4146
Rates of Evaporation from Swimming Pools in Active Use
Charles C. Smith, P.E.
George O.G. Löf, D.Sc., P.E.
Member ASHRAE
Fellow ASHRAE
ABSTRACT The rates of water evaporation from indoor and outdoor swimming pools in active use have been measured and compared with evaporation rates from unoccupied pools and with values calculated by the equation W=(95+0.425 v) (pwpa)Y, where W is evaporation rate, lb/h ft2; v is air velocity at water surface, ft/min.; pw is saturation vapor pressure at water temperature, in. Hg; pa is saturation vapor pressure at air dewpoint, in. Hg; and Y is latent heat at pool temperature, Btu/ lb. In undisturbed pools, evaporation rates were measured and found to be 74% of the rates obtained by use of the equation. Rates of evaporation from pools in active use increase with the number of swimmers, rising 40% to 70% above the rates from a quiet water surface. Measurements of evaporation from a pool in use by 15 to 20 swimmers per 1,000 ft2 were found to average 26% higher than the rate calculated by the equation.
ification system. Heat losses from outdoor pools are also largely by evaporation, but radiation and convection to the surroundings are typically 30% to 40% of the total loss. Prior to the current investigations, there have been no measurements of energy supply to swimming pools under well-controlled conditions. Equipment designers have commonly relied on a relationship originally formulated by W. H. Carrier (1918) and presented in ASHRAE Applications (1995, 1991, 1987). The equation is W = ( 95 + 0.425 v ) ( p w – p a ) ⁄ Y
where W v pw pa
INTRODUCTION The design of equipment for heating water in indoor and outdoor swimming pools and for heating ventilation air in indoor pools requires reliable information on rates of heat loss from the pools. Such information is also needed for predicting energy quantities and costs and for estimating the savings obtainable by use of energy conservation measures. Proper sizing of water heaters, air heaters, ventilation fans, heat exchangers, dehumidification systems, and numerous accessories and the evaluation of heat recovery systems, pool covers, and other energy saving equipment are directly involved. In indoor pools, virtually all the heat supplied to the pool water is dissipated to air in the natatorium by evaporation. Radiation and convection transfers are usually negligible. Moisture entering the air must be removed either by ventilation (requiring fresh-air heating when outdoor temperatures are appreciably below 80°F) or by condensation in a dehumid-
Randy W. Jones, P.E.
Y
(1)
evaporation rate, lb/h⋅ft2; air velocity at water surface, ft/min.; saturation vapor pressure at water temp, in. Hg; saturation vapor pressure at air dew point., in. Hg; also partial pressure of water in pool atmosphere; = latent heat at pool temperature, Btu/lb. = = = =
This formula was based on the results of measurements of evaporation from a shallow pan of water over which air was passed in a wind tunnel. Water losses were correlated with vapor pressures, humidities, and air velocity. Investigations of evaporation from open outdoor tanks by Rohwer (1931), from outdoor Florida pools by Root (1983), from five outdoor pools in Switzerland by Molinaux et al. (1994), and from measurement of condensate recovery from dehumidifier systems in German pools by Labohm (1971), Biasin and Krumme (1974), and Reeker (1978) have produced widely differing results. Variations in test conditions, uncertain measurement accuracy, and departures from typical pool designs have prevented significant use of any of these findings, thus leaving the ASHRAE relationship generally used for estimating pool evaporation and the requirements for heating and ventilation.
Charles Smith is a research scientist and George Löf is professor emeritus and founding director of the Colorado State University Solar Energy Applications Laboratory, Fort Collins, Colo. Randy W. Jones is a federal energy program specialist with the U.S. Department of Energy, Golden, Colo.
514
ASHRAE Transactions: Research
There has recently been disagreement, however, on the pool conditions to which the equation applies. In the 1987 ASHRAE Handbook, the equation is stated to represent evaporation from a quiet pool surface (Carrier), increasing as much as 50% when in active use. But in the updated ASHRAE Handbook (1991), without supporting evidence or explanation, the equation was stated to apply to “public pools at high to normal activity” and that “other pool uses may have up to 50% less moisture evaporation.” In the 1995 ASHRAE Handbook, the equation is reported to apply directly to pools “at normal activity;” “other pool uses may have more or less evaporation.” The lack of data on rates of evaporation from swimming pools, and the need for that information for equipment design and energy requirements, has stimulated a series of four Figure 1 Rate of evaporation from quiet indoor pool based on level evaporation measurement programs in institu(water loss) measurements and energy input measurements tional pools. In the order of performance, these compared with rate computed by Equation 1 as a function of tests have been conducted on (1) an unoccupied water vapor pressure difference. (Adjusted for pump energy indoor pool, (2) an unoccupied outdoor pool, and heat losses other than evaporation.) (3) an indoor pool in active use, and (4) an velocity varied enough for quantifying its effect on evaporaoutdoor pool in active use. In each of these projects, the two tion rate. The velocity coefficient, 0.35 (Equation 2) is based vapor pressures in Equation 1 were determined by measureon the results of the outdoor pool tests. During two periods ment of water and air temperatures and air humidity over time when no heat was supplied to the pool heater, water temperaintervals ranging from 2 hours to 68 hours. Airflow rate was ture decreases were used in heat balances, which showed 56% also measured. Evaporation from the unoccupied outdoor pool of total energy loss was by evaporation, 26% by radiation, and was determined by measurement of water loss rate, whereas 18% by convection. water loss and heat supply rate were both measured in the tests The determination of evaporation rates from pools in on the unoccupied indoor pool. The results of the indoor invesactive use was made by measuring the rate of change of water tigation have been published by Smith et al. (1993), and the level and by evaluating the heat loss rate by measuring the outdoor measurements and results were presented by Jones et decrease in water temperature when there were no heat addial. (1994). tions to the pool. Correlation of these evaporation rates with Rates of evaporation from the inactive indoor pool deterthe number of people in the pool provided the primary data for mined by the measurement of water level change during evaluating the influence of pool activity on evaporation rates extended time intervals are in satisfactory agreement with and energy losses. values based on the measurement of heat supply rates during Equation 1 shows the importance of vapor pressures in the same period. Figure 1, based on results by Smith et al. controlling pool evaporation rates. Reduction in water temper(1993), shows both sets of data, and for comparison, the result ature and maintenance of higher natatorium dewpoint, i.e., of using the Carrier/ASHRAE Equation 1 at the measured higher air temperature and relative humidity, can minimize the pool conditions is shown. The final evaporation rate equation vapor pressure gradient and evaporation rate, but conditions for a quiet indoor pool, based on level change measurements must be acceptable to swimmers and spectators. Relative and adjusted to apply to altitudes less than 1000 ft above sea humidity appreciably above 50% is not only uncomfortable level, is but can cause corrosion and structural damage by excessive (2) W = ( 69 + 0.35 v ) ( p w – p a ) ⁄ Y . condensation on cooler surfaces. Dewpoint control by regulation of ventilation air supply, exhaust fan use, and/or dehuEvaporation rates for the quiet pool computed by the use of midifier operation is, therefore, essential. Both indoor pools Equation 2 are 74% of those obtained by the use of Equation involved in these evaporation studies had automatic control of 1, the “ASHRAE equation.” air dewpoint, thereby minimizing vapor pressure variations. Rates of evaporation from the inactive outdoor pool, The facility descriptions, procedures, results, and concludetermined by the measurement of water level change (Jones sions for tests on indoor and outdoor pools in active use follow. et al. 1994) differed less than 2% from those in the indoor pool at comparable conditions. In contrast with the indoor tests, air
ASHRAE Transactions: Research
515
TEST FACILITIES Indoor Pool A municipal facility in Fort Collins City comprises three pools; a 1,200 ft2 wading/play pool, a 900 ft2 therapy pool, and a 13,000 ft2 athletic/fitness pool (Figure 2). The three pools are mechanically independent, having separate water recirculation, heating, chemical treatment, and make-up water systems. The pools share the same natatorium space and equipment area. The large athletic pool selected for this study has a total water volume of 526,000 gallons (4.38 million pounds). The pools were open to swimming and other activities each day for 8-12 hours. The large pool served a number of activities at one time, such as swimming, diving, and aquatic exercise. The number of people in the pool varied from 1-2 and up to more than 150. Pool-water temperatures were thermostatically controlled normally at 80°F - 82°F. The room air was normally at 85°F and 50% relative humidity. Automatic humidity control regulated the supply of fresh air and the operation of exhaust fans. The entire complex is served by the same heating equipment, so fuel used specifically for pool heating could not be measured.
Figure 3 The 4,000 ft2 outdoor community pool. indicate energy input is approximately 8 million Btu per day without covering and 5.5 million Btu per day when covered for about 12 hours overnight. The outdoor pool activity was similar to that in the large indoor pool. This pool was open to all types of activity for 45 minutes per hour and then limited only to swimming for 15 minutes. The pool water in both the indoor and outdoor facilities is circulated continuously by conventional means through sand filters, chlorinators, and natural gas-fired boilers.
Outdoor Pool The site for testing the outdoor pool in active use was the same as used earlier for the inactive pool tests. The pool is operated by a neighborhood association and is open for approximately three months in the summer. Its total surface area is 4125 ft2 and contains 144,000 gallons of water (1.2 million pounds). Buildings, trees, and fences are set back at least 20 ft, so the pool is relatively open to wind and solar radiation exposure (Figure 3). Radiation losses from the pool are directly to the sky. The pool is maintained at temperatures near 83°F by a thermostat in the return water line. Natural-gas billing records
EXPERIMENTAL PROCEDURE Measurement of Temperatures, Humidity, and Air Velocity The rate of evaporation from a water surface is proportional to the difference between the vapor pressure of the liquid water and the partial pressure of water vapor in the immediately adjacent air. Determination of these two quantities requires the measurement of water temperature, air temperature, and air humidity (or dew point). Air and water temperatures were measured with calibrated T-type thermocouples, with voltage recorded at six-minute intervals by use of a desktop computer-controlled data-acquisition unit. Differences between sensors and between repeated measurements with the same sensor did not exceed 0.1°F. Air humidity was obtained by monitoring the dew point temperature with a dew-point hygrometer. This instrument was calibrated against a secondary dew-point temperature standard immediately prior to use. The limit of departure of 0.2°F corresponds to a humidity difference of approximately 0.6%. Outdoor wind speed was obtained by the use of a rotating cup anemometer located at the edge of the pool, 1 ft above the water surface. Determination of Evaporation Rate
Figure 2 The 13,000 ft2 indoor athletic/fitness pool used for activity testing.
516
Two methods have been used for determining evaporation rates. One procedure involves the precise measurement of the change in water level during an extended period when no ASHRAE Transactions: Research
water additions or liquid losses occurred. The other method is based on water temperature measurements during a time interval when heat additions and losses are either measured or calculated. An energy balance then indicates heat dissipation by evaporation and the quantity involved. In an inactive pool, the change in water level is an exact measure of evaporation. In an active pool, however, splashing onto deck areas, water running over pool rims into drains, and water carried out on the wet skins of swimmers leaving the pool make level changes appreciably greater than that caused by evaporation alone. For that reason, evaporation from the active indoor pool was determined by temperature measurements and energy calculations. In the outdoor pool, doubtful accuracy of computed heat losses other than by evaporation made it necessary to use water level changes for estimates of the evaporation rate. Measurement of Heat Loss Rates The rate of evaporation is determined by measuring pool water-temperature change over an extended time interval, computing enthalpy increase or decrease, adding measured energy gains such as heater inputs, if any, and deducting losses other than by evaporation. The resulting energy quantity is that which was transferred to water vapor escaping from the pool surface at a rate computed by applying the heat of vaporization, 1040 Btu/lb.
Measurement of Water Loss Rates Rates of water loss from the outdoor pool were determined by the measurement of the decrease in pool water level over time intervals of several hours. No water additions occurred in these periods, but small quantities of water were lost by splashing. At a typical hourly evaporation rate of 0.1 lb/ft2 in an outdoor pool, the water level decreases about 0.02 in./h. In order to measure water levels with sufficient accuracy, a micrometer gauge was rigidly mounted to the side of a small basin adjacent to the pool. A hydrostatic tube maintained a water level in the basin identical to that in the pool. A small quantity of salt was added to the water to increase its electrical conductivity. Contact between the water surface and a metal point on the gauge was indicated by closure of an electric circuit. A Vernier scale on the gauge was read to the nearest 0.001 in. If errors at the start and end of a three-hour test are additive, a 0.06 in. change can be determined with an accuracy of about 3%. A photograph of this instrumentation is shown in Figure 4. Shorter intervals were sometimes necessitated by changes in conditions such as the number of pool users and wind velocity. Measurement of Activity in Pool Since there was no regulation of the number of people in the pools during these tests, the activity level was variable.
The heat-loss measurement method was used only for the indoor pool because solar energy gains and large radiative and convective losses from an outdoor pool impose inaccuracies in computing energy differences resulting from evaporation. Water temperatures in the indoor pool were measured during periods when heaters were shut down. Calibrated thermocouples in the return lines carrying water from evenly spaced points around the pool perimeter provided measurements with 0.1°F accuracy. With no external source of heat and negligible convective and radiative transfer in the active indoor pool (air and water temperatures are nearly equal), typical rates of temperature decrease of 0.2°F/h to 0.3°F/h result from evaporative heat loss. Considerable and frequent variation in pool occupancy restricted the duration of tests to a few hours; hence, total temperature changes were usually less than 2°F. The accuracy of these results is discussed below. In addition to evaporative heat loss, there was an estimated steady heat loss from the heat exchanger and pipework of 0.1 Btu/h⋅ft2 pool surface and a steady energy addition of 3.9 Btu/h⋅ft2 pool surface resulting from recirculation pump work. Evaporative heat loss was, therefore, determined by adding 3.8 Btu/h⋅ft2 to the measured hourly enthalpy decrease. ASHRAE Transactions: Research
Figure 4 Precision water level gauge (micrometer) to monitor pool water loss. 517
The method for activity measurement was to count people in the pool, regardless of the type of activity, each 15 minutes during test periods. The sum of the 15-minute counts over the testing period was divided by the number of periods, thus giving the approximate average number of people using the pool during the test. The water surface disturbance caused by different types of activity occurring at the same time could not be quantified. However, it was assumed that the combination of all activity effects upon evaporation was consistent with time. Activity in and around a pool causes disturbance of the water/air interface and creates an additional wet surface when people leave the water or otherwise remove water as liquid. Surface disturbances increase the mass-transfer coefficient and the water surface area. Six-inch waves at 3 ft wave intervals have about 20% more surface area than smooth water. Random turbulence causes further increase in surface area. Energy for the pool surface component of evaporation is supplied by pool water heating, whereas water evaporating from other surfaces within the pool enclosure requires energy from the ventilationair heat source, rather than from the pool water heater. EVAPORATION RESULTS—INFLUENCE OF POOL ACTIVITY Indoor Pool
rate from a quiet water surface when numerous swimmers are using the pool. Figure 5, based on the data in Table 1, shows evaporation rates increasing in proportion to pool occupancy. Measured temperatures and humidities were used with psychrometric data for 5000 ft elevation to obtain vapor pressures, which were then used in Equation 2, multiplied by an altitude correction (0.98) to obtain evaporation rates for the unoccupied pool. Actual evaporation divided by those computed values provides the ratios shown in Figure 5. The average departure of the measured ratio values from the regression line is 0.05, which represents ±3.8% in the range of 1 - 15 persons/1000 ft2. A few observations (not shown) of pool occupancy as high as 20 people/1000ft2 indicate an approximate upper limit on activity effects corresponding to the 15 people /1000 ft2 count. In a typical institutional pool 40 ft × 75 ft (3000 ft2), about 50 swimmers would correspond to the upper extreme of activity measured in these tests. As indicated in the discussion of water-temperature measurements, frequent changes in pool activity levels prevented lengthy test intervals and substantial temperature changes from the start to the end of a test. A typical change of 1°F, subject to 0.1°F uncertainty of each measurement, can produce a 20% maximum error in the result of that test. The probable error is, however, about half that figure. The error in a particular temperature measurement falls between 0°F and 0.1°F, or at a probable level of 0.05°F. The probable error in the difference of the two temperatures is also reduced. Only if one measurement is erroneously high and the other erroneously low are the errors additive. If, however, both measurements are, for example, 0.05°F low, the error in the difference is zero. The probable error in the measured temperature change in a particular test, therefore, should not exceed about 0.05°F.
Under active conditions, energy, rather than water loss, is the more reliable gauge of evaporation since water is partially lost by splashing onto deck areas. Water-level measurements were also made for estimating liquid losses but were not used in determining rates of evaporation from the indoor pool. Rates of evaporation from the active indoor pool, determined by calculations based on measured water-temperature decreases, are shown in Table 1. Also tabulated are vapor pressure differences, test durations, the average count of pool users, and, by the use of Equation 2 adjusted for the 5000 ft elevation of the site, the calculated rates of evaporation from an inactive pool at the same conditions of temperature and humidity. Nearly equal water and air temperatures make convective heat transfer negligible, and radiation to or from walls and ceiling is essentially zero. The rate of evaporation is, therefore, equal to the rate of heat loss plus the small gain from pump work, divided by the latent heat, 1040 Btu/lb. Evaporation from the active pool divided by evaporation from an inactive pool at the same conditions is based on the heat-loss measurements. Table 1 shows a strong dependence of evaporation rate on Figure 5 Rate of evaporation from indoor active pool relative to rate from inactive pool as a function of activity level (number of swimmers pool activity, rising 40% to 70% above the per 1000 ft2).
518
ASHRAE Transactions: Research
TABLE 1 Indoor Pool* Test reference number
*
Test duration, Temperature Swimmers/ h loss, °F 1000 ft2
Evaporation heat rate, Btu/h⋅ft2
pw - pa, in. HG
Inactive pool evaporation heat rate, Evaporation ratio, active/inactive Btu/h⋅ft2 (Equation 2)
1
4.1
.69
2.4
49.7
.603
44.0
1.13
2
3.9
.74
7.6
55.7
.521
38.0
1.47
3
3.7
.87
2.6
71.2
.881
64.3
1.11
4
4.2
.91
7.4
64.5
.621
45.3
1.43
5
2.8
.65
7.2
68.8
.692
50.4
1.36
6
9.4
1.43
.7
44.5
.544
39.7
1.12
7
3.2
.63
8.9
58.1
.554
40.4
1.44
8
3.6
.59
0.0
48.9
.633
46.2
1.06
9
13.0
1.67
0.0
37.9
.553
40.3
0.94
10
3.0
.48
1.6
48.2
.570
41.6
1.16
11
2.7
.45
12.7
50.7
.455
33.2
1.53
12
1.1
.25
13.0
69.1
.609
44.4
1.56
13
1.5
.36
12.2
71.0
.591
43.1
1.65
14
.75
.18
13.8
72.4
.585
42.7
1.69
Pool temperature, 81.5°F - 82.5°F; air temperature, 80°F - 83°F; air relative humidity, 45% - 55%.
Except in the tests of less than a two-hour duration, when temperature changes of less than one degree took place, probable errors in heat loss are, therefore, not more than 5%. Periods of high pool activity were of short duration, so evaporation rates under those conditions could not be measured with comparable accuracy. The four points representing those conditions are identified in Figure 5 and the corresponding portion of the graph is indicated by the dashed line. Although not as accurate as the data for less active conditions, the results conform with the trend and extend the results into the high pool occupancy range. The linear regression based on all 14 points is ER=1.04 + 0.046C, and if only the ten lower points are considered, the equation is ER=1.05 + 0.047C. The difference is relatively insignificant, and use of the equation based on all points is recommended. The logical value of the intercept on the evaporation ratio axis is 1.00, but the regression analysis yields 1.04. The discrepancy is due to the fact that water waves caused by even one swimmer in a large pool (0.08 swimmer/1000 ft2 in this 13,000 ft2 pool) result in a significant effect on the water-air interface and an increase in evaporation. Below one swimmer/ 1,000 ft2, the relationship is, therefore, not linear, as indicated by the dotted curve in Figure 5. When a pool is heavily used, (approaching 15 swimmers/ 1000 ft2), natatorium humidity will rise unless heating and ventilating equipment have capacities approximately 70% higher than necessary for an inactive pool. Equation 1 (ASHRAE), with the coefficients traditionally used, yields an ASHRAE Transactions: Research
evaporation rate 1.35 times that from a quiet water surface. Figure 5 shows that this rate is characteristic of a pool being used by about 6 people/1000 ft2. To provide full heating and ventilating capacity of equipment for maximum pool usage, i.e., 70% higher than for a quiet pool, and to use the ASHRAE equation, its result should be multiplied by 1.70 × 0.74 = 1.26. Evaporation from a pool in active use by numerous swimmers is, therefore, about 26% greater than computed by the ASHRAE equation. In summary, the ASHRAE equation in its widely used form shows an evaporation rate characteristic of a pool with about 6 swimmers/1000 ft2 of area. Evaporation from an unoccupied pool is 74% of the rate calculated by the equation; maximum evaporation, useful for equipment design requirements, is 26% higher than obtained from the equation, i.e., 1.26 times that value. Outdoor Pool Results of measurements in the outdoor pool in active use are shown in Table 2. Also tabulated are evaporation rates calculated by Equation 2 for an inactive pool, adjusted for altitude. Ratios of measured evaporation rates to those based on the equation for an inactive pool at the same conditions are also tabulated. Measured wind speeds were used in the equation, so the computed ratios show the specific influence of pool activity on evaporation. Evaporation rates reported in Table 2 are based entirely on measured changes in the pool level over the listed time peri519
TABLE 2 Outdoor Pool
Test Test reference duration, Swimmers/ 1000 ft2 number h
Average water temp. during test period
Measured depth Wind change velocity, (decrease), mph in.
Evap., lb/h⋅ft2
Equiv. evap. heat rate (1045xG), Btu/h⋅ft2
Calculated evap. heat rate from inactive pool at same pw - pa, wind speed, in. HG Btu/h⋅ft2
Evap. ratio, active/ inactive
1
5.3
7.5
82.2
2.2
0.147
0.144
151
0.811
112
1.34
2
4.0
6.3
82.9
1.3
0.091
0.118
123
0.871
96
1.28
3
5.9
6.5
81.7
1.1
0.105
0.092
96
0.721
75
1.29
4
3.4
6.5
83.5
0.5
0.063
0.096
100
0.772
65
1.54
5
3.6
1.9
84.0
1.5
0.058
0.083
87
0.690
80
1.08
6
4.0
1.2
82.7
2.8
0.096
0.125
131
0.809
128
1.03
7
6.5
2.2
81.2
2.0
0.083
0.128
134
0.840
111
1.21
8
4.2
8.0
83.2
1.4
0.082
0.101
106
0.751
85
1.25
9
3.8
3.9
82.0
2.8
0.110
0.150
157
0.822
130
1.21
10
5.6
4.1
81.9
2.9
0.148
0.137
143
0.754
121
1.18
11
4.8
3.9
83.1
2.7
0.136
0.147
154
0.773
120
1.29
12
4.0
4.4
83.0
2.2
0.094
0.122
128
0.668
92
1.40
the adjusted water-loss rates, divided by the rates computed by ods. Measurements were made shortly before and after swimthe use of Equation 2, for an unoccupied pool at the same mers were in the water, thereby avoiding effects of surface temperatures, humidity, and air speed as measured. Wind disturbances on water levels. Water disappearance other than velocity varied over a wide range, but its use in Equation 2 by evaporation is limited to splashing onto deck areas and yields results showing the specific effect of pool activity only. removal on skins of swimmers leaving the pool. It is estimated It is evident that the data points in Figure 7 for the outdoor that these physical water losses are less than 5 gal/h, equivapool are more widely scattered than those for the indoor pool lent to about 5% of the total measured disappearance. No in Figure 5. The “R” value for the outdoor pool data, 0.6448, correction for this estimated loss has been made, so evaporais considerably lower than the 0.9681 value for the indoor tion rates from the outdoor pool may be overstated by an pool. But it is seen that the equations for the best linear fit to amount approaching 5%. the two sets of data are in good agreement. The principal The effects of wind speed and pool activity on evaporation rate are shown graphically in Figures 6, 7, and 8. The data points in Figure 6 show that in active pools, regardless of the number of swimmers, evaporation rates are substantially higher than those in an unoccupied pool, shown by the “no activity” line based on the results of previous tests by Smith et al. (1993). The rates increase rapidly with wind speed. To compensate for differences in temperatures and humidity, the data are presented as evaporation rates per unit difference in water vapor pressure. The scatter of points is due to the wide variation in pool occupancy and the resulting influence on evaporation. In Figure 7, relative evaporation from the outdoor pool is correlated with the number of swimmers. Ordinate values are Figure 6 Effect of wind speed on evaporation from active pool.
520
ASHRAE Transactions: Research
and outdoor pools are in close agreement at very low outdoor wind velocity. In the present study of pools in active use, evaporation ratios (evaporation rates compared with those from inactive pools at the same conditions) are also comparable, slight differences probably resulting from unmeasured water losses. It is concluded that the higher consistency and quality of the indoor measurements support use of those results for the correlation equation, ER=1.04 + 0.046C. CONCLUSIONS Rates of evaporation from indoor and outdoor pools in active use have been determined by measuring rates of heat loss and water level change. These results are consistently higher than those previously obtained in quiet pools, the Figure 7 Outdoor pool evaporation as affected by activity level departure being proportional to the pool activity (swimmers/100 ft2). as represented by the number of users per unit area of pool surface. reasons for the scatter of data on the outdoor pool are the variIn indoor pools, disturbance and motion of the water ation in wind velocity, fluctuation in number of swimmers surface caused by typical swimming activity increase evapoduring a test period, and variable splashing losses. With fewer ration rates to levels approximately 70% higher than those than about 5 swimmers/100 ft2, the outdoor water-loss rate TABLE 3 was found to be slightly higher than the indoor rate (possibly Evaporation Relative to Rate in because splashing influenced the outdoor measurements). Unoccupied Pool and Zero Air Speed With ten swimmers, the highest use of the outdoor pool, the two pools show approximately equal water-loss rates. WIND SPEED - MPH
Combination of Indoor and Outdoor Results Figure 8 is a summary of pool testing results: inactive outdoor pool (line for zero swimmers), active indoor (four intercepts on the zero wind speed ordinate), and active outdoor (three lines for 5, 10, and 15 swimmers /1,000 ft2 of pool area). Figure 8 also shows the range of conditions that were not tested (dotted lines). In indoor pools, where air speeds are negligible, evaporation rates depend only on water and air conditions and the turbulence of the water as indicated by the number of swimmers. Air movement over outdoor pools, even at a comparatively low 3 mph (4.4 ft/sec) velocity has a strong additional effect, roughly doubling the rate of evaporation that occurs in an indoor pool. The combined effect of wind speed and pool activity is indicated in Table 3. The values at zero wind speed are for the indoor pool and at other wind speeds for the outdoor pool.
2
Persons /1000 ft
0
0.5
1
2
3
0
1.00
1.23
1.46
1.93
2.40
5
1.28
1.57
1.87
2.47
3.07
10
1.47
1.81
2.16
2.86
3.55
15
1.665
2.06
2.45
3.24
4.03
Difference in Indoor and Outdoor Evaporation Rates Previously published results by Smith et al. (1993) and Jones et al. (1994) of evaporation measurements in inactive indoor ASHRAE Transactions: Research
Figure 8 Relative evaporation as affected by wind speed and activity level.
521
from quiet water surfaces. Comparable increases are observed in outdoor pools where increased air movement causes additional evaporation losses. At wind speeds of 3 mph, evaporation rates are typically twice the rates from pools in still air. The activity of 15 people/1000 ft2 area of pool over which there is a 3 mph wind results in evaporation rates nearly four times those from an unoccupied pool in still air. Constant humidity can be maintained in an indoor pool being used by 15 to 20 persons/1000 ft2 if the design of heating and ventilation facilities is based on evaporation rates computed by use of the traditional ASHRAE equation, Equation 1, W = (95 + 0.425 v) (pw − pa)/Y, to which a 1.26 multiplier is applied. Use of the equation with a multiplier of 0.74 provides reliable evaporation rates from quiet (unoccupied) indoor pools.
1.
Study more than one indoor pool and use the most direct method of measurement possible, such as the use of a mechanical dehumidifier. the condensate can then directly be measured.
2.
Energy losses to the ground must be account for. They can very between 3% and 15% of the heat lost by the pool water.
REFERENCES ASHRAE. 1987. 1987 ASHRAE Handbook—HVAC Applications, p. 4.7. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 1991. 1991 ASHRAE Handbook—HVAC Applications, p. 4.7. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE. 1995. 1995 ASHRAE Handbook—HVAC Applications, p. 4.7. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Biasin, Von K., and W. Krumme. 1974. Evaporation in an indoor swimming pool. Electrowarme International, pp. a115-a129. May (Germany). Carrier, W.H. 1918. The temperature of evaporation. ASHRAE Transactions 24: 25. Jones, R., C. Smith, and G. Löf. 1994. Measurement and analysis of evaporation from an inactive outdoor swimming pool, Solar Energy 53(1): 3. Labohm, G. 1971. Heating and air conditioning of swimming pools. Gesundheits Ingenieur, pp. 72-80. March (Germany). Molinaux, B., B. Lachal, and O. Guisan. 1994. Thermal analysis of five outdoor swimming pools heated by unglazed solar collectors. Solar Energy 53(1): 21. Reeker, J. 1978. Water evaporation in indoor swimming pools. Klima & Kalte Ingenieur, no. 1, pp. 29-32. January (Germany). Rohwer, D. 1931. Evaporation from free water surfaces. Tech. Bulletin no. 271, U.S. Dept. of Agriculture. Root, D. 1983. How to determine the heat load of swimming pools. Solar Age, pp. 20-23. November. Smith, C.C., R. Jones, and G. Löf. 1993. Energy Requirements and Potential Savings for Heated Indoor Swimming Pools. ASHRAE Transactions 99(2): 864.
Ground heat losses depend on: •
soil condition
•
ground water table
•
pool insulation
•
existing crawl space around pool
•
impermeability of inside pool finishing
•
pool water temperature
3.
Number of bathers or swimmers and pool activity are not synonymous. Aquafit sessions have a high number of bathers and little or no activity. Water polo has only a few swimmers but a very high activity level.
4.
The condition of the deck area affects greatly the evaporation rate of an indoor pool. Wet deck areas that retain water add considerably to the evaporation rate, that does not reflect in the heating requirement of the pool water.
5.
Do the authors of this publication make recommendations for sizing dehumidifier-heat pumps for indoor pools?
Randall Jones: 1. The scientific value of this study can be best assessed in the context of the whole series of pool evaporation rate experiments conducted by the authors. The series investigated evaporation rates in indoor and outdoor pools, under quiet and active conditions. Quiet indoor and outdoor pool results have been reported in ASHRAE Transactions DE-93-12-3 and Solar Energy Journal, July 1994, respectively. The purpose was to aid engineering professionals in sizing and designing pool HVAC equipment, estimating pool energy use, and predicting savings from pool energy conservation measures. In all, 2 indoor and 1 outdoor facilities were used. One of the indoor facilities contained 3 separate pools, so a total of 5 pools were investigated. For the quiet pool studies, evaporation rate measurement was determined by the most direct method possible, measurement of volume loss by high precision measure of water level change. Secondary measurements of pool energy inputs from the heating system and losses by measurement of temperature drop and radiation and calculation losses for the outdoor pool were used to confirm water level measurements. Our confidence in this method was aided by the following results: •
Quiet pool evaporation rates determined by water level change from all 5 pools were virtually the same, i.e., 74% of that predicted by the equation in ASHRAE Applications.
•
Energy balance measurements and calculations were consistent and correlated with the water level change measurements.
DISCUSSION Reinhold Kittler, Chairman, Dectron, Inc., Montreal, Quebec: The scientific value of this study would be enhanced with the following: 522
ASHRAE Transactions: Research
•
The possibility of water loss from leakage was investigated in the first quiet pool experiment by measuring water level before and after a pool cover was applied over night to eliminate evaporation. Water level was unchanged, indicating no leakage. For the active indoor and outdoor pools, loss of water by splashing and exiting swimmers made water level change a less accurate measurement of evaporation rate. For active pools, energy balance measurements and calculations were the primary methods of assessing evaporation rate, with water level change used to check the results. Evaporation can not be directly or accurately measured from dehumidification system condensate recovery. The accuracy of this approach would depend on an assumption that all moisture evaporated from the pool would wind up in the system condensate. This does not account for loss of evaporated moisture through building surface condensation, transport through building materials, and infiltration and exhaust ventilation either directly outdoors or to other parts of the building. It would be interesting, however, to conduct evaporation experiments on a pool with a dehumidification system and correlate results. 2. Most references we have found indicate a loss of 1-5% to the ground, but there seems to be a lack of corroborating data. In our studies, ground losses were assumed to be negligible compared to the magnitude of other losses. Results bore out this assumption. There are several mechanisms that we recognized in this study as potential causes of heat and water loss other than by evaporation (including and in addition to this list). Two methods were used to resolve these questions: for example, the calculation of the radiation exchange between the pool surface and natatorium walls. Secondly, background measurements were taken as in the case of potential ground losses (addressed
ASHRAE Transactions: Research
in the question). The quiet pool in this study was monitored with a pool cover in place resulting in negligible loss relative to the evaporation quantities. The active pool could not be covered, however it agreed with the first pool while in the quiet state. 3. We agree that the type of activity as well as other factors such as water attractions impact evaporation. In this study, number of swimmers was used as the measurement of activity because it is the only parameter that was reasonably measurable and repeatable. 4. We agree that wet deck areas are a source of additional evaporation, but not to an extent comparable with that from water in the pool. Lack of a heat supply to water on the deck results in rapid cooling of these comparatively thin water layers toward the wet-bulb temperature, at which evaporation is substantially reduced. This is one of the factors which the authors considered in choosing to base evaporation loss from active pools on energy balance measurements on the pool water alone. 5. The information is presented to aid engineering professionals in sizing and design pool HVAC equipment, estimating pool energy use, and predicting savings from pool energy conservation measures. To maintain natatorium humidity levels at design conditions, the authors recommend that sizing of heating, ventilation, and dehumidification equipment be based on use of the evaporation equation in ASHRAE Applications, increased by a factor of 1.1 to 1.25, representing expected maximum or near maximum pool occupancy and use. For calculating evaporation heat losses in a quiet pool the authors recommend use of the evaporation equation in ASHRAE Applications, decreased by multiplication of a factor of.74. The authors wish to express their appreciation to Dr. Kittler and others for their comments and suggestions.
523
