1. Introduction
Air movement can play an foremost role in the thermal comfort of man and beast. A breeze on a humid summer day can make a significant variation to one's thermal comfort. recent strategies for improving energy-efficiency in buildings exertion to take list of the cooling effects of air movement from natural ventilation. When the construction envelope is done for air conditioning, local air movement is kept below 40 ft/min. This ignores the option of increased air movement to sacrifice the cooling energy in air conditioned space. This paper explores opportunities for rescue energy by utilizing the effects of indoor air movement.
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2. Cooling energy savings in air conditioned space from elevated air speed
The current edition of Ansi/Ashrae appropriate 55-2004 Thermal Environmental Conditions for Human Occupancy (Ashrae, 2004), provides for minuscule increases of summer thermostat climatic characteristic settings by increased local air speed. Form 1 is derived from Form 5.2.3 in the appropriate 55-2004.
The curves of equal heat loss from the skin for combinations of operative climatic characteristic and air movement are referenced to the upper limit of the comfort zone (Pmv= +0.5). Limits of 160 fpm and 5.4ºF are set for sedentary activity, 1.0 to 1.3 met. Large private differences in preferred air speed
requires that occupants have personal control of air speed in increments of 30 ft/min.
The appropriate states that it is appropriate to interpolate between these curves. Air speed is more effective at offsetting increases in climatic characteristic when mean radiant climatic characteristic is greater than the mean dry bulb air temperature.
It should be noted that there are two errors in Form 5.2.3 of the Standard. The "18°C" should read "18°F" and there is a scaling error between the fpm and m/s scales.
Five cut off curves are provided to adapt climatic characteristic differences of -18°F, -9°F, 0.0°F, +9°F, and +18°F between mean radiant temperature, tr , and mean dry bulb air temperature, ta. The writer fitted equations to the quantum of the curves minuscule to sedentary performance of 160 fpm and 5.4°F for 1.0 met to 1.3 met and 0.5 to 0.7 clo.
The writer also fitted equations to the quantum of the curves for performance beyond the sedentary limits. Cooling succeed limits for these equations fitted to curves in Form 5.2.3 in the appropriate 55-2004 were 300 fpm and 8°F.
2.1 Curve for tr - ta = 0.0 K
For tr - ta = 0.0°F, an air speed of 160 fpm permits a thermostat set point increase of 4.4°F limit for light sedentary performance (1 to 1.3 met) and 0.5 to 0.7 clo.
V = 40 + 6.8”t 1.85 (1)
Where V is the mean relative air speed in fpm and ”t is the cooling succeed in °F.
In most thermostatically controlled air conditioned spaces, wall, ceiling and floor surfaces temperatures are close to air temperature. That is tr - ta = 0°F. Conditions when tr -- ta is not zero comprise spaces with poorly insulated windows, walls or ceilings where the outer face is exposed to direct solar radiation or cold winter conditions.
2.2 Curve for tr - ta = +9°F
For tr - ta = +9°F an air speed of 160 fpm permits a thermostat set point increase of 5.4°F limit for light sedentary performance (1 to 1.3 met) and 0.5 to 0.7 clo.
V = 40 + 1.26”t 2.85 (2)
Where V is the mean relative air speed in fpm and ”t is the cooling succeed in °F.
2.3 Curve for tr - ta = +18°F
For tr - ta = +18°F an air speed of 126 fpm permits a thermostat set point increase of 5.4°F limit for light sedentary performance (1 to 1.3 met) and 0.5 to 0.7 clo.
V = 40 + 1.28”t 2.7 (3)
3. Beyond Sedentary performance limits
The appropriate is not clear on constraints for the portions of the curves up to 89°Fand 300 fpm, beyond the limits set for sedentary activity. Studies have measured the cooling succeed of air movement up to 600 fpm in warm atmosphere conditions (Khedari et al, 2000, Tanabe and Kimura, 1994, and Scheatzie et al, 1989). Air movement higher than 160 fpm is used in air conditioned gymnasia and shopping malls to augment cooling of occupants. The writer has fitted equations to the quantum of the curves for performance beyond the sedentary limits
For tr - ta = 0.0°F an air speed of 300 fpm indicates the thermostat set point increase could be 6.6°F at performance levels higher than 1.3 met.
V = 40 + 2.52”t 2.5 (4)
Limits for Equation 4 are 160 fpm to 300 fpm and 4.4 F to 6.6 F
For tr - ta = +9ºF an air speed of 276 fpm permits a thermostat set point increase of 8ºF at performance levels higher than 1.3 met.
V = 40 + 5.7”t 1.8 (5)
Limits to Equation 5 are 160 fpm to 280 fpm and 5.4ºF to 8ºF .
For tr - ta = +18ºF an air speed of 211 fpm indicates the thermostat set point increase could be 8ºF at performance levels higher than 1.3 met.
V = 40 + 6.3”t 1.59 (6)
Limits for Equation 6 are 132 fpm to 209 fpm and 5.48ºF to 8ºF.
4. Estimating Cooling energy Savings
The electrical Us utility corporation Exeloncorp (2005), suggests that domestic air conditioning cooling costs can be reduced by 3% to 4% for each ºF that the thermostat setting is raised in summer.
Occupants can offset an increased thermostat setting of 4.7ºF by providing 160 fpm of low-cost air flow from circulator fans and enjoy normal comfort while rescue air conditioning operating cost. On the basis of the Exeloncorp (2005) recommendation, an increase in the thermostat setting of 4.7ºF would contribute cooling energy savings from 14% to 19%. In gymnasia where higher air movement is appropriate the savings from a thermostat increase of 8ºF could be from 24% to 32%. A detailed pathology of reduction in residential cooling loads due to air flow was performed for six Us cities in a range of atmosphere zones (Byrne and Huang, 1986)
5. Comparison of fans and room air conditioners
A detailed comparison of the energy required to assert the same thermal comfort in a 141.5 ft2 bedroom in Townsville, Hope (2003), was conducted using a 55 inch diameter residential ceiling fan and a Vf100C Carrier window/wall room air conditioner, sized for the room by engineers at the local distributor. The measured rate of power consumption of a 55 inch diameter ceiling fan operating at its top speed was 0.068kW or 0.48 W/ft2 of floor area. This is 8.7% of the power used by the room air conditioner to perform the same thermal comfort. The rate of power consumption of the window/wall room air conditioner was 0.78 kW, or 5.51 W/ft2 of floor area. This is 11.5 times the power used by the ceiling fan.
6. Destratification
In heated spaces in winter, indoor air tends to stratify with the hottest, less dense, air accumulating under the roof due to the gravity force. This condition creates two problems. Firstly the hottest air is not contributing to the thermal comfort of occupants near floor level, and secondly, it creates a high climatic characteristic variation between the underside of the roof and the face of the roof that increases heat losses straight through the roof.
Destratification is the process of completely mixing indoor so that air climatic characteristic near the floor is the same as the air climatic characteristic under the roof, or no more than 2ºF difference. This is done using circulator fans. In a typical Us distribution storehouse with a 30 ft high ceiling, the seasonal heating energy savings from effective destratification is nearby 20% to 30%. To be effective about one half of the total volume of air in the space needs to be moved from ceiling level to floor level per hour.
To be effective in destratification the fan should be no more than 1 diameter below the ceiling and the jet from the fan must impact on the floor in order to perform effective circulation. Jets from ceiling fans have an effective throw of 5 to 6 diameters.
In large buildings with high ceilings such as churches, commercial buildings or distribution warehouses, a large volume of air needs to be circulated. In order to avoid complaints of drafts from occupants, the local air velocity at head height needs to be kept less than 40 ft/min.
Circulator fans are much more energy-efficient at low speeds, so large diameter, slow moving, fans are well considerable for destratification. One 24 ft diameter commercial ceiling fan operating at top speed of 42 rpm uses 1.67 kW of electrical power but only 0.06 kW operating at 14 rpm its peak efficiency. At 42 rpm this fan delivers nearby 337,700 cfm of air and 76,670 cfm at 14 rpm. An added benefit of operating large fans at low speed compared to smaller fans at higher speeds is the reduction in fan noise. Large slow piquant fans are virtually silent.
7. Estimating Destratification energy Savings
A recommended recipe for estimating heating energy savings from destratification is to determine the lumped seasonal heat change rate for the construction envelope and determine the variation in heat loss before and after destratification (Pignet and Saxena, 2002).
The lumped seasonal heat change rate for the construction envelope in Watts can be calculated using:
A x U = qbd / (ti -to) (7)
Where: A is the face area of the construction envelope in ft2; U is the lumped heat change coefficient for the construction envelope in Btu/ft2.h.ºF; qbd is the rate of heat loss straight through the construction envelope in Btu/h before destratification; and ti -to is the average heating season indoor to outdoor air climatic characteristic variation in ºF.
The total heat lost from the construction is the sum of heat released from furnaces plus heat released in the space from other sources such as lighting, people, machinery or manufacturing processes. The heat released from the furnaces can be thought about from the fuel bills for the season, the caloric value of the heating fuel and the ideas efficiency. The caloric value of natural gas is nearby 1000 Btu/ft3. The time used in these calculations is the heating season associated with the measured fuel consumption.
Forced air furnaces with flues have efficiencies nearby 0.7. Radiant heaters without flues have an efficiency of 0.8. Electrical heaters have an efficiency of 1.0. Heat from other sources is estimated in the normal way as set out in Hvac handbooks (Ashrae, 2005).
With the widespread heat loss U x A for the heating season before destratification determined, the reduction in heating after destratification, qad can be thought about from:
qad = U x A x (tibd - tiad) (8)
Where: qad = Reduced heat load after destratification in Btu/hr; U = Lumped time-averaged heat loss rate for the construction envelope in Btu/hr.ft2.ºF ; A = face area of the construction envelope, ft2; tibd = Heating season average indoor air climatic characteristic before destratification,,°F;; This depends on vertical climatic characteristic profile. This should be measured on site because the shape of the climatic characteristic profile can vary substantially depending on type of heaters, their height above floor level, and how ventilation is provided; tiad = Heating season average indoor air climatic characteristic after destratification, °F. This is taken as the thermostat set point as the indoor air climatic characteristic throughout the space is close to uniform after destratification.
The reduced heating load due to destratification can be converted into a quantity of fuel taking into list the efficiency of the heating ideas and the caloric value of the fuel. The heating fuel cost rescue typically between 20% and 30% is calculated using the unit cost of fuel.
8. Thermal comfort in Non-air Conditioned Space
The Ansi/Ashrae 55-2004 appropriate offers a recipe for determining an appropriate range of indoor operative climatic characteristic in occupant-controlled, naturally conditioned spaces. Occupant-controlled, naturally conditioned spaces are defined as spaces where thermal conditions of the space are regulated primarily by the occupants straight through opening and conclusion windows. These are spaces with no refrigerated air conditioning, radiant cooling, or desiccant cooling. Fans can be used when natural ventilation does not contribute adequate air movement.
In such spaces, occupants have separate expectations of thermal comfort and accept wider ranges of thermal conditions in both winter and summer than occupants of air conditioned spaces. This recipe is intended for climates where mean monthly air temperatures fall in the range of 50°F to 92°F. This recipe is ordinarily described as the Adaptive Model (de Dear and Schiller (2001).
Using the adaptive approach, the first step is to determine the average monthly climatic characteristic for each month of the cooling season for the location. In ventilated buildings without air conditioning, climatic characteristic for operative comfort toc, is based on mean monthly outdoor air climatic characteristic tout, and can be calculated using the following equation (Ashrae, 2005).
toc = 66 + 0.255(tout - 32) (9)
The comfort zone range of operative climatic characteristic to satisfy 80% of acclimatized citizen can be read of a graph in the appropriate or by adding and subtracting 6.3 ºF to the operative comfort temperature.
With a mean daily air climatic characteristic of 83.6ºF in the city of Houston while July, toc = 66 + 0.255(83.6 -32)= 79.2 ºF. The thermal comfort zone to satisfy 80% of citizen in July is then 72.9ºF to 85.5ºF.
Given the long term average monthly outdoor air climatic characteristic for Houston Tx in July is 83.6ºF, this presents the average need for a cooling succeed from air movement in January of 83.6ºF - 79.2ºF or 4.4ºF to restore the operative climatic characteristic to the norm. The question now is how much air movement is needed to perform a cooling succeed of 4.4ºF? Using the data from Khedari et al (2000), for a warm humid atmosphere with a relative humidity of 75% indicates 87 fpm is needed for a 4.4ºF cooling effect.
9. Cooling effects of air movement in naturally conditioned spaces
The Us Naval medical Command (1988) in a lesson on relieving heat stress published data on the relative cooling succeed of air movement Form 7. These data do not contribute a quantitative cooling succeed but are beneficial in that they indicate the maximum cooling succeed occurs with air movement nearby 1,500 fpm.
In naturally conditioned space, there is no control of humidity. As the cooling succeed of air movement in warm environments relates to evaporative cooling from sweating, it has been shown that as humidity increases, the cooling succeed of air movement decreases. The reduced cooling succeed is much greater in warm humid environments when air movement needed for thermal comfort exceeds 295 fpm, Form 6 (Khedari et al, 2000). It is foremost to use cooling succeed data derived from local atmosphere and cultural conditions. These data will good reflect the thermal comfort expectations of local citizen taking into list local dress and typical levels of metabolic activity.
A range of approaches have been taken by researchers to quantify the cooling effects of air movement. Cooling effects of air movement can effective in hot arid environments were evaporative cooling of the skin is not encumbered by high humidity (Scheatzle et al, 1989).
Another equation derived from some studies (Szokolay, 1998) that is widely used for estimating the cooling effects of air movement from 40 ft/min to 400 ft/min is:
”t = 10.8((V/197.85)-0.2)-1.8((V/197.85)-0.2)2 (11)
Where V is in ft/mim and ”t is in ºF.
Using this equation, air movement of 400 ft/min provides a cooling succeed 13.7 ºF. This is equivalent to Khedari et al cooling succeed for 400 ft/min at 57% relative humidity in Thailand.
10. Indoor air movement for livestock
Dairy farmers have learned from university studies that thermally comfortable cows' milk production, reproductive condition and increase are much good than those of cows subjected to summer heat stress (Sanford, 2004). while hot summer periods dairy farmers have installed small high speed circulator fans to perform the recommended air movement of 177 ft/min to 433 ft/min. Ten 36 inch diameter fans operating at 825 rpm use 3.73 kW of electrical energy. Farmers have found they can replace 10 of these 36 inch diameter fans with a particular 24 ft diameter fan operating at 42 rpm that uses only 1.6 kW of electrical energy while providing the same air movement. Added cooling can be achieved in drier atmosphere regions using misting water sprays for evaporative cooling.
11. Discussion
All the descriptions of air movement described so far in this document have referred to the average velocity of air movement. Olesen (1985) refers to a study by Fanger and Pedersen of the chilling succeed of winter draughts. It was observed in the study that the chilling succeed of gusting air flow reached a peak nearby a gust frequency of 0.5Hz.
More recently researchers in China (Xia et al,2000) repeated these studies inwarm, humid conditions with temperatures fluctuating from 79ºF to 87ºF and relative humidity between 35% and 65%. These experiments showed that the preferred gust frequency for cooling air movement was between 0.3Hz and 0.5Hz. Almost 95% of subjects preferred gust frequencies below 0.7Hz. Natural breezes and air flow from large low-speed circulator fans have a significant quantum of their energy spectral density nearby this frequency of 0.5Hz. Olesen (1985) suggested the use of an equivalent uniform air velocity, Table 1, to list for this succeed but this enhanced cooling succeed has not been specifically accounted for in cooling effects of air movement to date.
12. Conclusions
Current air conditioning form provides for uniform air climatic characteristic and humidity throughout a space, with indiscernible local air movement in the occupied zone of less than 40 ft/min. This conventional form is based on air conditioning heating and cooling loads that ignore the mammoth savings to be gained from increased indoor air movement from circulator fans.
Recent Ashrae acceptance of an adaptive thermal comfort model clearly shows that citizen who live in air conditioned houses, drive air conditioned cars, work in air conditioned offices impair their natural thermal comfort adaptation. This impairment results in unnecessarily high summer cooling loads.
Where naturally conditioned buildings are acceptable, indoor thermal comfort can be achieved with mammoth energy savings by good utilization of indoor air movement.
The cooling succeed of air movement has been well established by a number of researchers. There remains a need for Added investigate on the cooling effects of air movement on construction occupants to adapt performance levels beyond 1.3 met, higher air velocities for non-sedentary activity, and lighter clothing levels than 0.5 clo. This investigate is needed in both air conditioned and naturally conditioned spaces.
Research on the cooling effects of air movement has been presented in many forms. The chart produced by Khedari et al (2000) is one the good formats. Added investigate is needed to form a form which presents data in a way that makes it more no ifs ands or buts used by engineers to improve energy efficiency with increased indoor air movement.
The same circulator fans used to improve summer thermal comfort can be used to destratify indoor air to save heating energy in winter. This particularly applies to commercial or commercial spaces with high ceilings.
References
Ashrae (2005) Ashrae 2005 Handbook of Fundamentals, Ashrae, Atlanta, Ga. Page 26.11.
Ashrae (2004) Ansi/Ashrae appropriate 55-2004 Thermal Environmental Conditions for Human Occupancy. Ashrae, Atlanta, Ga.
Byrne, S. And Huang, V.(1986) The impact of wind-induced ventilation on residential cooling load and human comfort. Ashrae Trans. Vol.92, Pt. 2, 793-802.
de Dear, R. And Schiller Brager, G. (2001) The adaptive model for thermal comfort and energy conservation in the built environment. Int. J. Biometeorology, 45: 100-108.
Exeloncorp (2005) Controlling Temperatures is accessible on the internet at:
http://www.exeloncorp.com
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Khedari, J., Yamtraipat, N., Pratintong, N. And Hinrunlabbh, J. (2000) Thailand ventilation comfort chart. energy and Buildings, Vol. 32, pp. 245-249.
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[http://www.vnh.org/PreventiveMedicine/Pdf/P-5010-3.pdf]
Olesen, B. (1985) Local thermal discomfort. Bruel & Kjaer Technical Review, No.1, Denmark, pp.3-42.
Pignet, Tom and Saxena, Umesh (2002) estimation of energy savings due to destratification of air in plants, energy Engineering, Vol 99, No. 1, 69-72.
Sanford, S. (2004) energy conservation in agriculture: Ventilation and cooling systems for animal housing. University of Wisconnsin Cooperative prolongation publication A3784-6, pp.3.
Scheatzle, D., Wu, H. And Yellott, J.(1989) Extending the summer comfort envelope with ceiling fans in hot, arid climates. Ashrae Trans. Vol.100, Pt. 1, 269-280.
Szokolay, S. (1998) Thermal comfort in the warm-humid tropics, Proceedings of the 31st yearly conference of the Australian and New Zealand Arch. Science Association, Uni. Of Queensland., Brisbane, Sept.29-Oct.3, pp. 7-12.
Tanabe, S and Kimura, K. (1994) significance of air movement for thermal comfort under hot and humid conditions. Ashrae Trans. Vol. 100, Pt. 2, 953-969.
Xia, Y., Zhao, R. And Xu, W. (2000) Human thermal sensation to air movement frequency. Reading, Uk. Proceedings of the 7th International conference on Air Distribution in Rooms. Vol.1, pp. 41-46.
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