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Evaporator air-side fouling: effect on performance of room air conditioners and impact on indoor air quality.

INTRODUCTION

Heat exchangers are the main components of room air-conditioning systems. Thus, even fractional performance degradation due to fouling has the potential to cause further energy consumption. Furthermore, heat exchangers are directly in contact with the indoor airstream and have extended surfaces. They are typically a fin-and-tube configuration in order to increase the heat transfer rate between the refrigerant and the indoor air. However, a certain material that deposits on the condensate over the surface of these heat exchangers will react with other indoor airborne contaminants and produce mold compounds. These contaminants include airborne particulate matter and dusts. If some of those deposited materials are biological in nature, they will multiply and spread into the indoor air. Fouling of indoor air-side fin-and-tube heat exchangers, particularly air-conditioner evaporators of room units, leads to a reduction in airflow and to discomfort for occupants a few years after installation, particularly in dusty regions. The available literature concerning air-side heat exchanger fouling of room or unitary air conditioners or related subjects are summarized as follows. Krafthefer and Bonne (1987) investigated the cost-effectiveness of using high-efficiency air cleaners instead of more commonly used dust stop filters in heat pumps. Rossi and Braun (1996) evaluated the maximum cost savings associated with using optimal service scheduling for the cleaning of heat exchangers in packaged air-conditioning equipment. Bultman et al. (1993) simulated the effect of partially blocked condensers of vapor compression units and reported that the coefficient of performance (COP) was predicted to decrease by 7.6% when the airflow across the condenser is reduced by 40% for a constant-speed fan. Siegel (2002) and Siegel and Nazaroff (2003) conducted an experimental study to verify their model of predicting particle deposition on HVAC heat exchangers. Yang et al. (2004) investigated experimentally the impacts of air-side fouling on the performance of coil and filter combinations, which include filter pressure drop, coil air-side pressure drop, and coil air-side effective heat transfer coefficient. Breuker and Braun (1998) conducted an experiment on condenser fouling of a 3 ton rooftop unit by simulating the fouling in the test by blocking the condenser coil with strips of paper; the evaporator fouling was simulated by reducing the airflow. Pak et al. (2005) investigated experimentally the impacts of air-side fouling and cleaning on the performance of various condenser coils used in unitary air-conditioning systems.

Despite the potential importance and impact of air-side fouling of room air-conditioner unit evaporator coils on indoor air quality (IAQ) as well as the overload energy consumption during the cooling period, there has been relatively little research concerning this topic. Also, it was found from the available literature that experiments on fouling of HVAC heat exchangers and filters are done by using pure dust particulates. In practice, most of these dust particulates are usually trapped by the unit filter normally located in the air inlet side before reaching the indoor evaporator coil surfaces. In addition, most of the previous work focuses on the effect of the evaporator coil fouling on the air-side pressure drop. In the work of Siegel (2002), a picture taken using an optical microscope for real fouling material on the evaporator face was presented. The image showed a mixture of animal and human hair, textile fibers, and other spherical supermicron particles as some of the constituents of fouling material. Therefore, this study aims to investigate experimentally, by using real fouling materials collected from dirty evaporator coils, the impact of evaporator coil fouling on IAQ. Also, the fouling materials' effect on the measured performance of the room air-conditioner unit for the cases of a partially fouled and completely fouled evaporator coil (compared with a clean coil) are also investigated. This will be done to clarify the degradation of the unit's COP at the standard air face velocity of 1.53 m/s as well as at different inlet air face velocities.

EXPERIMENTAL APPARATUS, INSTRUMENTATION, AND PROCEDURE

Apparatus and Measurements

A test apparatus was designed to investigate the performance of a room air conditioner under different fouling stages and various airflow rates. A schematic diagram of the apparatus used is shown in Figure 1. The apparatus consists mainly of air-handling sections, a fouling-material injector, a test section, and the appropriately equipped instruments. The air-handling sections include a nozzle meter, a variable-speed airflow fan, an inlet air distribution plenum, a flow straightener (honeycomb), and a filter. A divergent duct having a length of 0.46 m connects the fan outlet to a rectangular duct of 0.36 m in width and 0.195 m in height. The flow straightener provides uniform air velocity distribution at the inlet to the evaporator coil (test section). The evaporator coil frontal finned area dimensions are 0.254 x 0.1905 m with depth in the flow direction of 0.1524 m. It is a plate fin-and-tube heat exchanger. The fins are made of aluminum alloy and the tubes are made of copper. Four tube rows with four tubes in depth are arranged in a staggered manner (2[d.sub.o] x 2.5do). The tubes are 19.05 mm (0.75 in.) in outer diameter, the fin pitch is 4.23 fin/mm, and the fin thickness is 0.6 mm. The test coil (evaporator) itself is a component of a compression refrigeration cycle using R-12 as a refrigerant (see Figure 1). The other main components of the refrigeration cycle include a compressor, an air-cooled condenser, a rotameter (with an uncertainty of [+ or -]0.01 L/min) for refrigerant mass flow measurements, a capillary tube for refrigerant expansion, a hermetic compressor, and four pressure gauges at the inlet and outlet of both the evaporator and condenser with an uncertainty of [+ or -]0.05 kPa. The input electric power to the compressor was measured by a Wattmeter with an uncertainty of [+ or -]5 W.

[FIGURE 1 OMITTED]

A variable electric resistor controlled the input power to the airflow fan motor; therefore, the airflow rate was controlled by the fan speed. The throat velocity of the air passing through the nozzle was determined by measuring the static pressure drop across the nozzle using four static pressure taps, each 1 mm in diameter and located flush with the inner wall of the nozzle, and by using a water manometer with an accuracy of [+ or -]0.5 mm.

A data acquisition system having a resolution of 0.1[degrees]C was used for the temperature measurements and recording. All temperatures were measured by using type T (copper/constantan) thermocouples with 0.5 mm diameters and an uncertainty of [+ or -]0.5[degrees]C. The thermocouples were used to measure the temperature at the following locations: air dry-bulb and wet-bulb temperatures before the evaporator coil at 12 points; on the evaporator coil tubes and fins at six points, including two points at the refrigerant inlet and outlet; and ambient air dry-bulb and wet-bulb temperatures at two points. The face air velocity at the air inlet to the evaporator was measured using a portable digital anemometer with an uncertainty of [+ or -]0.01 m/s.

Experimental Procedures

Real fouling materials were collected by the refrigeration and air-conditioning maintenance unit at the Faculty of Engineering, Assiut University, Egypt. More than 350 g of fouling material from dirty evaporator coils of window-type air conditioners was collected. The collected fouling material was prepared for injection in the test duct at the Mining and Metallurgy Laboratory. Preparation was done at various mesh sizes. The sieve analysis showed that the particles having diameters ranging from 0 to 63 [micro]m represented 40.2% of the total weight, diameters ranging from 64 to 80 [micro]m represented 8.75%, diameters ranging from 81 to 90 [micro]m represented 9.33%, diameters ranging from 91 to 100 [micro]m represented 11.54%, diameters ranging from 101 to 125 [micro]m represented 18.22%, diameters ranging from 126 to 160 [micro]m represented 7.54%, and diameters ranging from 161 to 200 [micro]m represented 4.4% of the total weight. Experiments were performed with one clean and three fouled coils at various fouling stages. Three hundred grams of fouling material were injected into the air-handling section of the test apparatus at three doses, each increasing by 100 g, during the fouled coil experiments. This amount represents a one-year quantity of fouling material flowing into the heat pump unit as reported by Yang et al. (2004) and Pak et al. (2005).

In order to determine the organic compositions of the fouling material, the following procedures were carried out. A sample of 0.3 g was weighed accurately and then diluted in 200 mL of pure water with a percentage of 1:600. One milliliter from the solution was then added to a biological media (Czapek's rose bengal agar) solution in a petri plate. The petri plate was kept at 20[degrees]C for four to five days. Then, after its appearance, the amount of fungi was determined from the average of all petri plates. The amount of fungi is based on a 1 g sample. The total accounted fungi number was multiplied by 600, which is the lighting percentage.

Tests were performed at four different conditions: 1) at clean condition (base case), 2) at fouled condition after injection of 100 g of the fouling material, 3) at fouled condition after injection of 200 g of the fouling material, and 4) at fouled condition after injection of 300 g of the fouling material. Injection of the fouling material was done as follows. The refrigeration machine worked at a low airflow rate until the evaporator coil became wet. At this time, a manual air blower was used to inject the fouling material dose in pulses for a period of one hour into the duct. Air leaving the nozzle carried the injected fouling material into the fan inlet. The fan provided a good mixing of the fouling material with the air. At the end of the injection of the fouling material dose, the compressor of the refrigeration cycle was turned off while the airflow rate was kept at the same airflow rate as during the injection for another hour. By the end of the fouling injection process, the preformance experiments were carried out for several days. After the last performance experiment, when a total of 300 g of fouling material had been injected, the test facility duct was cleaned to collect the residuals of the fouling material. The collected fouling material was weighed and reduced from the initial value of 300 g. Experiments were carried out in a quasi-steady-state condition for cleaned and fouled evaporators at different airflow rates corresponding to face velocities ranging from 0.1 m/s to more than 5 m/s. The stabilization of temperature readings to [+ or -]0.1[degrees]C in all thermocouple sensors was considered an indication of reaching quasi-steady-state condition for each experimental run. At this time, the average of the temperature values were read and recorded by the data acquisition system. In addition, readings of the refrigeration cycle pressure gauges and rotameter as well as the manometer for the pressure difference across the nozzle were taken at that time. The measurements were performed at the Heat Laboratory, Assiut University.

DATA REDUCTION

The measured data were used to determine the COP of the refrigeration cycle at clean and fouled evaporator coil conditions. The COP is defined as a ratio of evaporator coil cooling capacity, [Q.sub.avg], in W to compressor input electric power, [W.sub.c], in W by Equation 1:

COP = [[Q.sub.avg]/[W.sub.c]] (1)

The evaporator coil cooling capacity [Q.sub.avg] was obtained by performing energy balance on both refrigerant and air sides. It is given by

[Q.sub.avg] = [([Q.sub.a] + [Q.sub.r])]/2 (2)

where [Q.sub.a] is the air-side capacity of the evaporator coil and is calculated using the following equation:

[Q.sub.a] = [m.sub.a]([h.sub.a,i] - [h.sub.a,o]) (3)

where the air enthalpies [h.sub.a,i].and [h.sub.a,o], at the inlet and outlet of the evaporator coil, respectively, are calculated using the formula from ASHRAE Handbook--Fundamentals (ASHRAE 2005), and the air mass flow rate,[m.sub.a], at the nozzle throat is calculated from the following equation:

[m.sub.a] = [C.sub.d][A.sub.n][square root of[2[rho][DELTA]P]/[1-[([A.sub.n]/[A.sub.p]]).sup.2]]] (4)

where [C.sub.d] is the discharge coefficient and has a value of 0.98 for the nozzle used, [rho] is the air density at the inlet to the nozzle, [DELTA]P is the pressure drop across the nozzle, and [A.sub.n] and [A.sub.p] are the nozzle and inlet pipe areas, respectively.

[Q.sub.r] is the refrigerant-side cooling capacity of the evaporator coil and is calculated using the following equation:

[Q.sub.r] = [m.sub.r]([h.sub.r,o] - [h.sub,r.i]) (5)

where the refrigerant enthalpies [h.sub.r,i] and [h.sub.r,o] at the inlet and outlet of the evaporator coil, respectively, are obtained from ASHRAE Handbook--Fundamentals (ASHRAE 2005) as functions of the temperatures and pressures of state points. The refrigerant mass flow rate,[m.sub.r], is calculated from the rotameter reading and the refrigerant density. The experimental data that satisfy the condition of ([Q.sub.r] - [Q.sub.avg])/[Q.sub.avg] [lesser than or equal to] 0.05 are only considered in the determination of COP in Equation 1.

ASHRAE (2005) relations were used to obtain the air humidity ratio, [omega], at the inlet and outlet of the evaporator coil from the measured air dry-bulb temperature, [T.sub.a], and air wet-bulb temperature, [T.sub.w]. The uncertainties of the results calculated through data reduction of the experimental measurements were estimated based on the formula for computing overall errors of Doeblin (1990). The uncertainty estimated in the determination of the COP ranged from 9.5% to 13.6%.

RESULTS AND DISCUSSION

Fouling Behavior, Material Contents, and Impact on IAQ

At the end of the experiments, which took more than two months, photographs were taken of the evaporator coil's frontal face side (inlet air side) and rear side as well, after returning it to clean condition in order to compare visually the buildup fouling on the coil. The photos are shown in Figure 2. It can be seen from this figure that the fouling on the frontal surface is a mix of dust and fibers in non-uniform distribution. Also, visual inspection of the coil reveals that the fouling decreases gradually from the front face to the first tube row, while for the rear face the dust was in uniform distribution over the fins. In addition, it can be seen from the figure that the fouling was much more severe on the frontal face area than on the rear face. The fouling distribution on the rear side could be explained as follows. Most of the dust particles that do not impinge on the frontal fin surface edges are carried by the airflow through the fins. Some stick on wet fins and few may emerge from the rear side of the coil. The fiber and non-dusty material stick on the frontal face fins, which are wet, as the condensate falls down over the fin edges. A question may arise about the cause of the presence of fiber on the frontal face. The fiber and non-dusty materials are components of the fouling material, which were collected from other evaporator coils on a university campus, as previously mentioned, that have air filters before the front. The total collected amount of fouling material that did not deposit onto the evaporator coil was about 110 g. However, the net deposited amount of fouling material was about 190 g, with the ratio of the net deposited amount to the supplied amount at 63.3%.

[FIGURE 2 OMITTED]

Specimens from both face surface and rear side fouling material were collected at the end of the experiments to investigate their organic and non-organic components through chemical and biological analyses to gain knowledge of the fouling material's thermal properties. The analysis procedures were carried out as follows: five grams of the deposit under investigation were accurately weighed and the solid material was extracted from the sample by diluting it several times in petroleum ether and diethylether. The residual solid material was then collected by evaporating the solvents, and the non-organic material was weighed. The results of the frontal face sample showed a weight of 4.093 g for the non-organic residual solid materials (81.86% of the total sample). For the rear face sample, 4.94 g of non-organic residual solid materials (98.8%) were obtained. From these results it is clear that the organic material on the front face of the evaporator coil is 18.14% and on the rear face is 1.2%. Energy dispersive x-ray (EDX) analysis was used to define the chemical compositions of the non-organic solid materials of the fouling specimens, and a scanning electron microscope (SEM) was used for imaging the fouling specimens.

The SEM images are shown in Figure 3. It is clear from this figure that the front face fouling sample contains fibers and dust particles while the rear sample is almost free from fiber. These results confirm the previously mentioned chemical analysis results. The results of the EDX analysis for the non-organic fouling compositions on the front and rear faces of the coil are presented in Table 1.

[FIGURE 3 OMITTED]
Table 1. Non-Organic Fouling Compositions

 Element Frontal Sample, % Rear Sample, %

Aluminium 9.79 6.00
 Silicon 28.14 20.09
Phosphorus 4.21 0.34
 Sulphur 7.96 20.63
Potassium 3.51 2.89
 Calcium 18.74 33.46
 Titanium 0.9 1.06
Manganese 0.11 0.34
 Iron 18.25 11.02
 Copper 4.75 2.56
 Zinc 3.65 1.62
 Total % 100 100


At the end of the experiments, analyses using gas chromatography were also performed to determine the organic compositions of the fouling material in samples taken from the front and rear faces of the coil. The measurements gave the following results. The front face specimen had 17 organic components, while the rear face specimen had 6 organic components. However, as there are organic materials of 18.14% and 1.2% in the front and rear fouling samples, respectively, examination of biological contamination was performed in order to clarify its impact on IAQ. The results of analyses of the front and rear face samples are shown in Figure 4. As seen from the figure, the frontal face sample shows heavy colonies of Aspergillus fungi of different species: Aspergillus niger, Aspergillus flavus, and Aspergillus terreus. These fungi are normally spread from the evaporator to the building occupants with the cold air at the outlet. A search on the World Wide Web for the Aspergillus family gave the following information (DF 2007).

[FIGURE 4 OMITTED]

Aspergillus flavus is a mold fungus associated with aspergillosis of the lungs and is sometimes believed to cause corneal, otomycotic, and nasoorbital infections. It is believed to be allergenic. Aspergillus niger is less likely to cause human disease than some other Aspergillus species, but if large amounts of spores are inhaled, a serious lung disease, aspergillosis, can occur. Aspergillus terreus is associated with aspergillosis of the lungs and/or disseminated aspergillosis and can produce the toxins patulin and citrinin, which may be associated with disease in humans and animals. This result is important to fill in the knowledge gaps between biological deposition, colonization, and occupant problems of staying in an environment with a fouled evaporator. In addition, showing the growth of some types of biological materials on HVAC heat exchangers in typical buildings is important to help building operators implement solutions to biological fouling of such units.

Effect of Fouling on Performance

The presented results of this study are based on a quasi-steady-state condition. However, as the experiments were carried out during the summertime inside an unconditioned laboratory, the variation of inlet air dry-bulb temperatures during all tests was within 3[degrees]C, while the inlet air wet-bulb temperature variations were within 1.5[degrees]C. Therefore, these variations have influence on the COP within the uncertainty mentioned previously. The measured values of variation of inlet and outlet air-dry bulb temperatures over the evaporator coil, evaporator fin average surface temperature, and determined air humidity ratio [omega], calculated from the measured wet-bulb and dry-bulb temperatures at various face air velocities are shown in Figure 5. In the figure, each data point represents an experimental run when thermocouple sensor temperature readings reached stabilization. These results were obtained for four evaporator coil conditions: (a) clean, (b) after injection of one third of the fouling material (100g), (c) after injection of two thirds of the fouling materials (200 g), and (d) after injection of the final 100 g of fouling materials. As shown in Figure 5, the variation of inlet air temperature at the four presented cases is within 2[degrees]C to 3[degrees]C at any face velocity value. As the fouling value increases, the temperature level of the evaporator surface decreases, as expected. This is because the fouling material builds up thermal resistance over the evaporator coil surface, which leads to this effect. This was followed by an increase in the outlet air dry-bulb temperature level.

[FIGURE 5 OMITTED]

The effect of fouling stages on room air-conditioner COP as a function of the face velocity is shown in Figure 6. These results show that the COP values of the refrigeration unit increase with increasing air face velocity, as expected. After fouling loading, the COP decreased as a direct effect of the coil heat transfer capacity decrease due to the air-side fouling. The results also show that the significance of the impact of air face velocity on the COP is reduced as the fouling is increased. Quantitative comparison of the effect of fouling on the COP should be at the standard air face velocity of 1.53 m/s for unitary units, as reported by Pak et al. (2005). From the data of Figure 6, at face velocity of 1.53 m/s, the value of COP for the clean evaporator coil was 2.82. It decreased to 1.89 (67%) after injection of 100 g of the fouling materials, dropped to 1.79 (63.4%) after injection of 200 g of the fouling materials, and fell to 1.23 (43.6%) after injection of 300 g of the fouling materials. This degradation in COP values is a direct effect of the increase in coil fouling material thickness leading to an increase in the thermal resistance between the cold evaporator coil surface and the airstream and, consequently, decreasing the coil overall heat transfer coefficient. In addition, the results demonstrate that the predominant effect of fouling is to cause a significant degradation in room air conditioner COP.

[FIGURE 6 OMITTED]

Through the aforementioned literature, particularly the study of Pak et al. (2005), researchers reported that the predominant effect of fouling was to cause a more significant increase in air-side pressure drop than degradation in heat transfer performance. In addition, they reported that the heat transfer performance decreased by 7% to 12% at the standard air face velocity, which is much lower than the decrease detected in this present study.

CONCLUSION

This study presented the results of an experimental investigation of the effects of real fouling material at the air side of the evaporator heat exchanger coil on the performance of a room air-conditioning unit as well as the impact of some fouling material compositions on indoor air quality. The main findings of the present study can be summarized as follows:

* The fouling on the front face of the coil is a mix of dust and fibers in non-uniform distribution and decreases gradually from the front face to the first tube row. On the rear face, the fouling is in uniform distribution over the fins.

* Examination of biological contamination of the deposit materials on the evaporator coil reveals that the coil frontal face material has heavy colonies of Aspergillus fungi of different species. This result is important to fill in the knowledge gaps between biological deposition, colonization, and expected health problems of occupants staying in environments with fouled evaporators.

* The results demonstrate that the predominant effect of fouling is to cause significant degradation in room air-conditioner coefficient of performance.

REFERENCES

ASHRAE. 2005. 2005 ASHRAE Handbook--Fundamentals. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Breuker, M.S., and J.E. Braun. 1998. Common faults and their impacts for rooftop air conditioners. HVAC&R Research 4(3):303-18.

Bultman, D.H., L.C. Burmeister, V. Bortone, and P.W. TenPas. 1993. Vapor-compression refrigerator performance degradation due to condenser air flow blockage. American Society of Mechanical Engineers, New York. 93-HT-34, pp. 1-13.

DF. 2007. Doctor Fungus. www.doctorfungus.org/thefungi/Aspergillus_spp.htm.

Doeblin, E.O. 1990. Measurement Systems, Application and Design. New York: McGraw-Hill Book Co.

Krafthefer, B.C.R., and D.R. Bonne. 1987. Air-conditioning and heat pump operating cost savings by maintaining coil cleanliness. ASHRAE Transactions 93(1):1458-73.

Pak, B.C., E.A. Groll, and J.E. Braun. 2005. Impact of fouling and cleaning on plate fin and spine fin heat exchanger performance. ASHRAE Transactions 111(1):496-504.

Rossi, T.M., and J.E. Braun. 1996. Minimizing operating costs of vapor compression equipment with optimal service scheduling. HVAC&R Research 2(1):3-26.

Siegel, J.A., and W.W. Nazaroff. 2003. Predicting particle deposition on HVAC heat exchangers. Atmospheric Environment 37:5587-96.

Siegel, J. 2002. Particle deposition on HVAC heat exchangers. PhD dissertation, Department of Mechanical Engineering, University of California, Berkeley.

Yang L., J.E. Braun, and E.A. Groll. 2004. The role of filtration in maintaining clean heat exchanger coils. Final report, ARTI-21CR/611-40050-01. Arlington, VA: Air-Conditioning and Refrigeration Technology Institute.

Received March 22, 2007; accepted September 3, 2007

Ahmed Hamza H. Ali, PhD Ibrahim M. Ismail, PhD, PE

Ahmed Hamza H. Ali is an associate professor in the Mechanical Engineering Department and Ibrahim M. Ismail is a professor and the dean of the Faculty of Engineering, Assiut University, Assiut, Egypt.
COPYRIGHT 2008 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Author:Ali, Ahmed Hamza H.; Ismail, Ibrahim M.
Publication:HVAC & R Research
Article Type:Technical report
Geographic Code:7EGYP
Date:Mar 1, 2008
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