Recents in Beach

Correct heat load estimation factors

Correct heat load estimation

Correct estimation of heat load components of the space to be air conditioned is a basic requirement for a realistic estimate of cooling and heating loads. Following factors must be considered while estimating heating and cooling load of a building:

1. Orientation of building with respect to compass points, sun and wind effects.
2. Space usage – Office, hospital, department store, specialty shop, machine shop, factory, assembly plant, etc.
3. Physical dimensions of space - Length, width, and height.
4. Ceiling height - Floor to floor height, floor to ceiling, clearance between suspended ceiling and beams.
5. Construction materials - Materials and thickness of walls, roof, ceiling, floors and partitions, and their relative position in the structure.
6. Surrounding conditions - Exterior color of walls and roof, shaded by adjacent building or sunlit. Attic space - unvented or vented,
gravity or forced ventilation. Surrounding spaces conditioned or unconditioned temperature of non-conditioned adjacent spaces, such as furnace or boiler room, and kitchens. Floor on ground, crawl space, basement.
7. Windows - Size and location, wood or metal sash, single or double hung. Type of shading device. Dimensions of reveals and overhangs.
8. Doors - Location, type, size, and frequency of use.
9. Stairways, elevators & escalators - Location, temperature of space if open to unconditioned area. Horsepower of machinery, ventilated or not.
10. People - Number, duration of occupancy, nature of activity, any special concentration. At times, it is required to estimate the number of people on the basis of square feet per person, or on average traffic.
11. Lighting - Wattage at peak. Type incandescent, fluorescent, recessed, exposed. If the lights are recessed, the type of air flow over the lights, exhaust, return or supply, should be anticipated. At times, it is required to estimate the wattage on a basis of watts per sq ft, due to lack of exact information.
12. Motors – Location, nameplate and brake horsepower, and usage. The latter is of great significance and should be carefully evaluated. The power input to electric motors is not necessarily equal to the rated horsepower divided by the motor efficiency. Frequently these motors may be operating under a continuous overload, or may be operating at less than rated capacity. It is always advisable to measure the power input wherever possible. This is especially important in estimates for industrial installations where the motor machine load is normally a major portion of the cooling load.
13. Appliances, business machines, electronic equipment – Location, rated wattage, steam or gas consumption, hooded or unhooded, exhaust air quantity installed or required, and usage. Greater accuracy may be obtained by measuring the power or gas input during times of peak loading. The regular service meters may often be used for this purpose, provided power or gas consumption not contributing to the room heat gain can be segregated. Avoid pyramiding the heat gains from various appliances and business machines. For example, a toaster or a waffle iron may not be used during the evening, or the fry kettle may not be used during morning, or not all business machines in a given space may be used at the same time. Electronic equipment often requires individual air conditioning. The manufacturer’s recommendation for temperature and humidity variation must be followed, and these requirements are often quite stringent. 

You can use E20 excel sheet or HAP software to calculate heat load. Interface of E20 excel sheet is shown in figure below



Heat load estimate

Heat load is estimated to provide the basis for selecting the conditioning equipment. It must take into account the heat coming into the space from outdoors on a design day, as well as the heat being generated within the space. A design day is defined as:
1. A day on which the dry-and wet-bulb temperatures are peaking simultaneously
2. A day when there is little or no haze in the air to reduce the solar heat
3. 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. Actually, the situation of having all of the loads peaking at the same time will very rarely occur. To be realistic, various diversity factors should be applied to some of the load components

Outdoor loads

The loads from outdoors consist of:
1. The sun rays entering windows - for which the solar heat gain through glass should be estimated by using proper solar heat gain tables. The solar heat gain is usually reduced by means of shading devices on the inside or outside of the windows. In addition to this reduction, all or part of the window may be shaded by reveals, overhangs, and by adjacent buildings. A large portion of the solar heat gain is radiant and will be partially stored  and storage factors to be applied to solar heat gains in order to arrive at the actual cooling load imposed on the air conditioning equipment. These storage factors should be applied to peak solar heat gains.
2. The sun rays striking the walls and roof - These, in conjunction with the high outdoor air temperature, cause heat to flow into the space. Using proper tables to get equivalent temperature differences for sunlit and shaded walls and roofs.
3. The air temperature outside the conditioned space - A higher ambient temperature causes heat to flow thru the windows, partitions, and floors. The temperature differences used to estimate the heat flow through these structures.

Internal loads

The internal load, or heat generated within the space, depends on the character of the application. Proper diversity and usage factor should be applied to all internal loads. As with the solar heat gain, some of the internal gains consist of radiant heat which is partially stored, thus reducing the load to be impressed on the air conditioning equipment. Generally, internal heat gains consist of some or all of the following items mentioned below:
1. People - The human body thru metabolism generates heat within itself and releases it by radiation, convection, and evaporation from the surface, and by convection and evaporation in the respiratory tract. The amount of heat generated and released depends on surrounding temperature and on the activity level of the person
2. Lights - Illuminants convert electrical power into light and heat. Some of the heat is radiant and is partially stored
3. Appliances - Restaurants, hospitals, laboratories, and some specialty shops (beauty shops) have electrical, gas, or steam appliances which release heat into the space. Proper recommended values of heat gain for most appliances when not hooded should be selected. If a positive exhaust hood is used with the appliances, the heat gain will be reduced.
4. Electric motors - Electric motors are a significant load in industrial applications and should be thoroughly analyzed with respect to operating time and capacity. It is frequently possible to actually measure this load in existing applications, and should be so done where possible.



Design conditions

For most of the comfort system the recommended indoor design conditions like temperature and relative humidity are as follows

Dy bulb temperature (DBT) = 22.78˚C to 26.11˚C and Relative humidity (RH) = 50% for summer
Dry bulb temperature (DBT) = 22.11˚C to 22.22˚C and Relative humidity (RH) = 20-30% for winter

The cooling load of the building should be selected is based on 23˚C (74˚F) dry bulb temperature and 50% relative humidity indoor design conditions

Outdoor design conditions are determined from published data for the specific location, based on weather bureau or airport records

Following formulas must be used while determining heat load for some particular space

Heat load through walls & roof is given by formula
Q-Wall = U * A * CLTD --------- (1)
where
 Q Wall = Load through the walls in Btu/hr
U = Thermal Transmittance for walls in Btu/ (h ft2.F)
A = area of walls in ft2
CLTD = Cooling Load Temperature Difference for walls in °F
 
Heat load though glass is given by formula
Q-Glass Conductive = U x A x CLTD --------- (2)
Q-Glass Solar = Ax SCx SCL --------- (3)

where
Q Conductive = Conductive load through the glass in Btu/hr
Q Solar = Solar transmission load through the glass in Btu/hr
U = Thermal Transmittance for glass in Btu/ (h ft2.F)
A = area of glass in ft2
CLTD = Cooling Load Temperature Difference for glass in °F
SC = Shading coefficient
SCL = Solar Cooling Load Factor
 
Heat load through partitions, floor & ceiling is given by formula
Q = U x A x (Ta - Ti) --------- (4)
where
A = area of partition, ceiling or floor in ft2
Ta = Temperature of adjacent space in °F that is outside temperature minus 5 °F
Ti = Inside design temperature of conditioned space in °F

Heat load due to People is given by formula
Q sensible due to people = N x (QS) x (CLF) --------- (5)
Q latent = N x (QL) --------- (6)

Where
N = number of people in space
QS, QL = Sensible and Latent heat gain from occupancy
CLF = Cooling Load factor by hour of occupancy of people in space

Lighting load is given by formula
Q = 3.41 x W x BM x (CLF) --------- (7)
Where
W = Watts input from electrical lighting plan or lighting load data
BM = lighting ballast multiplier normally it's 1.02 to1.08
CLF = Cooling Load factor by hour of occupancy in space

Equipment load is given by formula
Q = 3.41 x W x (CLF) --------- (8)
Where
W = Watts input from electrical lighting plan or lighting load data
CLF = Cooling Load factor by hour of occupancy in space

Infiltration Air load is given by formula
Q sensible = 1.08 x CFM x (To – Ti) --------- (9)
Q latent = 4840 x CFM x (Wo – Wi) --------- (10)

 
Where
CFM = Infiltration air flow rate
To & Ti = Outside/Inside design dry bulb temperature in °F
Wo & Wi = Outside/Inside design humidity ratio in lb water/lb dry air
ho & hi = Outside/Inside design air enthalpy in Btu per lb (dry air)

So you have to calculate space sensible heat, solar & transmission gain through wall & roof, transmission gain through glass, partition, ceiling & floor, infiltration & by pass air load, Internal heat gain load due to people, lighting & equipment's, room latent heat load due to infiltration, outside air & people. After calculating all the loads you have to calculate total sensible & total latent heat load and get effective sensible heat ratio (ESHR) by formula given below

ESHR = Total sensible heat load / (Sensible heat load + Latent head load) --------- (11)

Once you get the ESHR then you will calculate total required load in btu/hr
let's say if we get total heat load of 30000 btu/hr then tonnage will be given as

Tonnage = Total heat load / 12000 --------- (12)
Tonnage = 30000/12000
Tonnage = 2.5

After tonnage you have to calculate apparatus dew point (ADP) and in the end you can calculate dehumidified CFM by given formula

Dehumidified CFM = Total sensible heat / (1.08 x dehumidified rise) --------- (13)
Where
 
Dehumidified rise = Contact factor x (Room temperature - ADP) --------- (14)

Contact factor = (1- by pass factor) --------- (15)
              
where by pass factor is normally 0.12 which is a standard value So contact factor can be calculated by using equation 15
Contact factor = (1- 0.12) = 0.88

let's say we got 54 deg F apparatus dew point (ADP) based on 75 deg F inside room temperature, 50% relative humidity & effective sensible heat ratio is 0.92 and total sensible heat is 24000 Btu/hr after performing load calculation

So dehumidified rise is given by equation 14

Dehumidified rise = 0.88 x (75 - 54) = 18.48 deg F

then dehumidified CFM is given by equation 13

Dehumidified CFM = 24000 / (1.08 x 18.48)

Dehumidified air flow rate = 1203 CFM

Air conditioning equipment selection

After the load is evaluated, the equipment must be selected with capacity sufficient to offset this load. The air supplied to the space must be of the proper conditions to satisfy both the sensible and latent loads estimated. Psychrometrics analysis should be done to determine the criteria from which the air conditioning equipment is selected.



Juulia_2


Juulia_3






Post a Comment

2 Comments

  1. Hello
    could you send me this excel if that is possible

    ReplyDelete
  2. Can I get the excel spread sheet for this one. Please sent me " winnweko@gmail.com" Thank you so much sir

    ReplyDelete