Cooling Load Estimation - Complete step by step guide in determining HVAC load

Cooling Load Estimation - Complete step by step guide

 After reading this post, you will understand everything that is related to cooling load estimation in HVAC. The main topics covered in this post are:

* Building survey/study

* Type of loads and load components

* Factors and tables(For all the tables refer ASHRAE tables from data hand book)

* Practiced factors

* Calculation procedure

* Practice as per standard load estimate formats

Objective of this article:

    After studying this article, you will be able to define heat loads and identify its sources for both heating and cooling of spaces. You will be able to determine heat loads through the use of u-values or r-values, through area(square footage), and design temperature charts. You will also be able to know the amount of heat required to remove from the conditioned space to maintain the desired conditions in the space.

        Next, lets know some of the important terminologies and their definitions.

Heating System:

            Heating systems require an installation that will produce enough heat. The heat must keep the occupied space at a comfortable temperature and relative humidity. In United States of America 70°F inside temperature when it is 0°F outside temperature with a 15 mph wind blowing.

Degree Days:

            Degree days is a measure used to help indicate the heating or cooling needed for a given region. Calculations are based on a temperature of 65°F. The degree-day is computed as follows. The mean (average) of the highest temperature and the lowest temperature is taken for a day. Then this average is subtracted from 65°F.

Formula = 65°F - [(High temperature + Low temperature)] by 2.

Example: The lowest recorded temperature for a certain day was 28°F. The highest recorded temperature for the same day was 36°F.

Calculate the degree-day using above formula.?

Heat Gains:

            Heat added to the space being cooled is known as heat gain.

Heat Lag:

            The time it takes for heat to travel though a substance heated on one side.

Heat Load:

            Amount of heat removed during a period of 24 Hours.

Heat Loss:

            Loss of warm air, resulting in a lower temperature.

Heat Leakage:

            Heat leakage is the heat that is conducted through the walls, ceilings, and floors.

Heat Transfer Coefficient(U-value):

            A measure of the amount of heat that a material or combination of materials will allow through.

R-value:

            The thermal resistance of a given material.

Heat transfer rate (Q):

            The amount of heat transfer through a given material per unit time.

Thermal Conductivity:

            The ability of the material to transfer heat.

Ponded roof:

            Flat roof designed to hold a quantity of water, which acts as a cooling device.

Infiltration:

            Passage of outside air into building through doors, cracks, windows, and other openings.

Exfiltration:

            Flow of air from a building to the outdoors.

Conduction:

            The flow of heat between substances by molecular vibration.

Convection:

            Transfer of heat by means of movement or flow of a fluid or gas.

Radiation:

            Transfer of heat by heat rays.

Aeration:

            Act of combining a substance with air.

Ventilation:

            Airflow from one area to the other.

Heat loads:

            An air-conditioning heating system must heat a space enough to make up for heat losses during heating. An air-conditioning cooling system must remove as much heat from a conditioned space for satisfactory human comfort. Heat energy will flow from a higher to a lower temperature level.

In heating system it is necessary to retard heat flow as much as possible. In cooling system it must remove the amount of heat gained. The maximum heat load (loss or gain) is determined for a period of one hour.

Major heat loads:

Walls, ceilings and floors (through conduction).

Leakage (through ventilation air).

Solar source (through sun load).

Energy devices such as light fixtures, electric motors, electric or gas stoves etc.

People's load and also animals load.

In all cases, the heat load can be described as either sensible heat load (temperature change) or latent heat load (moisture), evaporating or condensing.

Heat loads for heating:

            Heat loads for heating include all means by which heat will be lost from a building. They also include heat loss due to warming of cooler substances brought into the building.

Major Heat losses:

            Conduction through walls, ceilings and floors. Air leaking out of the building (exfiltration), and that, which leaks into the building (infiltration). Combustion air leaving the flue gas or oil furnaces, or from fireplaces. Normally all other heat losses are ignored. They are too small, relatively, to affect the size of the unit to be installed.

Determination of heat leakage:

            First, determine the area of each type of surface through which heat is leaking. Find the u-value for each type of surface. Total heat leakage is found by multiplying the heat leakage areas by their respective u-values (Heat transfer coefficient). Heat leakage can also be computed using the thermal resistance of the structure. Thermal resistance is known as the r-value. It is the reciprocal of conductance (c) or the overall heat transfer (U). The thermal conductivity (k) is a measure of how quickly heat can travel through a material. The unit of k is normally Btu/ft sq/°F/hr.

The letter c is used to indicate the heat transfer through a wall made of different substances

                        1/c = (x1/k1) + (x2/k2) + (x3/k3)

x is the thickness of material in inches.

                        c = 1/ ((x1/k1) + (x2/k2) + (x3/k3))

Infiltration:

            Buildings are not airtight. Air leaks through openings. There is a difference in air pressure. Air pressure is caused by the wind. During heating season, cold air that enters must be heated. During cooling season, warm air that enters must be cooled. A way to prevent unwanted infiltration is to maintain a positive air pressure within the building. The pressurized air will filter out through cracks and openings in the building. With this practice, a special fresh air intake is needed. Incoming air can be conditioned before it is admitted to the rooms in the building.

Infiltration calculations can be based on the total volume of the building, or they can be based on the length and size of all the cracks in the buildings.

Number of air changes in the building:

            A building with a volume of 10,000 cubic feet of fresh air infiltration per hour. If six people occupy this place, there is 10,000 / 6 or 1667 cubic feet per hour for each person, or 1667 / 60 = 27.8 cfm (0.79 cubic m / min).

The air change will be reduced considerably if this building is constructed with vapor barriers. Fitting all doors and windows with weather-stripping will also reduce the air change. It may even be reduced to the point of unsafe ventilation. There will be too little oxygen in the air.

Heat Transfer Rate:

            The heat transfer rate is the amount of heat conducted through a structure for a given unit of time. It is usually represented by the letter Q and expressed in Btu/hr. The total heat transfer rate is found as follows.

The heat transfer coefficient (U-value) is multiplied by the temperature difference and the area.

Heat transfer rate = heat transfer coefficient x area x temperature difference

                            Q = (U x area) x (T­­₀ - T_i)

Where T­­₀= Outside Temperature

             T_i  = Inside Temperature

Heat Load Calculation Formula:

                             Q = U x A x [T­­₀ - T_i]

Heat Transfer Coefficient (U-value):

            To measure the amount of heat that a material or combination of materials will allow through the value of U, however, includes the additional insulating effect of an air film. This air film always exist on each side of the surface.

fi is the heat transfer through the inside air film &

fo  is the heat transfer through the outside air film.

U-value is a term indicating the amount of heat transferred through a structure (wall) in Btu /sq ft / °F / hour .

1 / U = (1 / fi) + (x1/k1) + ..... (1 / fo)

U- values are based on a 15 mph wind on the outside and a 15 fpm ( 1 / 6 mph) draft on the inside wall surface.

The U-value can be obtained from the Ashrae Hand book.

R-Value:

            The thermal resistance of a given material. It is also called as R resistance unit. Thermal resistance is the reciprocal of the heat transmission coefficient. R-values can be found by taking reciprocals of the heat conductance ( c value from tables which you can find in ASHRAE Data Book).

Windows and doors:

            Knowledge of the area of each window is needed in a heat leakage calculation. It is determined by measuring the opening in the wall. This would be the distance to the brick edges. Window construction varies considerably. Windows may be single-pane, double-pane using a storm window, or permanent double-pane. Energy-efficient windows may even have three permanent layers of glass.

The permanent window (single or double-pane) is called the primary window. An additional framed pane of glass may be set into place to provide added insulation. This is called a storm window or sash.

  The most efficient window construction is the permanent double or triple-pane. Two or three panes of glass with sealed air spaces between panes provide excellent insulation. This air space is dehydrated and evacuated. Then it is usually filled with nitrogen or some other dry gas to prevent sweating (condensation). Vinyl-clad aluminium frames are used to minimize frosting. The frame around windows may be made of wool or metal. Warm air will condense on cold surfaces. This becomes a problem with windows and walls. Condensation occurs due to a combination of both temperature and relative humidity (dew-point).

            To prevent condensation, reduce the relative humidity or raise the temperature of the glass surface. Lowering the relative humidity in the home may not be practical or comfortable. To prevent condensation better, add a vapor barrier between the inside surface of the window and the room air. It will also serve the added insulation. Large sliding glass doors are called patio doors. They are responsible for 20% of solar heating and heat leakage in some homes. Proper door-to-wall sealing is very important to minimize heat leakage. The use of rubber weather stripping is common practice. When computing the wall heat leakage area, add the area of the doors in the outside walls to the area of the windows. Then, subtract this amount from the total wall area.

Ceilings:

            Ceilings generally are made by fastening dry walls to the joists. Heat leakage will be considerable if the joists do not have a floor over them. It will also be considerable if there is no insulation between the joists. It is advisable to install insulation in the top floor of the building to prevent solar gain. The insulation thickness depends upon the solar gain. 2 inches, 4 inches, or 6 inches insulation used (fiber-glass, rock-wool, animal hair-felt, thermocole etc).

Basement Heat Loss:

            Deeper the basement lesser the heat loss. It is usually assumed that a basement is at 60°F.

Leakage through the basement floor is usually not calculated.

The heat leakage load is calculated through the first floor of the building (the basement ceiling). Buildings built on a concrete slab have different heat losses than those built with a basement. 

Crawl apace:

            A space left between the floor and the ground to allow access. The earth floor of a crawl space should have a vapor barrier on it. (This could be plastic sheeting, roofing paper, etc.) In addition, the floor can be insulated from underneath to provide maximum thermal protection. A crawl space must have sufficient venting to minimize moisture problems in the summer. The venting also minimizes the amount of cold air entering in the winter. Recent construction practices use vapor barriers (plastic sheeting) between the basement walls and the surrounding ground.

Sun Heat Load:

            Heat energy from the sun adds considerable heat load during the summer. The sun's rays in the northern hemisphere shine of the east, and west walls. Compute the heat load on the east wall in the morning. Compute the heat load on the south wall all day long. Compute the heat load on the west wall in the afternoon. The sun releases different amounts of heat to surfaces. The approximate maximum heat gain from the sun is 330 Btu / hr / sq ft. (97 watts / sq feet. or 1040 w / meter square). This is for a black surface at right angles to the sun's rays near the equator. Any other color surface at an angle to the sun's rays will receive less heat. Much of the heat is reflected back into the atmosphere. Windows must be protected with swings, use a temperature of 15°F higher than outside ambient temperature for correct results.

Heat Lag:

            It takes time for the heat to travel through a substance that is heated on one side. Heat Lag is the time needed for heat to travel through a substance that is heated on one side. The sun heats the outside wall of the building. However, several hours pass before this heat reaches the inner surfaces of the wall. In normal building, this time varies between three and four hours. With well-insulated or thick walls, the sun may be gone by the time the heat soaks through.

            If the walls are quite thick then, the sun heat moves into the wall when the sun shines. The wall is thick enough to prevent the heat from reaching the interior. During the night, when outdoor temperature falls below those as inside, the heat floe reverses itself and travels outward through the wall. The heat lag causes the rooms to be heated even after the sun goes below the horizon and the outdoor temperature drops.

Heat released by Human Beings:

            The heat released by one person weighing about 150 pounds (68 kg) is 253 Btu per hour (74 watts) when at rest. It is about 440 watts, (1,500 Btu /hour) when the same person is working. Stoves, lights, electric motors, iron boxes etc.

Humidifier Heat load:

        Window heat load for cooling. For ordinary glass, it is approximately three times as great as flow through ordinary residential roods and ceilings. To reduce the solar heat gain through glass, special type of glass with high-heat reflecting qualities may be used. Special heat-absorbing glass can reduce the solar heat load by as much as 30%. Another method is to use a glass tinted to a bluish gray, to reduce the solar glare and cooling load.

Roof extensions over a window will reduce the area exposed to the sun. Double-glazed windows exposed to sun-rays reduce solar heat absorption by 15%. Awnings to shade glass windows exposed to the sun can reduce the heat load by 55%. During the heating season, water vapor must be added to the air for comfortable conditions. Heat to produce the water vapor may come from heated air, furnace heat, or electric heat.

The amount of heat needed is figured as follows: The number of volume changes per hour must be known. Generally, one change per hour is satisfactory for homes. The number of grains to be added per pound of air to obtain the required relative humidity must be known.

Formula:

                Pounds of air per 24 hours x increase in grains = grains / day.

                Gr. / day / 7000 gr./ lb = lb. of water / day

                Lb. of water / day / 8.34 lb / gal = Gal. / day.

To calculate:

Volume x changes/ hr. X gr1 - gr2 / 33, 000 = gal. / day

Example:

The windows in the shade have a U-value of 1.25. If the temperature difference is about 12°F, the multiplier becomes 15. (12 * 1.25 = 15). The following is a way to make a rough estimate.

The chart identifies COP, which is the ratio of output divided by input. The output is the amount of heat absorbed by the system. Input is the amount of energy put into the system. Remember that, on the average, a medium-size room needs 5000 to 6000 Btu / hr of cooling.

Air conditioner heat load:

The average window comfort-cooling unit will adequately handle the cooling loads as follows:

0 - 6000 Btu/hr  =  1/2 hp & cop = 4.71

6000 - 9000 Btu/hr  = 3/4 hp & cop = 4.71

9000 - 11,000 Btu/hr = 1 hp & cop = 4.32

Design Temperature:

        Always refer ASHRAE for data on design temperature.

Always choose the outdoor design temperatures (odt) on the low side. Heating plants that are over-worked cause excessive stack and chimney temperature and may cause fires. Oversized units will be less efficient and waste energy. The odt is never as low as the lowest temperatures recorder for the area. This is because, these extreme lows are usually of short duration. Residual heat in the building usually enables the furnace based on the design temperature to handle the load.

    When working with design temperatures follow the ASHRAE (odt) chart, the odt varies with latitude and elevation.

Insulation and vapor barriers:

            Insulation reduces heat loss. Thermo cole, rock wool, animal hair felt, easy flowing bulk-insulation, plaster based rigid insulation, flexible insulation (easy to install), and blankets of pulverized wood. Sufficient strength, deterioration, unpleasant odor, vermin-proof and fire resistant (properties). Barriers reduces moisture travel through the wall. Aluminium sheet and tarred paper (hygroscopic). Mima-mineral insulation manufacturer's association.

Ceilings  r-19 through r-28

Walls      r-11 through r-19

Floors over unheated spaces     r-11 through r-19

 Ponded roof, an ordinary roof may be heated by the sun to a temperature of 100°F to 150°F. Ceilings under such roofs will become warm and radiate this heat through the space below. Many buildings with flat roofs are provided with some summer comfort cooling by using ponded roofs. A 2-inch to 3-inch pond of water covers the roof surface. This type of cooling is well suited to one-story factory and market buildings. To be effective, the roof area should be large as the floor area. The cooling effect comes from the evaporation of water from the roof. Naturally, ponded systems are most effective in areas having a high temperature, low relative humidity, and bright sunshine. By ponding, roof temperature may be kept below that of the surrounding atmosphere.

Ponded roof with water:

            Ponded roofs require a means of maintaining a constant level of water on the roof. Drains are needed to take away excess water due to rain. If the roof is large, wave breakers are needed to prevent waves from forming under high winds. The waves could cause a large quantity of water to be blown off the roof edge. A ponded roof may reduce the required air conditioning capacity by as much as 30%. However, the added weight on the roof will result in higher construction costs. A ponded roof should never be added to an existing building unless the roof structure can support the additional weight of the water.

RH (Relative Humidity):

            Reducing heat loads. By installing efficient insulation and double-glazed windows. Electric heating requires a well-insulated and tight structure. Sometimes, however, this adds to the ventilation problem. If the building is occupied by many people, frequent air changes must be provided for ventilation purposes. 

Building insulation and ventilation for electric heating. Electric heating elements in the plenum chamber or the air duct system. Some systems have heating elements installed in each room. (Individual control system), Dehumidifying equipment may be needed in order to maintain RH in a conditioned space.

Energy conservation:

             Following are the steps that help in saving the energy:

Use insulation wherever possible (especially between roof and ceiling surfaces). Increase in r-value results in saving energy. Use the latest design temperatures when calculating required HVAC system capacity. Use proper inside design temperatures. Proper relative humidity and indoor design temperature will maximize overall comfort. Eliminate unnecessary heat leakage around doors and windows with good sealing techniques. Use secondary heat sources (motors, machinery etc.) as much as possible. Use the most efficient construction materials, such as shaded glass, thermo panes, and metal foam insulated doors.

Construction types and designs:

Single floor type (ranch style)

Two floor type (colonial style)

Combination of both (split level style)

Each style may be built over a basement, a crawl space, or a cement slab. Each type of structure requires careful consideration when installing or servicing an HVAC system. Materials used in construction (brick, aluminium, insulation etc.). You should be able to read construction prints or floor plans. These will provide information useful in making heating or cooling calculations. Recent designs and new materials have reduced heat losses and gains. 

Roof Design and Construction:

            Heat loss through the roof of a building is influenced by several factors. These include, the type of roof construction, ventilation and covering. The most common residential roof construction is the pitched roof using a prefabricated truss system. The slope of a pitched roof is the vertical rise or height of the roof compared to the horizontal run. Recently, roof pitch has become an aspect of energy conservation as well. Proper ventilation is also important in the roof-enclosed area. Roof coverings are chosen based on the surrounding climate. Dark asphalt shingles help to absorb the sun's heating effect during winter months.

Wall Construction:

            Care must be taken to avoid leakage of air and mixture passage through the wall. During heating season inside vapor barrier is necessary. In summer the outside vapor barrier is necessary. Where vapor barriers are used, they should be as tightly sealed as possible.

stucco wall, Thin coating of cement on wall. Hard wood siding in modern buildings. Aluminium siding is available in a variety of colors, textures, and styles.

Commercial construction:

            Buildings are usually made of concrete blocks set on a concrete slab. Or of a wood-stud construction. Roof on low-rise shopping center is usually flat with asphalt weather protection. water drainage on a flat roof is critical. High-rise buildings and large industrial buildings are much more complex in their construction. In these cases, you should become familiar with the general building design. Study the blueprints prior to doing any major HVAC work involving the building's structure wall heat leakage areas. In addition to finding the several u- or r- values for the building structure, the area of the walls will need to be = u x wall area x temperature.

            Areas to be measured are the outside dimensions of the building have slightly higher heat leakage loads than if inside dimensions are used. U-values based on outside dimensions are conservative. Windows and doors areas must be subtracted.

Objective of Load Estimation:

            To estimate the amount of heat required to remove from the conditioned space to maintain the desired conditions in the space, we need to consider the following factors:

Building survey

1. Orientation of the building

Location of space to be air-conditioned with respect to compass points.

Nearby Permanent structure.

Shading effects.

Reflective surfaces.

Water, sand, parking

2. Space use

Office, Hospital, Department store, Factory.

3. Physical Space Dimension

Length, width, and height.

4. Ceiling Height

Floor to floor and floor to ceiling height.

Clearance between suspended ceiling and beams.

5. Construction materials

Materials and thickness of walls, roof, floors and partitions.

6. Surrounding conditions

Exterior color of walls, roof.

Temperature, furnace, boiler room and kitchen.

7. Windows

Size and location.

wood or metal sash.

Type of glass-single or multiple.

Type of shading devices.

8. People

Number and duration of occupancy

Nature of activity.

9. Lighting

wattage

type - incandescent, fluorescent, LED

Wattage per sq. ft floor area.

10. Appliances

Location, rated wattage.

Steam and gas consumption

11. Ventilation

cfm per person

cfm per sq. ft floor area

Air change per hour according to usage.

12. Operation

Daily operation.

Hours of operation etc.

Occasional operation.

Design Day:

When dry and wt bulb temperature is peaking simultaneously. When all internal loads are normal.

Space heat gain

The rate at which heat enters or generates or generates within a space at a given moment is called "space heat gain". Solar radiation through transparent surfaces. Heat conduction through exposed walls and roofs. 

Heat conduction through interior partitions, ceilings and floors.

Heat generated within the space by occupants, lights and appliances.

Miscellaneous Heat gains

Space cooling load

The space-cooling load is the rate at which heat must be removed from space to maintain desired comfortable conditions in the space.

Type of heat gain (load)

Sensible heat gain (load)

A heat generating and entering in conditioned space increase by dry bulb temperature.

Latent heat gain (load)

Moisture generating and entering in conditioned space, increases humidity.

Classification of heat gain

Outside heat gain/ load

 Originates from outside or external heat source.

Internal heat gain / loads

Originates within conditioned space itself

Other heat gain / loads

Occurring from heat gains or losses associated with moving cool fluids.

Air flow

Water flow

 Outside heat gain / loads

Solar gain through glasses

Due to radiation heat

Solar and transmission gain through walls and roof

Due to sun rays striking external surfaces

Due to higher outdoor temperature

Transmission through glass, ceiling, partitions or floors.

Due to difference in temperature

Infiltration

Ventilation

Internal heat gain/ load

People

 Sensible heat

 latent heat.

Lights

Sensible Heat

Appliances / equipments

sensible or latent heat

Other heat gain / load

supply airside

supply duct heat gain

supply air fan heat

by passed outdoor air

return airside

return air heat gain

return air fan heat

outside or ventilation air

solar heat gain - through glasses

Solar heat through glasses = (glass area) x (sun gain) x (shade factor)

Glass area - this is a glass area (sq. ft) . In specific orientation / direction.

Sun gain - this includes the solar intensity, effect of adjustment factors and storage effect.

It is tabulated in ASHRAE data handbook for specific time and orientation .

It is multiplied by correction factors described in ASHRAE tables.

Shade factor - this is over all factor for solar heat gain through glass, the factors are tabulated in tables(in ASHRAE data handbook) for specific glasses.

Glass shading devices

Reduces amount of solar energy in reaching air conditioning space.

External

Internal

External are efficient than internal shading.

prevents direct strike.

Radiation get diffused.

Heat gain thru walls and roof

It is caused by:

Solar heat being absorbed at exterior surface.

Transmission heat by temperature difference between outdoor and indoor.

Both heats are evaluated together.

Solar and transmission gain through walls and roofs.

Formulae:

Wall / roof area x eq. temp diff X over all transmission coefficient (u-factor).

Wall / roof area - in sq ft.

Eq temp. diff - it is tabulated in table - 19 and 20 for respective exposure and wt of wall / roof.

Transmission coefficients are tabulated in tables(ASHRAE Data handbook) for most common type of constructions.

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 in tables(ASHRAE Data handbook)

It takes into account the different type of construction.

Exposures

Time of day and month

Location of building and outside design condition.

Inside and outside design condition.

Basis of tables(ASHRAE Data handbook).

Solar heat in july at 40° north latitude.

Out door dry bulb daily range 20°F

Maximum outdoor dry bulb temperature 95°F and indoor 80°F

Dark color walls and roofs.

Sun time.

Latitude changes:

Latitudes other than 40 deg.n and for other months with dif solar intensities. Etds from tables(ASHRAE Data handbook) are approximately correct for east and west wall during hottest weather.

Etds for north and south wall is approximately same as north or shade wall.

Hence correction is not needed to apply (approximately).

Overall transmission coefficient "u"

Transmission coefficient or u value is the rate at which heat is transferred through a building structure in Btu / hr. per sq ft and per °F temp difference.

"U" value for a wall / roof is the reciprocal of total resistance offered by wall/roof, in flow of heat.

Transmission coefficients are tabulated in tables(ASHRAE Data handbook) for most common type of constructions.

Total resistance r = r1 + r2 + r3 + r4 ............   r1, r2, r3, r4 are resistances offered by individual barrier.

Resistance of materials are listed in table 34

Thus transmission coefficient u = 1/r

Transmission heat gain through glasses

It is due to outside and inside temperature difference and is equal to:

glass area sq. ft x (outside temp - inside temp) x (u-value for the glass from table)

Transmission heat gain through partition, ceiling and floor:

It causes due to temperature difference between conditioned and conditioned space.

It is equal to: area in sq ft x (outside temp - inside temp - 5°F) x (u-value from table).

In case of kitchen or boiler room, then 

area, sq ft x (outside temp - inside temp + 15 to 25°F) x (u-value from tables(ASHRAE Data handbook)).

Ventilation

An adequate outside air is added to return air for proper ventilation and odor as per ventilation standards (table)

The outside air is at higher dry bulb temperature, hence adds heat to return air or load on cooling coil as per equation:

Heat gain (Btu/hr) = outside air, cfm x temp. diff, °F x 1.08

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 simply supplied to condition space. This partly air is called bypass air.

Bypass factor depends on coil construction, tubes arrangement and fins.

Coil face air velocity

Internal heat gain

people

sensible and latent heat

Tabulated in table is based on activities, applications and room dry bulb temperatures.

Add 30 Btu per hour to sensible and latent heat gain for restaurant due to food served.

Lights

Incandescent - total light wattage x 3.14 Btu per hour

Fluorescent - total wattage x 3.4 x 1.25 Btu per hour

Recommended steps

Define area or space

Based on type of load

Based on inside condition.

Select the time at which normal load occurs

Tabulate following:

External dimension.

Glass area.

Net wall area.

 Net ceiling and floor area.

Structural weights and colors.

Transmission factors.

Estimation procedure:

Calculate solar heat gain. Internal heat gain and transmission heat gain.

Calculate internal heat (sensible and latent) due to people, light and appliances.

Calculate sensible and latent load due to ventilation and infiltrated air.

Calculate other loads.

Add all loads to arrive at grand total heat load.

Add further safety if required.

Review for economy

Re-check whether the following load can be reduced or eliminated.

Solar gain through glass.

Solar and transmission gain through walls and roof.

Transmission gain through glass and partition, ceiling and floor.

In future post i will solve one example and demonstrate how to calculate Heating and cooling load with step by step guide.. SO keep checking this website on regular basis.

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