INTERNAL HEAT GAINS (IHG)
The sources of internal heat gains (IHG) include:
- PEOPLE (sensible and latent heat gain)
- LIGHTS (sensible heat gain only)
- EQUIPMENT
- (a) Receptacles or electrical plug loads (sensible heat gain only)
- (b) Processes such as cooking (sensible and latent heat gain)
IHG can be a major component of the total building cooling load. This is particularly true of non-residential (commercial, institutional and industrial) buildings. IHG for lights can be calculated if the type and number of lighting fixtures are known. This is also true for electrical equipment. IHG for people and process loads are approximate since the level of activity varies.
IHG loads for each hour of the year is estimated on the basis of percent of peak design load. Like the hourly weather data that affects energy loads due to the building envelope, infiltration and ventilation, internal loads can vary from hour to hour and year to year.
A range of IHG design hour values from low, average and high can estimated on the basis of type of building. This is the type of information that is available. Such estimates apply to a particular region, country, economy and society. After the building is designed and built, it can be under-used or over-used. The building can be used for purposes other than what it was designed for. In the case of office buildings, lighting loads have decreased due to more efficient lighting and equipment loads have increased due to computers and telecommunication equipment.
In the case of under-usage, building's control system will adjust the cooling system at the expense of inefficient use of the cooling equipment. In the case of over-usage, the building’s cooling capacity must be increased. Poor judgment in estimating IHG can result in unsatisfactory operation. As with building envelope loads, IHG estimating procedures are therefore rigorous and precise using the best information available for the given type of building.
Latent heat (moisture or water vapor) from people and equipment added to the space is an instantaneous cooling load. Sensible heat generated by internal heat sources (people, lights and equipment) is a time-delayed cooling load. As with solar radiation heat entering the space, part of sensible heat generated by internal sources is first absorbed by the surroundings and then gradually released into the air increasing it’s temperature. The air temperature is sensed by the control system (thermostat) which operates the cooling system and equipment. So there is a time-delay in the corrective action also.
To allow for the time delay due to thermal storage, Cooling Load Factors (CLF) were developed to estimate the heat gains from internal heat emitting sources. CLFs are based on the time (hour) when the internal source starts to generate heat load and the number of hours it remains in operation. This information is expressed as hourly internal load profiles (percent of design).
PEOPLE (P)
| Q-ps | = N-p * Fu * qs * CLF-h (sensible heat gain |
| Q-pl | = N-p * Fu * ql (latent heat gain) |
| Q-ps | = Sensible Heat Gain (SHG) from people |
| Q-pl | = Latent Heat Gain (LHG) from people |
| N-p | = Number of people (maximum or design from occupancy criteria for building) |
| Fu | = Diversity factor or percentage of maximum design for each hour of the day = 0 when there are no people in the room = 1 when the maximum design number of people are in the room 0 <= Fd <= 1 |
| qs | = sensible heat gain (SHG) per person for the degree or type of activity in the space (ASHRAE Table 8.18 ). Examples, 245 btu/hr per person when working in an office and 580 btu/hr per person performing heavy manual work in a factory. |
| ql | = latent heat gain (LHG) per person for the degree or type of activity in the space (ASHRAE Table 8.18 ). Examples, 155 btu/hr per person when working in an office and 870 btu/hr per person performing heavy manual work in a factory. |
| CLF-h | = Cooling Load Factor (CLF) for given hour. This depends on zone type, hour entering space, and number of hours after entering into space (ASHRAE Table 8.19). |
The sensible heat has to be first absorbed by the surroundings and then released into the air. The cooling load factor accounts for this time delay. The latent heat is an instantaneous cooling load so there is no cooling load factor associated with it. The following table gives examples of SHG and LHG from people. It demonstrates the range of heat gain values due to people. When this information is combined with design space occupancy density (25 ft2/person for an aerobics class and 250 ft2/person for an apartment) the heat gain from people becomes very significant.
Figure ??
Example
The following example is for the 5 zones of a rectangular building where the solar heat gain (24oN latitude) for each zone peaked at different hours. Assume that the design number of people enters the space at 8:00 AM and remains in the space until 6:00 PM (10 hours). In reality the number of people per hour will vary and this must also be taken into account.
Building Envelope (Walls and Windows) Cooling Loads
Cooling Load Factors for People
LIGHTS (L)
| Q-l | = (W * 3.412) * Fu * Fs * CLF-h (sensible heat gain) |
| Q-l | = Sensible Heat Gain (SHG) from lights |
| W | = Lighting power output in Watts (Btu/hr = W * 3.412) |
| Fu | = Usage factor or percentage of maximum design for each hour of the day = 0 when all lights are off = 1 when the maximum design number lights are on 0 <= Fu <= 1 Example Fu = 0.5 when 50% of lights are on. |
| Fs | = Service Allowance Factor or Multiplier (accounts for ballast losses in fluorescent lights and heat returned to return air ceiling plenum in the case of air-light fixtures) |
| CLF-h | = Cooling Load Factor (CLF) for given hour. This depends on zone type, total hours that lights are on, and number of hours after lights are turned on, |
Example
The following example is for the 5 zones of a rectangular building where the solar heat gain (24oN latitude) for each zone peaked at different hours.
Building Envelope (Walls and Windows) Cooling Loads
Cooling Load Factors for Lights
EQUIPMENT (E)
Equipment consists of three categories
- electric resistance sensible load (ex. Toaster)
- electric inductive sensible load (ex. Motor)
- sensible and latent loads (ex. Electric or Gas Tea Kettle)
Equipment Sensible Heat Gain
| Q-es | = (W * 3.412) * Fu * Fp * CLF-h (sensible heat gain from electric resistance, btu/hr) |
| Q-es | = 2545 * ( HP / Eff ) * Fu * Fp * CLF-h (sensible heat gain from electric motor, btu/hr) |
| W | = Equipment output in Watts (Btu/hr = W * 3.412) |
| Fu | = Usage factor or percentage of maximum design for each hour of the day = 0 when all equipment are off = 1 when the maximum design number equipment are on 0 <= Fu <= 1 Example Fu = 0.5 when 50% of equipment are on. |
| Fp | = Part load operating factor for motor type. Example, a compressor operating at 50% capacity might still use 80% of electric power. |
| HP | = Rated electrical horsepower of equipment motor (Btu/hr = HP * 2545) |
| Eff | = Motor Efficiency |
| CLF-h | = Cooling Load Factor (CLF) for given hour. This depends on zone type, total hours that lights are on, and number of hours after lights are turned on, |
The sensible heat has to be first absorbed by the surroundings and then released into the air. The cooling load factor accounts for this time delay. The heat generated by a motor is a cooling load only if the motor is located inside the air-conditioned space or in the ducted supply air stream and it is not directly exhausted away from the source.
Equipment Latent Heat Gain
| Q-el | = Mw * Hfg * Fu * Fp |
| Q-el | = latent heat gain from equipment (btu/hr) |
| Mw | = Mass (lbs) of water converted to steam (evaporated or boiled) |
| Hfg | = Heat (btu/hr) required to convert 1 lb of water to steam = 1075 at standard conditions |
The latent heat from equipment such as tea kettles and dish-washers is an instantaneous cooling load. Cooling Load Factors (CLF) do not apply to latent loads.
Usage Factors (Fu)
These are also referred to as operating schedules or profiles
Occupancy Schedule (Profile)
Lighting Schedule (Profile)
Equipment Schedule (Profile)
EXAMPLE (Internal Heat Gains)
Building Type: Factory
Dimensions: 600' Long, 300' Wide, 30' High. Area = 18,000 ft2, Volume = 540,000 ft3
Zone Types
| Heat Source | Solar | People | Lights | Equipment |
| Zone Type | A | C | D | C |
Occupancy Criteria
Occupancy density = 1 Person per 100 ft2 (100 ft2/P)
1/3 of occupants performing office work (seated). 250 SHG/P 200 LHG/P
2/3 of occupants performing factory work (heavy). 600 SHG/P 900 LHG/P
All occupants enter space at 8:00 AM and leave at 6:00 PM (10 hours total).
Occupancy Profile: 70% at 9:00 AM, 90% at 2:00 PM, 80% at 5:00 PM
Lighting Criteria
| 2000 | fluorescent 48" x 24" light fixtures with two 40 watt lamps per fixture. Lighting ballast factor = 1.2. |
| 1000 | fluorescent 48" x 24"return air light fixtures with two 40 watt lamps per fixture. 30% of light heat returned to ceiling plenum. Lighting ballast factor = 1.2. |
| 300 | incandescent 100 watt light fixtures. |
| Lighting Profile: 90% at 9:00 AM, 100% at 2:00 PM, 90% at 5:00 PM | |
Equipment Criteria
| 50000 | watts of miscellaneous electrical (plug in receptacle) loads |
| 100 | five (5) horsepower (HP) pieces of equipment (drills, etc.) |
| 1500 | lbs/hr of steam generated by various processes. |
| Equipment Profile: 50% at 9:00 AM, 80% at 2:00 PM, 60% at 5:00 PM | |
Calculate:
The Sensible and Latent heat gains from People, Lights and Equipment at:
(1) 9:00 am (2) 2:00 pm (3) 5:00 pm
Heat Gain from People
| Q-ps | = N-p * qs * Fu * CLF-h | (sensible heat gain) |
| Q-pl | = N-p * ql * Fu | (latent heat gain) |
No. of people = 180,000 ft2 / 100 ft2 / Person = 1,800 people
| Office: | No. of people at office work = 1800 * 1/3 = 600 = Np |
| SHG = 600 people * 250 btuh/person (qs) = 150,000 btuh = ( Np * qs ) | |
| LHG = 600 people * 200 btuh/person(ql) = 120,000 btuh = ( Np * ql ) | |
| Factory: | No. of people at factory work = 1800 * 2/3 = 1200 = Np |
| SHG = 1200 people * 600 btuh/person (qs) = 720,000 btuh = ( Np * qs ) | |
| LHG = 1200 people * 900 btuh/person (ql) = 1,080,000 btuh = (Np * ql) |
Total SHG = 150,000 + 720,000 = 870,000
Total LHG = 120,000 + 1,080,000 = 1,092,000
Zone Type = C (given). Total Hours in space (8:00 AM to 6:00 PM) = 10
| 9:00 AM | 2:00 PM | 5:00 PM | |
| Fu (fraction of max people) | 0.7 | 0.9 | 0.8 |
| LHG = 1,092,000 * Fu | 764,000 | 982,800 | 873,600 |
| CLF (Zone-C) | 0.62 | 0.86 | 0.92 |
| SHG = 870,000 * Fu * CLF-h | 377,580 | 673,380 | 640,320 |
Q-l = (W * 3.412) * Fu * Fs * CLF-h (sensible heat gain)
Zone Type = C (given). Total Hours that lights are ON (8:00 AM to 6:00 PM) = 10
Fu (fraction of max) and CLF vary with time. Fs is constant for given fixture.
(1) Fluorescent Fixtures (Fu and CLF not considered)
2000 fixtures with two 80 watt lamps. 2000 * 80 * 2 = 320,000 watts. Ballast Factor = 1.2 (Fs)
Sensible Heat Gain = W * Fs * 3.41 = 320,000 * 1.2 * 3.41 = 1,309,440 btu/hr.
(2) Air-Light Fluorescent Fixtures (Fu and CLF not considered)
1000 fixtures with two 40 watt lamps. 1000 * 40 * 2 = 80,000 watts
30% of heat to ceiling plenum, 70% (0.7) to space. Ballast Factor = 1.3. Fs = 1.3 * 0.7 = 0.91.
Sensible Heat Gain = W * Fs * 3.41 = 80,000 * 0.91 * 3.41 = 248,248 btu/hr.
(3) Incandescent Fixtures (Fu and CLF not considered)
300 fixtures at 100 watts each. 300 * 100 = 30,000 watts. Fs = 1.0 for incandescent
Sensible Heat Gain = W * 3.41 = 30,000 * 3.41 = 102,300 btu/hr.
| 9:00 AM | 2:00 PM | 5:00 PM | |
| Fu (fraction of max usage) | 0.9 | 1.0 | 0.9 |
| CLF (Zone-D) | 0.68 | 0.84 | 0.88 |
| (1) Fluorescent | 801,377 | 1,099,930 | 1,037,076 |
| (2) Air-Light Fluorescent | 151,928 | 208,528 | 196,612 |
| (3) Incandescent | 62,608 | 85,932 | 81,022 |
| TOTAL | 1,015,913 | 1,394,390 | 1,314,710 |
Heat Gain from Equipment
Q-es = (W * 3.412) * Fu * Fp * CLF-h (sensible heat gain from electric resistance, btu/hr)
Q-es = 2545 * ( HP / Eff ) * Fu * Fp * CLF-h (sensible heat gain from electric motor, btu/hr)
Q-el = Mw * Hfg * Fu * Fp (latent heat gain from equipment, btu/hr)
Zone Type = C (given). Total Hours equipment is ON (8:00 AM to 6:00 PM) = 10
Fu (fraction of max), Fp (part load efficiency) and CLF vary with time.
(1) Miscellaneous electrical (resistance) loads
50,000 watts. Sensible Heat Gain = 50,000 * 3.41 = 170,500 btu/hr.
(2) Motors (inductive) loads
100 five HP motors. Sensible Heat Gain = 100 * 5 * 2545 = 1,272,500 btu/hr.
(3) Steam
1500 lbs/hr. Latent Heat Gain = 1,500 lbs/hr * 1075 btu/lb = 1,612,500 btu/hr.
| 9:00 AM | 2:00 PM | 5:00 PM | |
| Fu (fraction of max usage) | 0.5 | 0.8 | 0.6 |
| CLF (Zone-C) | 0.62 | 0.86 | 0.92 |
| (1) Miscellaneous (resistance) | 52,855 | 80,705 | 89,001 |
| (2) Motors (inductive) | 394,475 | 875,480 | 664,245 |
| TOTAL (sensible) | 447,330 | 956,185 | 753,246 |
| (3) Steam (latent) | 499,875 | 1,109,400 | 841,725 |
Summary (Internal Cooling Loads)
| Sensible Heat Gain (SHG) | 9:00 AM | 2:00 PM | 5:00 PM |
| People | 377,580 | 673,380 | 640,320 |
| Lights | 1,015,913 | 1,394,390 | 1,314,710 |
| Equipment | 447,330 | 956,185 | 753,246 |
| Total SHG | 1,840,823 | 3,023,955 | 2,708,276 |
| Latent Heat Gain (LHG)) | 9:00 AM | 2:00 PM | 5:00 PM |
| People | 764,000 | 982,800 | 873,600 |
| Equipment | 499,875 | 1,109,400 | 841,725 |
| Total LHG | 1,263,875 | 2,092,200 | 1,715,325 |
| Tons = (SHG + LHG) / 12,000 | 258.7 | 426.3 | 368.6 |
