1. Introduction
Air movement can play an prominent role in the thermal comfort of man and beast. A zephyr on a humid summer day can make a considerable variation to one's thermal comfort. Up-to-date strategies for enhancing energy-efficiency in structure exertion to take list of the cooling effects of air movement from natural ventilation. When the building envelope is ended for air conditioning, local air movement is kept below 40 ft/min. This ignores the selection of increased air movement to cut the cooling power in air conditioned space. This paper explores opportunities for saving power by utilizing the effects of indoor air movement.
2. Cooling power savings in air conditioned space from elevated air speed
The current edition of Ansi/Ashrae thorough 55-2004 Thermal Environmental Conditions for Human Occupancy (Ashrae, 2004), provides for little increases of summer thermostat climatic characteristic settings by increased local air speed. Figure 1 is derived from Figure 5.2.3 in the thorough 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 individual differences in beloved air speed
requires that occupants have personal operate of air speed in increments of 30 ft/min.
The thorough states that it is thorough to interpolate in the middle of these curves. Air speed is more sufficient 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 Figure 5.2.3 of the Standard. The "18°C" should read "18°F" and there is a scaling error in the middle of the fpm and m/s scales.
Five cut off curves are in case,granted to adapt climatic characteristic differences of -18°F, -9°F, 0.0°F, +9°F, and +18°F in the middle of mean radiant temperature, tr , and mean dry bulb air temperature, ta. The writer fitted equations to the quantum of the curves little 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 effect limits for these equations fitted to curves in Figure 5.2.3 in the thorough 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 growth 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 effect 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 consist of spaces with poorly insulated windows, walls or ceilings where the outer covering 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 growth 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 effect 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 growth 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 thorough 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 effect of air movement up to 600 fpm in warm climate 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 growth 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 growth 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 growth 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 power 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 general comfort while saving air conditioning operating cost. On the basis of the Exeloncorp (2005) recommendation, an growth in the thermostat setting of 4.7ºF would furnish cooling power savings from 14% to 19%. In gymnasia where higher air movement is thorough the savings from a thermostat growth 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 variety of climate zones (Byrne and Huang, 1986)
5. Comparison of fans and room air conditioners
A detailed comparison of the power required to declare 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 accomplish 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 health 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 in the middle of the underside of the roof and the covering of the roof that increases heat losses 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 warehouse with a 30 ft high ceiling, the seasonal heating power savings from sufficient destratification is colse to 20% to 30%. To be sufficient 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 sufficient 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 accomplish sufficient circulation. Jets from ceiling fans have an sufficient throw of 5 to 6 diameters.
In large structure with high ceilings such as churches, market structure 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 distinguished for destratification. One 24 ft diameter market 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 colse to 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 curious fans are virtually silent.
7. Estimating Destratification power Savings
A recommended recipe for estimating heating power savings from destratification is to determine the lumped seasonal heat transfer rate for the building envelope and determine the variation in heat loss before and after destratification (Pignet and Saxena, 2002).
The lumped seasonal heat transfer rate for the building envelope in Watts can be calculated using:
A x U = qbd / (ti -to) (7)
Where: A is the covering area of the building envelope in ft2; U is the lumped heat transfer coefficient for the building envelope in Btu/ft2.h.ºF; qbd is the rate of heat loss through the building envelope in Btu/h before destratification; and ti -to is the mean heating season indoor to outdoor air climatic characteristic variation in ºF.
The total heat lost from the building 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 carefully from the fuel bills for the season, the caloric value of the heating fuel and the theory efficiency. The caloric value of natural gas is colse to 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 colse to 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 general way as set out in Hvac handbooks (Ashrae, 2005).
With the thorough heat loss U x A for the heating season before destratification determined, the reduction in heating after destratification, qad can be carefully 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 building envelope in Btu/hr.ft2.ºF ; A = covering area of the building envelope, ft2; tibd = Heating season mean 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 mean 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 theory and the caloric value of the fuel. The heating fuel cost saving typically in the middle of 20% and 30% is calculated using the unit cost of fuel.
8. Thermal comfort in Non-air Conditioned Space
The Ansi/Ashrae 55-2004 thorough offers a recipe for determining an thorough range of indoor operative climatic characteristic in occupant-controlled, simply conditioned spaces. Occupant-controlled, simply conditioned spaces are defined as spaces where thermal conditions of the space are regulated primarily by the occupants through chance and closing windows. These are spaces with no refrigerated air conditioning, radiant cooling, or desiccant cooling. Fans can be used when natural ventilation does not furnish sufficient air movement.
In such spaces, occupants have distinct 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 generally described as the Adaptive Model (de Dear and Schiller (2001).
Using the adaptive approach, the first step is to determine the mean monthly climatic characteristic for each month of the cooling season for the location. In ventilated structure 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 people can be read of a graph in the thorough 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 during July, toc = 66 + 0.255(83.6 -32)= 79.2 ºF. The thermal comfort zone to satisfy 80% of people in July is then 72.9ºF to 85.5ºF.
Given the long term mean monthly outdoor air climatic characteristic for Houston Tx in July is 83.6ºF, this presents the mean need for a cooling effect from air movement in January of 83.6ºF - 79.2ºF or 4.4ºF to restore the operative climatic characteristic to the norm. The ask now is how much air movement is needed to accomplish a cooling effect of 4.4ºF? Using the data from Khedari et al (2000), for a warm humid climate with a relative humidity of 75% indicates 87 fpm is needed for a 4.4ºF cooling effect.
9. Cooling effects of air movement in simply conditioned spaces
The Us Naval curative Command (1988) in a chapter on relieving heat stress published data on the relative cooling effect of air movement Figure 7. These data do not furnish a quantitative cooling effect but are useful in that they indicate the maximum cooling effect occurs with air movement colse to 1,500 fpm.
In simply conditioned space, there is no operate of humidity. As the cooling effect of air movement in warm environments relates to evaporative cooling from sweating, it has been shown that as humidity increases, the cooling effect of air movement decreases. The reduced cooling effect is much greater in warm humid environments when air movement needed for thermal comfort exceeds 295 fpm, Figure 6 (Khedari et al, 2000). It is prominent to use cooling effect data derived from local climate and cultural conditions. These data will good reflect the thermal comfort expectations of local people taking into list local dress and typical levels of metabolic activity.
A variety of approaches have been taken by researchers to quantify the cooling effects of air movement. Cooling effects of air movement can sufficient in hot arid environments were evaporative cooling of the skin is not encumbered by high humidity (Scheatzle et al, 1989).
Another equation derived from several 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 effect 13.7 ºF. This is equivalent to Khedari et al cooling effect 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 health and growth are much good than those of cows subjected to summer heat stress (Sanford, 2004). during hot summer periods dairy farmers have installed small high speed circulator fans to accomplish 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 single 24 ft diameter fan operating at 42 rpm that uses only 1.6 kW of electrical power while providing the same air movement. Additional cooling can be achieved in drier climate 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 mean velocity of air movement. Olesen (1985) refers to a study by Fanger and Pedersen of the chilling effect of winter draughts. It was observed in the study that the chilling effect of gusting air flow reached a peak colse to a gust frequency of 0.5Hz.
More recently researchers in China (Xia et al,2000) repeated these studies inwarm, humid conditions with temperatures ranging from 79ºF to 87ºF and relative humidity in the middle of 35% and 65%. These experiments showed that the beloved gust frequency for cooling air movement was in the middle of 0.3Hz and 0.5Hz. Practically 95% of subjects beloved gust frequencies below 0.7Hz. Natural breezes and air flow from large low-speed circulator fans have a considerable quantum of their power spectral density colse to this frequency of 0.5Hz. Olesen (1985) suggested the use of an equivalent uniform air velocity, Table 1, to list for this effect but this enhanced cooling effect has not been specifically accounted for in cooling effects of air movement to date.
12. Conclusions
Current air conditioning build 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 build is based on air conditioning heating and cooling loads that ignore the enormous savings to be gained from increased indoor air movement from circulator fans.
Recent Ashrae acceptance of an adaptive thermal comfort model clearly shows that people 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 simply conditioned structure are acceptable, indoor thermal comfort can be achieved with enormous power savings by good utilization of indoor air movement.
The cooling effect of air movement has been well established by a estimate of researchers. There remains a need for Additional explore on the cooling effects of air movement on building 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 explore is needed in both air conditioned and simply 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. Additional explore is needed to build a form which presents data in a way that makes it more absolutely used by engineers to improve power 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 power in winter. This particularly applies to market or market spaces with high ceilings.
References
Ashrae (2005) Ashrae 2005 Handbook of Fundamentals, Ashrae, Atlanta, Ga. Page 26.11.
Ashrae (2004) Ansi/Ashrae thorough 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 power 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
Fountain, M. (1995) An empirical model for predicting air movement beloved in warm office environments. Standards for thermal comfort: Indoor air temperatures for the 21st century. Edited by F. Nicol, M. Humphreys, O. Sykes and S. London, Roaf, E & F Spon. Pp. 78-85.
Hope, P (2003) power efficiency ratings: Implications for the building manufactures in the humid tropics. Scholar in Tropical Architecture dissertation, Australian build of Tropical Architecture, James Cook University, Townsville, Australia, pp. 377.
Khedari, J., Yamtraipat, N., Pratintong, N. And Hinrunlabbh, J. (2000) Thailand ventilation comfort chart. power and Buildings, Vol. 32, pp. 245-249.
Naval curative Command (1988) hand-operated Of Naval Preventive Medicine, chapter 3, page 3-7. Accessible on the internet at:
[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) assessment of power savings due to destratification of air in plants, power Engineering, Vol 99, No. 1, 69-72.
Sanford, S. (2004) power conservation in agriculture: Ventilation and cooling systems for animal housing. University of Wisconnsin Cooperative extension 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 annual seminar 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 seminar on Air Distribution in Rooms. Vol.1, pp. 41-46.
enhance vigor Efficiency - saving vigor With Indoor Air Movement