internal loads are normal.

The time of peak load can usually be established by inspection. Although, in some cases, estimates must be made for several different times of the day and the more realistic estimate should be concluded. All things considered, the circumstance of having every one of the heaps cresting simultaneously will seldom happen. To be realistic, various diversity factors must be applied to some of the load components.

Next we will see what are the different types of loads or heat gains.

Type of Load/ Heat Gain:

    Sensible Heat Gain/Sensible Load:

                The heat generating or entering in the conditioned space which increases the dry bulb temperature of air is called as Sensible Heat Gain or Sensible Load. The sensible load components are: * Solar heat through glasses, walls, roofs and skylights. 

* Heat gain through partitioned walls, ceilings, floors facing unconditioned space.

* Heat gain from people, lights and appliances.

* Outside air infiltration.

    Latent Heat/Latent Load: 

                The moisture vapor generating or entering into conditioned space increases the moisture contents in the air, thus increases the humidity. To reduce the moisture contents in the air or to reduce the humidity, the moisture vapors are condensed by removing its Latent Heat and the heat gain is called as Latent Heat Gain or Latent Load. The Latent load components are:

* People

* Outside air infiltration

* Steam or moisture generating appliances

* Swimming pools etc

* Vapor transmission through walls.

General classification of Heat Gains/Load follows three types:

* Outdoor Loads

* Indoor Loads and

* Other Loads.

We will see about these loads one by one.

Outdoor Loads:

                The outdoor loads originate from heat sources outside to the conditioned space such as:

1. Solar heat gain through glasses.

2. Transmission heat gain through glasses.

3. Solar and Transmission gain through walls and Roofs.

4. Transmission Heat Gain through Partition walls/ceiling and floors.

5. Outside air load/ Ventilation load.

Solar Heat Gain through glasses:

                    The sun rays entering the glass windows increases the temperature of air and thus adds sensible heat to the condition space. The sun oriented heat gain is normally decreased through concealing gadgets within or outside of the windows. The solar heat through glasses can be calculated by the equation:

                        Solar Gain = ( Glass Area) x ( Sun Gain) x ( Shade Factor)

Glass Area: This is a glass area exposed to sun rays in specific orientation ( East, West, North and South or Nort-East, South-East, North-West and South-West).

Sun Gain: This is per square feet instantaneous heat gain due to solar radiation based on Geographical location of space (city), Month of the year, Exposure, Sun Time.

Shade Factor: Shading and Shading devices reduce the amount of solar radiation reaching the conditioned space. The manufacturers of glasses and windows provide factors based on the tests conducted on the product.

Transmission Gain through Glasses(windows, skylights and glass block walls):

                    In addition to solar gain through glasses, we have to work out the transmission gain through glasses, which is due to Temperature difference between inside and outside. The same can be calculated by following equation:

                    Transmission Gain = Glass Area x (Outside Temperature - Inside Temperature) x (U-Factor).

Solar and Transmission Gain through Walls and Roofs:

                    The heat flows from one point to another whenever a temperature difference exists between the two points. The course of stream is dependably towards the lower temperature. Heat flow through building walls and roof is normally calculated at the time of greatest heat flow. It is caused by solar heat being absorbed at the exterior surface and by the temperature difference between outdoor and indoor air. Both heat sources are highly variable through out any one day, and therefore, result in unsteady state heat flow through the exterior construction. This unsteady state flow is difficult to evaluate for each individual situation. However, it can be handled best by means of an equivalent temperature difference across the structure.

            The Equivalent temperature difference is that temperature difference which results in the total heat flow through the structure as caused by the variable solar radiation and outdoor temperature. The same temperature contrast across the design should consider the various sorts of development and openings, season of day, area of the structure (scope), and configuration conditions. The heat flow through the structure may then be calculated, using the steady state heat flow equation with the equivalent temperature difference.

Solar and Transmission Gain through Walls and roofs = (Wall/Roof area) x (Equivalent temperature difference) x Overall Transmission coefficient (U-Factor).

Wall/Roof area - Net exposed wall or Roof area in a particular exposure.

Equivalent Temperature Difference:

                        It is the temperature difference, which results in the total heat flow through the wall/roof as caused by the variable solar radiation and outdoor temperature. It is tabulated based on Time of day and month, inside and outside design condition. It takes into account the different type of construction and Exposures.

Transmission Gain through Partition wall/ceiling and Floor:

                        The heat flow through the floors, ceilings and partition wall, is caused due to difference in temperature, of the air on both sides of the structure. This temperature difference is essentially constant through out the day, and therefore, the heat flow can be determined from the steady state heat flow calculation, using the actual temperatures on either side.

                Heat Gain = (Surface Area) x (Temperature difference) x (U-value)

In case of non-conditioned space, the non-conditioned space temperature is  generally assumed as 5 degree Fahrenheit lesser than outside temperature. It is always appreciated to obtain average actual temperatures. In case of Kitchen or Boiler room, the other side temperature is assumed as 15 degree to 25 degree Fahrenheit more than outside temperature. 

Outside Air Load/Ventilation Air Load:

                Outdoor air is necessary to maintain the odor level and to maintain indoor air quality in conditioned space. Since outside air has higher temperature and more moisture, hence imposes a cooling and dehumidifying load on the cooling apparatus. Generally, In small applications, the ventilation air is directly added to return air, thus increasing the load on apparatus directly. In this situation, the ventilation load shall be worked out and combined with space load to arrive at Total space load.

Thermal Conductivity or Transmission Coefficient "U":

                Transmission coefficient or U-value is the rate at which heat is transferred through a building structure in Btu/hr per square feet per inch thickness per degree Fahrenheit temperature difference. If U is the conductivity of material then 1/U is the resistance of the material of 1.00 sq. foot cross section and 1.0 inch thickness. If the thickness is T inches, the resistance becomes T/U per sq. feet.

                In electrical system, resistances connected in series are added in order to find the total resistance. Similarly, if a wall/roof is made up of several materials barrier, the total resistance shall be calculated by adding the resistance of individual barrier. If a wall is made up of three barriers having individual resistances, R1, R2 and R3 then the total resistance of wall barrier shall be:

                            R = R1 + R2 + R3

Film Resistance or Film Coefficient:

                    In addition to the resistance of the various barriers, we have to consider one more resistance, offered by a film of air in flow of heat, which clings on barrier surface inside and outside. The resistance is more when the air is still and relatively less when there is a wind velocity. Please note that this differs from the thermal conductivity, in the sense, it is not related to any film thickness as in case of materials.

                Overall Conductivity, U-Factor = 1/R

In large Buildings, A separate apparatus is provided for cooling and dehumidifying the outside air. The cooled and dehumidified air is then supplied to conditioned space directly. In this case, Ventilation load need not be combined with space load and as such Space Air Handling unit need not to have capacity to remove this heat gain due to ventilation.

Outside air requirement for conditioned space is calculated:

* Based on Cubic feet per minute (CFM) per person.

* Based on Cubic feet per minute (CFM) per sq. ft floor area.

* Based on specified Air Changes per Hour (ACHR).

* Based on client and consultant specific requirement.

* Based on ASHRAE and relevant Ventilation standards.

Outside Air Load can be calculated by following equations:

Sensible Heat Gain = (Outside air, CFM) x (Temperature Difference (°F)) x 1.08

Latent Heat Gain = (Outside air, CFM) x (Difference in Grains/Lb, of air) x 0.68

Internal Loads/Internal Heat Gain:

                    The internal load is the sensible and latent load released within the conditioned space by the occupants, Lights, appliances, machines etc. Appropriate diversity factor ought to be applied to all inward loads.

The following are the common load components:

* People:

                The human body through metabolism generates heat within itself and release it by radiation, convection, and evaporation from the surface, and by convection and evaporation in the respiratory tract. How much hotness produced and delivered, relies upon encompassing temperature and on the action level of the individual.

* Lights:

                Lights produce reasonable hotness by the change of the electrical power input into light and hotness. The heat is dissipated by radiation to the surrounding surfaces, by conduction into the adjacent material and by convection to the surrounding air.

The heat gain from the Lights can be calculated as:

* Fluorescent type : Total light watts x 1.25 x 3.4

* Incandescent type : Total light watts x 3.4

The fluorescent light wattage is multiplied by 1.25 to include the heat gain in blast.

* Appliances:

                        Most machines contribute both reasonable and inactive hotness to a space. Electric machines contribute dormant hotness, exclusively by ideals of the capacity they play out, or at least, drying, cooking, and so forth. Offices, Residences, restaurants, hospitals, laboratories have electrical, gas, or steam appliances which release heat into the space.

* Evaporation from a free water surface - Latent heat gain:

                Considering still air and Room temperature 75°F DB and 50% RH

Other Loads/ System Heat Gain:

                    Other loads or system gain is considered as the heat added or lost by the system components, such as ducts, piping, air conditioning fans and pumps, etc. This heat gain must be estimated and included in the load estimate, but can be accurately evaluated only after the system has been designed. To be practical as a general acceptable practice, a percentage is added to this effect.

Percentage added to Sensible Load:

* Due to supply duct heat gain : 2%

* Supply duct leakage loss : 2%

* Fan Horse power : 2%

* Safety factor : 4%

Percentage added to Latent Load:

* Supply duct leakage loss : 1%

* Safety factor : 4%

Percentage added to Grand Total Heat to arrive at Equipment Load:

* Return duct heat gain : 1%

* Return duct leakage loss : 1%

* HP Pump Loss : 1%

* Pipe Losses : 2%

Note: The above percentages are applied absolutely based on experience and can vary from application to applications.

Design Conditions:

                    The design conditions established determine the heat content of the air, both outdoors and indoors. They directly affect the load on the air conditioning equipment by influencing the transmission of heat across the exterior structure and the difference in heat content between the outdoor and inside air. Before starting your HVAC load calculation, the first step is to select a region and take their respective data. For example, if we have to start HVAC load calculation for Dubai and Abu Dhabi, in United Arab Emirates, we can select these values as shown in the table below.

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