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Evaluation of an air-to-air heat exchanger
Environment International, Vol. 8, pp. 453-459, 1982 Printed in the USA.
0160-4120/82/070453-07503.00/0 1982 Pergamon Press Ltd.
EVALUATION OF AN AIR-TO-AIR HEAT EXCHANGER A. Persily* Center for Energy and Environmental Studies, Princeton University, Princeton, New Jersey 08544, USA
A room-size, residential air-to-air heat exchanger was tested for effectiveness of heat recovery. The experiments were conducted in a small (16.6 m3), wood frame building, the Test Chamber, on the roof of a laboratory building. The Test Chamber is a controlled and instrumented structure for which we have the ability to predict within 1°70 the heat loss rate over a 6-h nighttime period. With the heat exchanger installed and operating in the Test Chamber, the heat recovery efficiency was determined by comparing the actual heat loss to that expected due to the mechanically induced ventilation. The heat exchanger recovered almost 50°70 of the heat contained in the outgoing air flow. Additional experiments quantified effects of fan power consumption and heat conduction through the case of the device. By considering these effects, we determined that the heat exchanger itself works at an efficiency less than the manufacturer's claim. Conducting the experiments in a real building has provided insights and evaluations not possible in an indoor laboratory.
Introduction
others are being developed. Research into the design and performance of these devices is taking place worldwide (Besant et al., 1978; Fisk et al., 1981a; Fisk et al., 1981b; Shoukri, 1979). Experiments were conducted to test the effectiveness of one particular model, the Lossnay model VL-1500 manufactured by Mitsubishi Electric Industrial Products. The Lossnay is a crossflow heat exchanger constructed of plates and fins made from treated paper capable of moisture transfer. An aluminum core may be used if moisture transfer is not desired. A schematic of the Lossnay is shown in Fig. 1. An electric motor drives two centrifugal blowers that draw air through the heat exchanger. The VL-1500 is the size of a small window air conditioner, requires no ductwork, and has a capacity suited to one or two rooms. The Lossnay was chosen as an example of a commercially available residential heat exchanger and not because of any unique design features. This paper reports on measurements of the in situ efficiency of the Lossnay in the Test Chamber (Persily and Blomsterberg, 1979). There is no standardized test method for measuring the efficiency o f residential airto-air heat exchangers. Most measurements have taken place in laboratory facilities under conditions which are very different from those in which these devices normally operate. Therefore, we chose to evaluate the Lossnay's performance when it was installed in a test building. The Test Chamber is a small structure with an interior volume of 16.6 m 3, on the roof of a two-story
Energy can be saved in residential buildings by decreasing the heat loss and gain due to infiltration through the building shell. Reducing the infiltration rate of a home may increase the indoor concentrations of many substances if no compensatory action is taken. Whether danger exists to the occupants a n d / o r building materials from moisture, combustion products, radon, tobacco smoke, or other substances depends on source strengths, house volume, pollutant decay rates, and infiltration rates. As homes become more airtight, there is increased concern for indoor environmental conditions and a need for methods to keep indoor air safe and comfortable. Techniques for controlling indoor air quality include exhaust fans at sources, air cleaning devices, dehumidifiers, and mechanical ventilation coupled with heat recovery or air-to-air heat exchangers. An air-to-air heat exchanger is a device that mechanically induces ventilation while transferring heat (or cold) and possibly moisture and water soluble pollutants from the outgoing air to the incoming air. In this manner, one obtains increased amounts of fresh air without paying a high energy penalty. Several air-to-air heat exchangers for residential applications have already reached the marketplace and
*Present address: Building 226, Room A313, National Bureau of Standards, Washington, DC 20234. 453
454
A. Persily
INSIDE
OUTSIDE
Feed Air
Exhaust Air
TSA
TEA
R ..... Ai[r , , ~ ~ \ ~ .
~
~
I
L
Fresh Air
TOA TRA
Electric r Centrifugal Blowers
Fig. 1. Schematic of the Lossnay VL-1500.
building. It is a highly controlled and instrumented building useful for precise experimental work.
Description of the Experiments The Lossnay air-to-air heat exchanger was evaluated by operating it in the Test C h a m b e r and performing a heat balance on the structure. Previous research has provided the ability to predict the Test C h a m b e r heat loss very accurately (Persily, 1982). Tests of the nighttime heat loss were made with essentially no air infiltra-
tion, and the heat loss rate was found to depend on the intensity of infrared radiation f r o m the sky, the wind speed, and the average of the inside air and the calculated outside ,;u;face temperatures. Effects of thermal inertia were also considered. All the measurements took place from midnight to 6 a.m. to avoid the need to consider solar gain. To determine the efficiency of the Lossnay, the predicted energy consumption was subtracted from the measured energy while the device was ope~ v.ting. The remainder was then related to the measured ventilation rate induced by the Lossnay to determine the efficiency of heat recovery. This procedure of efficiency determination is referred to as performing a "heat balance" on the Test Chamber. The ventilation rate was measured by tracer gas decay using ethane as a tracer and an infrared gas analyzer to measure the ethane concentration. The Lossnay efficiency was also determined by measuring the four temperatures in Fig. 1, and calculating a heat balance on the device. The Lossnay was installed in the Test Chamber's lower windows, as shown in Figs. 2 and 3. Fig. 2A, taken f r o m inside the Test Chamber, shows the Lossnay uncovered, revealing the centrifugal blower or "squirrel cage fan" that removes the inside air, and a triangular opening through which fresh enters the interior. Fig. 2B shows the Lossnay with its plastic cover in place. Figures 3A and B show the outside of the Test Chamber. In the close-up of the Lossnay (Fig. 3A), two openings are evident, one for exhausting the inside air and the other for drawing in outside air. Several different experiments were conducted to understand the performance of the Lossnay. The first tests were done without the Lossnay installed in the Test Chamber. During these experiments some infiltration was allowed to occur, either naturally or with a fan. The purpose was to determine whether we could account for the heat loss, Q1, due to infiltration, or ventilation. It is assumed that
B
A Fig. 2. Interior view of Lossnay, (A) uncovered, (B) covered.
Air-to-air heat exchanger
455
A
B
Fig. 3. (A) Exterior view of Lossnay, close-up; (B) Test Chamber with Lossnay installed. Q , = ~c~ v I A T,
where 0cp V I AT
(1)
= = = =
heat capacity of air (W h/m3°C); Test C h a m b e r volume = 16.6 m3; measured infiltration rate (h-'); instantaneous inside-outside temperature difference (°C). The next experiments measured the conduction through the Lossnay itself. By installing the Lossnay in the Test Chamber, we are replacing m a n y centimeters of insulated wall with a metal box, thereby increasing the conduction losses. After these preliminary tests we measured the efficiency of the Lossnay at the low and medium fan settings of the heat exchanger. The tests with the fan on low were done under two conditions: (1) with the Lossnay as is, and (2) with any obvious leaks in the Lossnay case taped up to see if the sealing would increase the efficiency. The tests with the fan on medium were done only under the retrofit conditions. The Lossnay efficiency was also measured using the four temperatures shown in Fig. 1, and performing a heat balance on the heat exchanger. This test method is plagued by errors in measuring the temperatures of the four air streams. Radiation effects and conduction along the thermistor stems m a y lead to errors in determining the efficiency in this manner.
Results The results of each experiment in the Test C h a m b e r are listed in Table 1. The first column gives the condi-
tions during the experiment. The second column is the measured ventilation rate minus the minute rate expected under normal conditions. In the next three columns are the total measured wattage input to the Test C h a m b e r (Q~), the predicted wattage due to conduction (Qc), and the remaining wattage (QR) after subtracting Qc from Q,~. F r o m our previous research, we are able to predict Q c within ± 1%. Next is the expected power consumed due to ventilation Q~ as calculated f r o m Eq. (1). In the appropriate cases, Q, is followed by the overall efficiency rio given by rio = 1 -
(Q,,/Q,).
(2)
Since Q, is the expected amount of heat loss due to the ventilation rate I and QR is the actual heat loss due to ventilation through the Lossnay, rio is the efficiency of heat recovery from the air leaving the Test C h a m b e r through the Lossnay. The first three experiments were done to test Eq. (I) to account for the heat loss due to infiltration. In these tests, the Lossnay was not installed, but some infiltration was allowed. In the first case, the infiltration rate was low, only 0.10 h -1 above the expected rate. The power input remaining QR after subtracting out conduction is very close to the expected power input Q,. In the next two tests, an exhaust fan was used to induce a larger amount of infiltration. In both cases the extra wattage measured (QR) is close to that expected (Q,). The results are even better when one notes that the exhaust fan consumes about 20 W of power and that little
456
A. Persily
c~ t
i
M
M i
~
~
~
~
II
o
O~
,.J
o
o
> .~a .ca
0
f~E ,..~ ~
0
~ ' ,-
o
~ o
o
o
0
o 0
~
0
~
Air-to-air heat exchanger
457
of this fan energy actually gets into the Test Chamber. From these three tests, one may conclude that Eq. (1) works well for quantifying the heat loss due to infiltration. The next three heat loss measurements were made to determine the increased conduction losses due to the Lossnay. In these tests the Lossnay was installed but not operated, and all the inlet and outlet vents were sealed. The expected additional heat loss Q, is zero in all three cases, but QR is clearly nonzero. Dividing QR by A T for each night, one obtains a value of the heat loss associated with conduction through the Lossnay. For the three nights the heat loss rates are 0.8, 0.5, and 1.0 W / ° C , which average out to approximately 0.8 W / ° C . This value is used to account for conduction through the Lossnay in subsequent analysis. After these six preliminary tests, we were able to study the overall efficiency of the Lossnay itself. The last row in Table 1 show the results of the six measurements of the heat exchanger efficiency. Two efficiency measurements were made for each operating condition. Table 1 lists the overall efficiency ~/o for each case and the results are very similar within each pair. ~/ois the efficiency o f the device as it performs in the Test Chamber, including losses due to conduction through the Lossnay itself and the power consumed by the fan. The results in Table 1 shows that the overall efficiency is 46070 when the fan is on low and the heat exchanger is unretrofitted. Retrofitting the device increases 7o by about 6o70 to 52°/o at the low fan setting. This result implies that there is some leakage in the unit, possibly be-
tween the incoming and outgoing air streams. Turning up the fan speed to medium decreases the efficiency to about 49°7o. While these efficiencies are good, they are significantly less than the manufacturer's claim of 70°/o. To understand the Lossnay performance further, two other factors were taken into account. We correct for conduction through the Lossnay by multiplying the inside-outside temperature difference A T by 0.8 W / ° C . We also correct for the power used by the fans to move the air. Using a watt-hour meter the fan was found to consume 18 W on low and 39 W on the medium setting. It is not clear how much of this fan power reaches the Test Chamber interior and how much is exhausted to the outside. Because the device is symmetrical (see Fig. 1), it seems reasonable to assume that one-half of the fan power is lost to the outside while the other half stays within the Test Chamber. In Table 2 the six efficiency tests are considered in detail by including fan power and conduction through the Lossnay case. Again, the predicted heat loss due to conduction through the Test Chamber is subtracted from the measured power QM. In addition, one-half of the fan power and the conduction through the Lossnay are also subtracted from QM. The remaining wattage Q~ is the heat loss associated with the induced ventilation alone. In Table 2, Q~ is used to calculate the heat exchanger efficiency ~/L similarly to the overall efficiency ~/o using (3)
rlL = 1 -- ( Q " /Q,~).
The heat exchanger efficiencies r/L are larger than the
T a b l e 2. H e a t e x c h a n g e r e f f i c i e n c y c a l c u l a t i o n s . Test Conditions Q,,,
=
- Qc
=
-339
Power
=
-
9
Conduction
=
-
16
Lossnay on low; unretrofitted -Fan -Lossnay
Q'R rtL =
1 -
Q'R/Q~
490
=
-Fan -Lossnay
Qu
=
508
- Qo
=
- 407
Power
=
-
9
Conduction
=
-
19
126
=
55%
Q'R ~IL =
1 -
Q'R/Q,
=
145
=
55o70
Lossnay on low; retrofitted QM -Q, -Fan -Lossnay
=
=
415
-Qc
Power
=
-
9
=
-
15
1 -
Q'./Q,
QM
-301
Conduction Qt R
~
=
~
90
=
63%
-Fan -Lossnay
"qL =
= =
576 -416
Power
=
-
9
Conduction
=
-
20
1 -
Q'R
=
131
Q,
=
60%
Q'R/
Lossnay on medium; retrofitted Q~,
~IL =
645
=
-364
Power
=
-
19
Conduction
=
-
17
-Fan -Lossnay
=
-Qc
1 --
Q'R/
Q'R
=
245
Q~
=
57070
Qu
=
Qc
=
-
Power
=
-
Conduction
=
-
-
-Fan -Lossnay
Q'R ~IL =
1 --
Q',,/Q,
710
388 19 19
=
284
=
55°70
458
A. Persily
corresponding overall efficiencies by about 7% to 9%. By accounting for the effects of fan power and conduction through the device, we have isolated the efficiency of the heat exchanger itself at about 60%, which is closer to, but still lower than, the manufacturer's claim of 70%. Our measurement of the Lossnay efficiency of heat recovery is also in agreement with independent measurements made at the Lawrence Berkeley Laboratory in a laboratory test facility (Fisk et al., 1981a). Experimental errors in temperature and energy consumption measurements lead to an uncertainty in the efficiency of ~L of about + 2%, but this is inadequate to account for the 10% difference between our efficiency measurements and the manufacturer's claimed efficiency. It is not clear how the manufacturer obtains the 70% efficiency for heat recovery. The heat exchanger efficiency was also determined by measuring the four air temperatures in Fig. 1: the incoming outside air, Toa; the feed air into the interior, TsA; the exhaust air, TEa; and the return air from the inside, TRA. These four temperatures were measured with thermistors while the infiltration rate of the Test C h a m b e r was monitored. As mentioned earlier, these temperatures are very difficult to measure, thus this efficiency determination is somewhat crude. Equating the energy flow into the Lossnay to the energy flow out, while assuming that the fan power is negligible and that the volumetric heat capacity for the four air streams is the same, one arrives at expressions for two efficiencies o f the heat exchanger. The heat transfer to the incoming air, or the feed side efficiency, is n = (TEA -- T , a ) l ( T o a
T,A).
-
(4)
The exhaust side efficiency or the efficiency o f heat transfer f r o m the outgoing air is n' = (TEA -- T , A ) / ( T o a
-
T,A).
(5)
The four temperatures were monitored through several nights and days, and the efficiencies ~ and ~' were calculated. For a typical night, the average feedside efficiency f r o m midnight to 6 a.m. was ~ = 72% and the average exhaust side efficiency was 7' = 61%. F r o m the design of the heat exchanger, one would not expect such a large difference between the two values. The discrepancy is due to the fan power which tends to w a r m both air streams. W a r m i n g the incoming air tends to m a k e the feed side efficiency larger while warming the exhaust stream decreases the exhaust side efficiency. Through a heat balance on the Lossnay one can estimate the power consumed by the fan qj using the four measured temperatures. Equating the heat flows into the device to the heat flows out, one obtains (mc~)oa Toa + (mcp),a Tna + q~ = (mcp)sa Tsa - (mc,)Ea TEa,
(6)
where (mcp) is the heat flow per unit of absolute temperature for the particular air stream noted, m being the mass flow rate and cp the heat capacity. Assuming that (mc,,) is the same for each stream, basing its value on the measured ventilation rate at the medium fan setting, and making conservative estimates of experimental errors, one obtains qj = 46 4. 10 W. The value measured with a watt-hour meter is 39 W. During the day the feed side efficiency is as high as 85% or 90% due to solar heat gain of the case of the Lossnay. In this way, the device acts as a solar collector. The feedside efficiency determined by the four temperatures at night is close to the manufacturer's claim. There may have been significant errors in the temperature measurements, but the magnitude of these errors is uncertain. Assuming an uncertainty of 4. 0.2 °C, within the thermistor specifications, the feed side efficiency is 72 4- 2%. Even with a spread of 4.4%, the feed side efficiency is significantly different from ~L = 56% as calculated f r o m a heat balance on the Test Chamber. A possible explanation for this discrepancy is imperfect mixing of the air within the Test Chamber. The Lossnay heat exchanger was designed with the interior supply and return vents built into the unit, only about 40 cm apart. This design eliminates the need for any additional ductwork in the home, but it may be a disadvantage because it makes the Lossnay basically a "local" device. The heat exchanger is involved predominantly with the air near the device and has less effect on the air at some distance. This m a y be more important in homes that are very airtight, in which there is little air flow through the structure and possibly less internal mixing than in a leakier structure. To model the effect of internal mixing on the Lossnay, one may consider the Test Chamber as two separate cells exchanging air with the outside at different rates. One cell of the Test C h a m b e r has a large ventilation rate induced by the heat exchanger and the other has only minimal infiltration through the shell. The measured ventilation rate was determined from the tracer concentration of an equally blended mixture from both cells. It has been pointed out that infiltration rates measured under these conditions may be lower than the actual rates (Hunt, 1980; Persily, 1982). If our infiltration measurements were low, then our calculated efficiency would also be low.
Conclusions The Lossnay air-to-air heat exchanger has been tested with it operating in the Test Chamber. Through a heat balance on the structure the efficiency of heat recovery was found to be 55% to 60%, depending on the fan speed. This e f f c i e n c y decreases to about 50°7o if one also considers the lost fan power and heat conduction through the metal case of the exchanger. Both the manufacturer's claimed efficiency and the efficiency as
Air-to-air heat exchanger m e a s u r e d b y d o i n g a heat b a l a n c e o n the device a r e close to 7 0 % . This d i s c r e p a n c y is believed to be d u e in p a r t to inc o m p l e t e m i x i n g within the Test C h a m b e r . Because o f i m p e r f e c t m i x i n g o f i n t e r i o r air, t h e L o s s n a y t e n d s to affect p r e d o m i n a n t l y a localized v o l u m e within the structure. This f i n d i n g m a y have i m p o r t a n t i m p l i c a t i o n s f o r the design o f a i r - t o - a i r heat e x c h a n g e r s , p a r t i c u l a r l y for the p l a c e m e n t o f the i n t e r i o r s u p p l y a n d r e t u r n vents. I n c o m p l e t e m i x i n g is less o f a p r o b l e m in l e a k i e r houses, b u t it is also h a r d to c o n t r o l the v e n t i l a t i o n o f a l e a k y dwelling. In a d d i t i o n , h o u s i n g t h a t is m o r e a i r t i g h t is m o r e likely to h a v e i n d o o r air q u a l i t y p r o b l e m s a n d b e in need o f a n a i r - t o - a i r h e a t e x c h a n g e r . By e v a l u a t i n g the L o s s n a y in the Test C h a m b e r we h a v e been a b l e to learn a b o u t effects t h a t w o u l d n o t be a p p a r e n t in a l a b o r a t o r y test facility. T h e c o n d u c t i o n o f heat t h r o u g h the L o s s n a y a n d the effects o f i m p e r f e c t m i x i n g on the h e a t e x c h a n g e r effectiveness w o u l d have been o v e r l o o k e d if the e x p e r i m e n t s were n o t d o n e in the Test C h a m b e r . Such a c c u r a t e , in situ testing is i m p o r t a n t for h e a t e x c h a n g e r s a n d o t h e r energy c o n s e r v i n g a n d p o l l u t a n t c o n t r o l devices. Acknowledgements--This work was supported by the Buildings Energy Science Branch, Building Systems Division, Office of Building Energy Research and Development, Conservation and Renewable Energy, United States Department of Energy, under Contract No. DE-AC02-77CS20062.
459
References Besant, R. W., Brooks, E. E., Schoenan, G. J., and Dumont, R. S. (1978) Design of low cost ventilation air heat exchangers," in Energy Conservation in Heating, Cooling, and Ventilating Buildings, C. J. Hoogendoorn and N. H. Afgan, eds. Series in Thermal and Fluids Engineering, Hemisphere Publishing Corp. Washington, DC. Fisk, W. J., Archer, K. M., Boonchanta, P., and Hollowell, C. D. (1981a) Performance measurements for residential air-to-air heat exchangers, presented at the International Symposium on Indoor Air Pollution, Health and Energy Conservation, Amherst, MA. Fisk, W, J., Roseme, G. D., and Hollowell, C. D. (1981b) Test results and methods: Residential air-to-air heat exchangers for maintaining indoor air quality and saving energy. Report LBL-12280, Lawrence Berkeley Laboratory, University of California, Berkeley, CA. Hunt, C. M. (1980) Air infiltration: A review of some existing measurement techniques and data, in Building Air Change Rate and Infiltration Measurement, C. M. Hunt, J. C. King, and H. R. Trechsel, eds., ASTM STP 719, American Society for Testing and Materials, Philadelphia, PA. Persily, A. (1982) Understanding air infiltration in homes. Report No. 129, Center for Energy and Environmental Studies, Princeton University, Princeton, NJ. Persily, A. and Blomsterberg, A. (1979) Test chamber handbook. Working Paper No. 48, Center for Energy and Environmental Studies, Princeton University, Princeton, NJ. Shoukri, M. (1979). The use of a regenerative air-to-air rotary heat exchanger for heat recovery in residential ventilation systems. Presented at the American Society of Mechanical Engineers, Annual Meeting, New York, NY.
0160-4120/82/070453-07503.00/0 1982 Pergamon Press Ltd.
EVALUATION OF AN AIR-TO-AIR HEAT EXCHANGER A. Persily* Center for Energy and Environmental Studies, Princeton University, Princeton, New Jersey 08544, USA
A room-size, residential air-to-air heat exchanger was tested for effectiveness of heat recovery. The experiments were conducted in a small (16.6 m3), wood frame building, the Test Chamber, on the roof of a laboratory building. The Test Chamber is a controlled and instrumented structure for which we have the ability to predict within 1°70 the heat loss rate over a 6-h nighttime period. With the heat exchanger installed and operating in the Test Chamber, the heat recovery efficiency was determined by comparing the actual heat loss to that expected due to the mechanically induced ventilation. The heat exchanger recovered almost 50°70 of the heat contained in the outgoing air flow. Additional experiments quantified effects of fan power consumption and heat conduction through the case of the device. By considering these effects, we determined that the heat exchanger itself works at an efficiency less than the manufacturer's claim. Conducting the experiments in a real building has provided insights and evaluations not possible in an indoor laboratory.
Introduction
others are being developed. Research into the design and performance of these devices is taking place worldwide (Besant et al., 1978; Fisk et al., 1981a; Fisk et al., 1981b; Shoukri, 1979). Experiments were conducted to test the effectiveness of one particular model, the Lossnay model VL-1500 manufactured by Mitsubishi Electric Industrial Products. The Lossnay is a crossflow heat exchanger constructed of plates and fins made from treated paper capable of moisture transfer. An aluminum core may be used if moisture transfer is not desired. A schematic of the Lossnay is shown in Fig. 1. An electric motor drives two centrifugal blowers that draw air through the heat exchanger. The VL-1500 is the size of a small window air conditioner, requires no ductwork, and has a capacity suited to one or two rooms. The Lossnay was chosen as an example of a commercially available residential heat exchanger and not because of any unique design features. This paper reports on measurements of the in situ efficiency of the Lossnay in the Test Chamber (Persily and Blomsterberg, 1979). There is no standardized test method for measuring the efficiency o f residential airto-air heat exchangers. Most measurements have taken place in laboratory facilities under conditions which are very different from those in which these devices normally operate. Therefore, we chose to evaluate the Lossnay's performance when it was installed in a test building. The Test Chamber is a small structure with an interior volume of 16.6 m 3, on the roof of a two-story
Energy can be saved in residential buildings by decreasing the heat loss and gain due to infiltration through the building shell. Reducing the infiltration rate of a home may increase the indoor concentrations of many substances if no compensatory action is taken. Whether danger exists to the occupants a n d / o r building materials from moisture, combustion products, radon, tobacco smoke, or other substances depends on source strengths, house volume, pollutant decay rates, and infiltration rates. As homes become more airtight, there is increased concern for indoor environmental conditions and a need for methods to keep indoor air safe and comfortable. Techniques for controlling indoor air quality include exhaust fans at sources, air cleaning devices, dehumidifiers, and mechanical ventilation coupled with heat recovery or air-to-air heat exchangers. An air-to-air heat exchanger is a device that mechanically induces ventilation while transferring heat (or cold) and possibly moisture and water soluble pollutants from the outgoing air to the incoming air. In this manner, one obtains increased amounts of fresh air without paying a high energy penalty. Several air-to-air heat exchangers for residential applications have already reached the marketplace and
*Present address: Building 226, Room A313, National Bureau of Standards, Washington, DC 20234. 453
454
A. Persily
INSIDE
OUTSIDE
Feed Air
Exhaust Air
TSA
TEA
R ..... Ai[r , , ~ ~ \ ~ .
~
~
I
L
Fresh Air
TOA TRA
Electric r Centrifugal Blowers
Fig. 1. Schematic of the Lossnay VL-1500.
building. It is a highly controlled and instrumented building useful for precise experimental work.
Description of the Experiments The Lossnay air-to-air heat exchanger was evaluated by operating it in the Test C h a m b e r and performing a heat balance on the structure. Previous research has provided the ability to predict the Test C h a m b e r heat loss very accurately (Persily, 1982). Tests of the nighttime heat loss were made with essentially no air infiltra-
tion, and the heat loss rate was found to depend on the intensity of infrared radiation f r o m the sky, the wind speed, and the average of the inside air and the calculated outside ,;u;face temperatures. Effects of thermal inertia were also considered. All the measurements took place from midnight to 6 a.m. to avoid the need to consider solar gain. To determine the efficiency of the Lossnay, the predicted energy consumption was subtracted from the measured energy while the device was ope~ v.ting. The remainder was then related to the measured ventilation rate induced by the Lossnay to determine the efficiency of heat recovery. This procedure of efficiency determination is referred to as performing a "heat balance" on the Test Chamber. The ventilation rate was measured by tracer gas decay using ethane as a tracer and an infrared gas analyzer to measure the ethane concentration. The Lossnay efficiency was also determined by measuring the four temperatures in Fig. 1, and calculating a heat balance on the device. The Lossnay was installed in the Test Chamber's lower windows, as shown in Figs. 2 and 3. Fig. 2A, taken f r o m inside the Test Chamber, shows the Lossnay uncovered, revealing the centrifugal blower or "squirrel cage fan" that removes the inside air, and a triangular opening through which fresh enters the interior. Fig. 2B shows the Lossnay with its plastic cover in place. Figures 3A and B show the outside of the Test Chamber. In the close-up of the Lossnay (Fig. 3A), two openings are evident, one for exhausting the inside air and the other for drawing in outside air. Several different experiments were conducted to understand the performance of the Lossnay. The first tests were done without the Lossnay installed in the Test Chamber. During these experiments some infiltration was allowed to occur, either naturally or with a fan. The purpose was to determine whether we could account for the heat loss, Q1, due to infiltration, or ventilation. It is assumed that
B
A Fig. 2. Interior view of Lossnay, (A) uncovered, (B) covered.
Air-to-air heat exchanger
455
A
B
Fig. 3. (A) Exterior view of Lossnay, close-up; (B) Test Chamber with Lossnay installed. Q , = ~c~ v I A T,
where 0cp V I AT
(1)
= = = =
heat capacity of air (W h/m3°C); Test C h a m b e r volume = 16.6 m3; measured infiltration rate (h-'); instantaneous inside-outside temperature difference (°C). The next experiments measured the conduction through the Lossnay itself. By installing the Lossnay in the Test Chamber, we are replacing m a n y centimeters of insulated wall with a metal box, thereby increasing the conduction losses. After these preliminary tests we measured the efficiency of the Lossnay at the low and medium fan settings of the heat exchanger. The tests with the fan on low were done under two conditions: (1) with the Lossnay as is, and (2) with any obvious leaks in the Lossnay case taped up to see if the sealing would increase the efficiency. The tests with the fan on medium were done only under the retrofit conditions. The Lossnay efficiency was also measured using the four temperatures shown in Fig. 1, and performing a heat balance on the heat exchanger. This test method is plagued by errors in measuring the temperatures of the four air streams. Radiation effects and conduction along the thermistor stems m a y lead to errors in determining the efficiency in this manner.
Results The results of each experiment in the Test C h a m b e r are listed in Table 1. The first column gives the condi-
tions during the experiment. The second column is the measured ventilation rate minus the minute rate expected under normal conditions. In the next three columns are the total measured wattage input to the Test C h a m b e r (Q~), the predicted wattage due to conduction (Qc), and the remaining wattage (QR) after subtracting Qc from Q,~. F r o m our previous research, we are able to predict Q c within ± 1%. Next is the expected power consumed due to ventilation Q~ as calculated f r o m Eq. (1). In the appropriate cases, Q, is followed by the overall efficiency rio given by rio = 1 -
(Q,,/Q,).
(2)
Since Q, is the expected amount of heat loss due to the ventilation rate I and QR is the actual heat loss due to ventilation through the Lossnay, rio is the efficiency of heat recovery from the air leaving the Test C h a m b e r through the Lossnay. The first three experiments were done to test Eq. (I) to account for the heat loss due to infiltration. In these tests, the Lossnay was not installed, but some infiltration was allowed. In the first case, the infiltration rate was low, only 0.10 h -1 above the expected rate. The power input remaining QR after subtracting out conduction is very close to the expected power input Q,. In the next two tests, an exhaust fan was used to induce a larger amount of infiltration. In both cases the extra wattage measured (QR) is close to that expected (Q,). The results are even better when one notes that the exhaust fan consumes about 20 W of power and that little
456
A. Persily
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Air-to-air heat exchanger
457
of this fan energy actually gets into the Test Chamber. From these three tests, one may conclude that Eq. (1) works well for quantifying the heat loss due to infiltration. The next three heat loss measurements were made to determine the increased conduction losses due to the Lossnay. In these tests the Lossnay was installed but not operated, and all the inlet and outlet vents were sealed. The expected additional heat loss Q, is zero in all three cases, but QR is clearly nonzero. Dividing QR by A T for each night, one obtains a value of the heat loss associated with conduction through the Lossnay. For the three nights the heat loss rates are 0.8, 0.5, and 1.0 W / ° C , which average out to approximately 0.8 W / ° C . This value is used to account for conduction through the Lossnay in subsequent analysis. After these six preliminary tests, we were able to study the overall efficiency of the Lossnay itself. The last row in Table 1 show the results of the six measurements of the heat exchanger efficiency. Two efficiency measurements were made for each operating condition. Table 1 lists the overall efficiency ~/o for each case and the results are very similar within each pair. ~/ois the efficiency o f the device as it performs in the Test Chamber, including losses due to conduction through the Lossnay itself and the power consumed by the fan. The results in Table 1 shows that the overall efficiency is 46070 when the fan is on low and the heat exchanger is unretrofitted. Retrofitting the device increases 7o by about 6o70 to 52°/o at the low fan setting. This result implies that there is some leakage in the unit, possibly be-
tween the incoming and outgoing air streams. Turning up the fan speed to medium decreases the efficiency to about 49°7o. While these efficiencies are good, they are significantly less than the manufacturer's claim of 70°/o. To understand the Lossnay performance further, two other factors were taken into account. We correct for conduction through the Lossnay by multiplying the inside-outside temperature difference A T by 0.8 W / ° C . We also correct for the power used by the fans to move the air. Using a watt-hour meter the fan was found to consume 18 W on low and 39 W on the medium setting. It is not clear how much of this fan power reaches the Test Chamber interior and how much is exhausted to the outside. Because the device is symmetrical (see Fig. 1), it seems reasonable to assume that one-half of the fan power is lost to the outside while the other half stays within the Test Chamber. In Table 2 the six efficiency tests are considered in detail by including fan power and conduction through the Lossnay case. Again, the predicted heat loss due to conduction through the Test Chamber is subtracted from the measured power QM. In addition, one-half of the fan power and the conduction through the Lossnay are also subtracted from QM. The remaining wattage Q~ is the heat loss associated with the induced ventilation alone. In Table 2, Q~ is used to calculate the heat exchanger efficiency ~/L similarly to the overall efficiency ~/o using (3)
rlL = 1 -- ( Q " /Q,~).
The heat exchanger efficiencies r/L are larger than the
T a b l e 2. H e a t e x c h a n g e r e f f i c i e n c y c a l c u l a t i o n s . Test Conditions Q,,,
=
- Qc
=
-339
Power
=
-
9
Conduction
=
-
16
Lossnay on low; unretrofitted -Fan -Lossnay
Q'R rtL =
1 -
Q'R/Q~
490
=
-Fan -Lossnay
Qu
=
508
- Qo
=
- 407
Power
=
-
9
Conduction
=
-
19
126
=
55%
Q'R ~IL =
1 -
Q'R/Q,
=
145
=
55o70
Lossnay on low; retrofitted QM -Q, -Fan -Lossnay
=
=
415
-Qc
Power
=
-
9
=
-
15
1 -
Q'./Q,
QM
-301
Conduction Qt R
~
=
~
90
=
63%
-Fan -Lossnay
"qL =
= =
576 -416
Power
=
-
9
Conduction
=
-
20
1 -
Q'R
=
131
Q,
=
60%
Q'R/
Lossnay on medium; retrofitted Q~,
~IL =
645
=
-364
Power
=
-
19
Conduction
=
-
17
-Fan -Lossnay
=
-Qc
1 --
Q'R/
Q'R
=
245
Q~
=
57070
Qu
=
Qc
=
-
Power
=
-
Conduction
=
-
-
-Fan -Lossnay
Q'R ~IL =
1 --
Q',,/Q,
710
388 19 19
=
284
=
55°70
458
A. Persily
corresponding overall efficiencies by about 7% to 9%. By accounting for the effects of fan power and conduction through the device, we have isolated the efficiency of the heat exchanger itself at about 60%, which is closer to, but still lower than, the manufacturer's claim of 70%. Our measurement of the Lossnay efficiency of heat recovery is also in agreement with independent measurements made at the Lawrence Berkeley Laboratory in a laboratory test facility (Fisk et al., 1981a). Experimental errors in temperature and energy consumption measurements lead to an uncertainty in the efficiency of ~L of about + 2%, but this is inadequate to account for the 10% difference between our efficiency measurements and the manufacturer's claimed efficiency. It is not clear how the manufacturer obtains the 70% efficiency for heat recovery. The heat exchanger efficiency was also determined by measuring the four air temperatures in Fig. 1: the incoming outside air, Toa; the feed air into the interior, TsA; the exhaust air, TEa; and the return air from the inside, TRA. These four temperatures were measured with thermistors while the infiltration rate of the Test C h a m b e r was monitored. As mentioned earlier, these temperatures are very difficult to measure, thus this efficiency determination is somewhat crude. Equating the energy flow into the Lossnay to the energy flow out, while assuming that the fan power is negligible and that the volumetric heat capacity for the four air streams is the same, one arrives at expressions for two efficiencies o f the heat exchanger. The heat transfer to the incoming air, or the feed side efficiency, is n = (TEA -- T , a ) l ( T o a
T,A).
-
(4)
The exhaust side efficiency or the efficiency o f heat transfer f r o m the outgoing air is n' = (TEA -- T , A ) / ( T o a
-
T,A).
(5)
The four temperatures were monitored through several nights and days, and the efficiencies ~ and ~' were calculated. For a typical night, the average feedside efficiency f r o m midnight to 6 a.m. was ~ = 72% and the average exhaust side efficiency was 7' = 61%. F r o m the design of the heat exchanger, one would not expect such a large difference between the two values. The discrepancy is due to the fan power which tends to w a r m both air streams. W a r m i n g the incoming air tends to m a k e the feed side efficiency larger while warming the exhaust stream decreases the exhaust side efficiency. Through a heat balance on the Lossnay one can estimate the power consumed by the fan qj using the four measured temperatures. Equating the heat flows into the device to the heat flows out, one obtains (mc~)oa Toa + (mcp),a Tna + q~ = (mcp)sa Tsa - (mc,)Ea TEa,
(6)
where (mcp) is the heat flow per unit of absolute temperature for the particular air stream noted, m being the mass flow rate and cp the heat capacity. Assuming that (mc,,) is the same for each stream, basing its value on the measured ventilation rate at the medium fan setting, and making conservative estimates of experimental errors, one obtains qj = 46 4. 10 W. The value measured with a watt-hour meter is 39 W. During the day the feed side efficiency is as high as 85% or 90% due to solar heat gain of the case of the Lossnay. In this way, the device acts as a solar collector. The feedside efficiency determined by the four temperatures at night is close to the manufacturer's claim. There may have been significant errors in the temperature measurements, but the magnitude of these errors is uncertain. Assuming an uncertainty of 4. 0.2 °C, within the thermistor specifications, the feed side efficiency is 72 4- 2%. Even with a spread of 4.4%, the feed side efficiency is significantly different from ~L = 56% as calculated f r o m a heat balance on the Test Chamber. A possible explanation for this discrepancy is imperfect mixing of the air within the Test Chamber. The Lossnay heat exchanger was designed with the interior supply and return vents built into the unit, only about 40 cm apart. This design eliminates the need for any additional ductwork in the home, but it may be a disadvantage because it makes the Lossnay basically a "local" device. The heat exchanger is involved predominantly with the air near the device and has less effect on the air at some distance. This m a y be more important in homes that are very airtight, in which there is little air flow through the structure and possibly less internal mixing than in a leakier structure. To model the effect of internal mixing on the Lossnay, one may consider the Test Chamber as two separate cells exchanging air with the outside at different rates. One cell of the Test C h a m b e r has a large ventilation rate induced by the heat exchanger and the other has only minimal infiltration through the shell. The measured ventilation rate was determined from the tracer concentration of an equally blended mixture from both cells. It has been pointed out that infiltration rates measured under these conditions may be lower than the actual rates (Hunt, 1980; Persily, 1982). If our infiltration measurements were low, then our calculated efficiency would also be low.
Conclusions The Lossnay air-to-air heat exchanger has been tested with it operating in the Test Chamber. Through a heat balance on the structure the efficiency of heat recovery was found to be 55% to 60%, depending on the fan speed. This e f f c i e n c y decreases to about 50°7o if one also considers the lost fan power and heat conduction through the metal case of the exchanger. Both the manufacturer's claimed efficiency and the efficiency as
Air-to-air heat exchanger m e a s u r e d b y d o i n g a heat b a l a n c e o n the device a r e close to 7 0 % . This d i s c r e p a n c y is believed to be d u e in p a r t to inc o m p l e t e m i x i n g within the Test C h a m b e r . Because o f i m p e r f e c t m i x i n g o f i n t e r i o r air, t h e L o s s n a y t e n d s to affect p r e d o m i n a n t l y a localized v o l u m e within the structure. This f i n d i n g m a y have i m p o r t a n t i m p l i c a t i o n s f o r the design o f a i r - t o - a i r heat e x c h a n g e r s , p a r t i c u l a r l y for the p l a c e m e n t o f the i n t e r i o r s u p p l y a n d r e t u r n vents. I n c o m p l e t e m i x i n g is less o f a p r o b l e m in l e a k i e r houses, b u t it is also h a r d to c o n t r o l the v e n t i l a t i o n o f a l e a k y dwelling. In a d d i t i o n , h o u s i n g t h a t is m o r e a i r t i g h t is m o r e likely to h a v e i n d o o r air q u a l i t y p r o b l e m s a n d b e in need o f a n a i r - t o - a i r h e a t e x c h a n g e r . By e v a l u a t i n g the L o s s n a y in the Test C h a m b e r we h a v e been a b l e to learn a b o u t effects t h a t w o u l d n o t be a p p a r e n t in a l a b o r a t o r y test facility. T h e c o n d u c t i o n o f heat t h r o u g h the L o s s n a y a n d the effects o f i m p e r f e c t m i x i n g on the h e a t e x c h a n g e r effectiveness w o u l d have been o v e r l o o k e d if the e x p e r i m e n t s were n o t d o n e in the Test C h a m b e r . Such a c c u r a t e , in situ testing is i m p o r t a n t for h e a t e x c h a n g e r s a n d o t h e r energy c o n s e r v i n g a n d p o l l u t a n t c o n t r o l devices. Acknowledgements--This work was supported by the Buildings Energy Science Branch, Building Systems Division, Office of Building Energy Research and Development, Conservation and Renewable Energy, United States Department of Energy, under Contract No. DE-AC02-77CS20062.
459
References Besant, R. W., Brooks, E. E., Schoenan, G. J., and Dumont, R. S. (1978) Design of low cost ventilation air heat exchangers," in Energy Conservation in Heating, Cooling, and Ventilating Buildings, C. J. Hoogendoorn and N. H. Afgan, eds. Series in Thermal and Fluids Engineering, Hemisphere Publishing Corp. Washington, DC. Fisk, W. J., Archer, K. M., Boonchanta, P., and Hollowell, C. D. (1981a) Performance measurements for residential air-to-air heat exchangers, presented at the International Symposium on Indoor Air Pollution, Health and Energy Conservation, Amherst, MA. Fisk, W, J., Roseme, G. D., and Hollowell, C. D. (1981b) Test results and methods: Residential air-to-air heat exchangers for maintaining indoor air quality and saving energy. Report LBL-12280, Lawrence Berkeley Laboratory, University of California, Berkeley, CA. Hunt, C. M. (1980) Air infiltration: A review of some existing measurement techniques and data, in Building Air Change Rate and Infiltration Measurement, C. M. Hunt, J. C. King, and H. R. Trechsel, eds., ASTM STP 719, American Society for Testing and Materials, Philadelphia, PA. Persily, A. (1982) Understanding air infiltration in homes. Report No. 129, Center for Energy and Environmental Studies, Princeton University, Princeton, NJ. Persily, A. and Blomsterberg, A. (1979) Test chamber handbook. Working Paper No. 48, Center for Energy and Environmental Studies, Princeton University, Princeton, NJ. Shoukri, M. (1979). The use of a regenerative air-to-air rotary heat exchanger for heat recovery in residential ventilation systems. Presented at the American Society of Mechanical Engineers, Annual Meeting, New York, NY.