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International Refrigeration and Air Conditioning
Conference
School of Mechanical Engineering
2006
An Experimental Study of the Air-Side Particulate
Fouling in Fin-and-Tube Heat Exchangers of Air
Conditioners
Youngchull Ahn
Pusan National University
Seongir Cheong
Pusan National University
Youngman Jung
Pusan National University
JaeKeun Lee
Pusan National University
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Ahn, Youngchull; Cheong, Seongir; Jung, Youngman; and Lee, JaeKeun, "An Experimental Study of the Air-Side Particulate Fouling in
Fin-and-Tube Heat Exchangers of Air Conditioners" (2006). International Reigeration and Air Conditioning Conference. Paper 818.
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R020, Page 1
International Compressor Engineering Conference at Purdue, July 17-20, 2006
An Experimental Study of the Air-side Particulate Fouling in
Fin-and-Tube Heat Exchangers of Air Conditioners
Young Chull AHN*
1
, Seong-Ir CHEONG
2
, Young Man JUNG
2
, Jae Keun LEE
2
1
Research Institute of Mechanical Technology, Pusan National University, San 30, Jangjeon-Dong,
Kumjung-Ku, Busan, 609-735, Korea,
Tel: +82-51-510-2492, Fax: +82-51-512-9835, E-mail: ycahn@pusan.ac.kr
2
School of Mechanical Engineering, Pusan National University, San 30, Jangjeon-Dong, Kumjung-Ku,
Busan, 609-735, Korea
Tel: +82-51-510-2455, Fax: +82-51-512-5236, E-mail: jklee@pusan.ac.kr
ABSTRACT
An experimental study is conducted to investigate the chronological performance variation such as pressure drop
across a heat exchanger and cooling capacity due to the air-side particulate fouling of fin-and-tube heat exchangers
for air conditioner evaporators used. 30 samples of air conditioners used in the field such as inns, restaurants, and
offices are collected in chronological used order. This study is intended to provide factual long-term fouling data
under actual operating conditions. It is found that the important parameters to influence the fouling of heat
exchangers are the concentration and size of indoor pollutants, the filter efficiency, hydrophilicity of fin surfaces and
fin spacing. The pressure drop of heat exchangers increases due to the deposition of indoor pollutants larger than 1
μm in size and increases up to 44% in the samples used for 7 years. The air-side particulate fouling degrades the
cooling capacity by 10-15% in the samples.
1. INTRODUCTION
Fouling may be defined as the accumulation of unwanted deposits on the surfaces of heat exchangers. The presence
of these deposits reduces the heat transfer, increases the pressure drop across a heat exchanger, and increases the
maintenance cost of cleaning. The foulant may be crystalline, biological material, the products of chemical
reactions including corrosion or particulate matter (Bott, 1995).
Fouling of heat exchangers used in heating, ventilating, and air conditioning (HVAC) system is important because
of their widespread use in commercial, residential and industrial HVAC applications. Indoor and outdoor air
contaminants foul these heat exchangers. The air-side fouling of air conditioners, the deposition of indoor dusts and
other particulate matters on evaporator heat exchangers, increases system pressure drop, decreases system air flow
and air conditioner performance. Moreover because the fouling materials act as bacteria cultivator, the fouling must
be prevented and eliminated in the indoor heat exchangers in the HVAC applications (Ahn et al., 2002).
Air-side particulate fouling can be described as the accumulation of solid particles suspended in the air onto a heat
transfer surface. In HVAC system, dust particles carried forward in the air are potential foulants for the heat
exchangers. The theory associated with the transport of particles towards and onto surfaces is extensive and
complex (Epstein, 1988). Transport of particles to the heat exchanger surfaces in the particulate fouling can occur
by Brownian or laminar diffusion, turbulent diffusion, gravitational settling, inertial impaction, thermophoresis, or
electrophoresis. In actual practice, two or more of these mechanisms will probably occur simultaneously. The sizes
of particles have a large influence on the dominant mechanism. For instance, very small particles would be
expected to be subject to Brownian diffusion and turbulent diffusion whereas the larger particles due to their mass
would move under inertia impaction. Having arrived at the surface the particle must stick or adhere if it is to be
regarded as part of the foulant layer residing on the surface.
The purpose of this study is to investigate the chronological performance variation such as pressure drop and cooling
capacity due to the air-side particulate fouling of fin-and-tube heat exchangers for air conditioner evaporators used.
R020, Page 2
International Compressor Engineering Conference at Purdue, July 17-20, 2006
30 samples of air conditioners used in the field such as inns, restaurants, and offices are collected in chronological
used order. The psychrometric calorimeter for measuring the cooling capacity and pressure drop of heat exchangers
in the air conditioners is used in this study.
2. EXPERIMENTAL
Fig. 1 shows a schematic diagram of the psychrometric calorimeter (3S System, 7,000×8,200×3,000 ㎣) for
measuring the pressure drop and the cooling capacity of HVAC heat exchangers. It consists of a constant
temperature and humidity chamber, a wind tunnel for measuring the performance of heat exchangers, an isothermal
water bath and a control panel. The constant temperature and humidity chamber is composed of a chiller, a heater, a
humidifier, and an air-handling unit. It keeps the temperature and the humidity inside the calorimeter constant. The
dry and the wet bulb temperatures inside the calorimeter are set at 27 ℃ and 19.5 ℃, respectively, based on the
Korean Standard “Air Conditioners” (KS C 9306, 2002). The wind tunnel, (310×210 ㎟ for window-type and
750×210 ㎟ for wall-mounted type of air conditioners), is composed of the heat exchanger for testing, two couples
of dry and wet bulb thermometer, an orifice and a suction fan. Two couples of dry and wet bulb thermometer
measure the air temperature and humidity upstream and downstream of the test heat exchanger to obtain the air-side
enthalpy variation. The air flow rate is monitored by the pressure drop of the orifice(65 ~ 150 ㎜ in diameter) and
can be controlled by adjusting the electric frequency of the suction fan motor.
TEST HEAT
EXCHANGER
CONDENSATE
TURBO FAN
ORIFICE
AIR
HANDLING
UNIT
PUMP
FLOW
METER
ISOTHERMAL
WATER BATH
AIR
AIR
T
DO
T
WO
T
DI
T
WI
AIR
SAMPLING
UNIT
CHILLER/HEATER
HUMIDIFIER
CONTROL
PANEL
DATA
ACQUISITION
SYSTEM
WIND TUNNEL
¥ÄP
Figure 1: Schematic diagram of the psychrometric calorimeter for measuring the cooling capacity and the pressure
drop of heat exchangers in the air-conditioner.
The isothermal water bath contains distilled water as a working fluid and the constant temperature (5 ℃) water is
supplied by pump to the heat exchanger at the flow rate of 430 kg/hr. The water-side enthalpy variation can be
calculated because the temperature of the return water changes due to the heat exchange with the air stream. The
deviation between the values of the air-side and the water-side enthalpy for measuring the cooling capacity of the
heat exchanger maintains within ±3% in order to stabilize the equipment and to give reliability.
The heat exchanger used in this study is the evaporator of the air conditioner which consists of a compressor, a
condenser, an expansion device, and the evaporator. The evaporator heat exchangers used in this study is the fin-
and-tube types with 15 ~ 18 fins per inch and the diameter of tubes is 7 ~ 9 mm. Table 1 shows specifications of the
evaporators tested in this study.
The face velocity of the heat exchanger is set at 1 m/sec for measuring the pressure drop. On the other hand, the
cooling capacity is measured at the initial pressure drop of the new heat exchanger. In that condition for measuring
pressure drop across the heat exchanger, the face velocity is slightly slower than 1 m/sec. That is, the higher the
R020, Page 3
International Compressor Engineering Conference at Purdue, July 17-20, 2006
pressure drop, the slower the face velocity to have the same pressure drop as the initial value of the clean heat
exchanger.
Table 1: Specifications of the test heat exchangers
Tube
Years used Fin shape Fins/Inch
Diameter Arrangement
3 Slitted plate 18 7 mm Staggered
4 Slitted plate 18 7 mm Staggered
6 Slitted plate 18 7 mm Staggered
7 Slitted plate 18 7 mm Staggered
9 Slitted plate 18 7 mm Staggered
13 Louvered plate 16 9 mm Staggered
14 Louvered plate 16 9 mm Staggered
15 Plane plate 15 9 mm Staggered
3. RESULTS AND DISCUSSIONS
3.1 Analysis of fouling particles
Fig. 2 shows photographs of the fouled evaporator heat exchangers used in seaside inns for 6 and 14 years,
respectively. The fouling materials consist of particulates and fibers. The particulates are mainly originated from
indoor dusts and the fibers are separated from clothes, bedclothes, papers, and fur of pets. In general, the evaporator
heat exchanger in the air conditioner repeats wet and dry cycles. The wet cycle (“ON” mode of air conditioners)
means the operation that the evaporator has condensed water on the fin surfaces of the heat exchanger by the
circulation of refrigerants, while the dry cycle (no refrigerant circulation and fan operation only) means the
operation that the evaporator doesn’t have condensed water on the heat exchanging surfaces. When the air
conditioner repeats wet and dry cycles, the fouling particles agglomerate with adjacent particles and grow by
condensation.
(a) (b)
Figure 2: Photographs of fouled evaporator heat exchangers. (a) Used for 6 years, (b) Used for 14 years
The size of the fouling particles on the evaporator heat exchangers used in the office, the restaurant and the inn is
analyzed using a particle analyzer (API, Aerosizer). The mean sizes of the fouling particles collected in the office,
the restaurant, and the inn are 6.6, 9.8, and 20.9 ㎛, respectively. The particle size of the inn sample is the largest
one because the air conditioner installed in the inn has been exposed to somewhat high dust concentration and
particle agglomeration due to long operation time. The air conditioners sampled from the inn are window-type,
while those of the office and the restaurant are wall-mounted type. Actually the window-type air conditioner is
installed 1.5 m above the floor and the wall-mounted air conditioner is installed 2 ~ 2.5 m above the floor. There
exists a significant difference of the suspended dusts concentration due to the human activity. Thatcher and Layton
(1995) investigated deposition and suspension of particles within a residence and showed that even light activity can
have a significant impact on the concentration of airborne particles with diameters greater than 5 ㎛. These particle
sizes also contribute the majority in the size distribution of indoor air quality based on particle mass. Therefore the
R020, Page 4
International Compressor Engineering Conference at Purdue, July 17-20, 2006
window-type air conditioner is exposed to relatively high dust concentration and shows the largest size of fouling
particles.
3.2 Chronological variations of pressure drop
Fig. 3 represents chronological variations for the pressure drop across the evaporator heat exchanger in air-
conditioners used in the seaside inn. For the wet cycle (“ON” mode with refrigerant circulation), the pressure drop
increases up to 145% compared with the initial pressure drop and shows the asymptotic increase. The pressure
drops in the wet cycle are higher than those of dry cycle (no refrigerant circulation and fan operation only). It is
believed that the water condensates formed on the fin and tube surfaces by condensing during the wet cycle result in
flow resistances.
Figure 3: Chronological variations for the pressure drop of evaporator heat exchangers in air-conditioners used in
the seaside inn (wet cycle: “ON” mode of air conditioners, dry cycle: no refrigerant circulation and fan operation
only).
Table 2 shows variations of static contact angles in the fin surface of heat exchangers in the chronological order.
Contact angle of fin surfaces influence the cooling capacity and pressure drop across the heat exchanger. Most heat
exchangers of air conditioner evaporators have hydrophilic fin surfaces at the beginning due to hydrophilic coating.
If the fin has hydrophilic surface, the condensed water flows down freely and the increase of the pressure drop
across the heat exchanger can be minimized. However, the hydrophilic coating cannot sustain the hydrophilicity for
long time due to fouling. The initial contact angle is 20 ~ 30˚ and shows good hydrophilicity, but the
hydrophilicity becomes weak by the repetition of wet and dry cycle. Finally, the contact angle of fin surfaces
increases up to 80˚ .
Table 2: Static contact angles of the fin surfaces in the evaporator heat exchanger
Years used Type
Static contact angle(˚ )
Initial - 20 ~ 30
3 Window 47 ~ 67
6 Window 81 ~ 88
6 Wall-mounted 61 ~ 64
7 Window 63 ~ 78
9 Wall-mounted 58 ~ 72
13 Wall-mounted 53 ~ 60
14 Window 45 ~ 65
15 Wall-mounted 45 ~ 81
* Window-type collected from the seaside inn.
* Wall-mounted type collected from the restaurant, office, and residence.
0123456789
100
110
120
130
140
150
Wet Cycle
Dry Cycle
Dry/Wet bulb : 27¡É/19.5¡É
Face velocity : 1 m/sec
Variations of pressure drop (%)
Years
