TECHNICAL STUDY PART 3

THERMAL INFLUENCE OF COATING SYSTEMS ON HEAT EXCHANGERS

1 Introduction

Heat exchangers are designed to exchange heat between media without direct contact between those media. Aluminum and copper are good materials for this purpose as they have high heat conductivity ratings. Standard liquid-to-air heat exchangers are made with copper tubes and aluminum fins. A weakness in this design is the joint between copper and aluminum. As long as the fins are tightly fixed to the copper tube without interference of organic layers or corrosion products the heat transfer will be optimal.

Coating protection to heat exchangers is realized by applying organic coatings to the metal surfaces. The different systems can be divided into two types: pre-coat and post-coat.

Pre-coated aluminum consist of a thin organic (usually vinyl or epoxy) layer that is applied to the aluminum sheets before fin fabrication. After cutting, bending and assembly there will be an organic layer between the copper tube and the aluminum fin. It is accepted that this pre-coat layer results into a capacity loss ranging from 10 -15 % of heat exchange capacity.

Post coat systems consist of an organic layer that is applied after the heat exchanger is produced. An organic layer seals the copper-aluminum joint and the rest of the metals.

The effect of the Blygold post coat systems on the initial heat exchange capacity is summarized in this document.

2 Test

Pressure drop and thermal resistance of three types of heat exchanger were measured before and after a Blygold PoluAl treatment.

Heat exchanger 1 (PoluAl)

Type :                louvered fin (7 elements)

Thickness :         1 row, ± 2 cm

FPI :                  15

Tube :               3/8”, copper

Heat exchanger 2 (PoluAl)

Type :               extruded spiral tubes louvered fin

Thickness :         4 rows, ± 18 cm

FPI :                  11

Tube :               1 ”, copper

Heat exchanger 3 (PoluAl)

Type :               louvered fins

Thickness :         3 rows, ± 7 cm

FPI :                  14

Tube :               3/8”, copper

One type of heat exchanger was tested before and after a Blygold Plus Tropic treatment.

Heat exchanger 4 (Tropic)

Type :                louvered fin (7 elements)

Thickness :         1 row, ± 2 cm

FPI :                  15

Tube :               3/8”, copper

3 Results

Heat exchanger 1 (PoluAl)

Type :              louvered fin (7 elements)

Thickness :       1 row, ± 2 cm

FPI :                15

Tube :             3/8”, copper

Pressure Drop Before and After Coating

Air velocity in m/s

Thermal resistance before and after PoluAl treatment

Air velocity in m/s

Thermal resistance decrease at standard face velocity (3 m/s): 2% of initial resistance

Heat exchanger 2 (PoluAl)

Type :                 extruded spiral tubes louvered fin

Thickness :          4 rows, ± 18 cm

FPI :                   11

Tube :                1 ”, copper

Pressure Drop Before and After PoluAl Treatment

Frontal airspeed in m/s

Pressure drop increase at standard face velocity (3 m/s): 5% of initial pressure drop

Global heat exchange capacity before and after Blygold treatment

Air capacity in kg/(m2.s)

Thermal resistance increase at standard face velocity (3 m/s): 3% of initial resistance

Heat exchanger 3 (PoluAl)

Type :                louvered fins

Thickness :         3 rows, ± 7 cm

FPI :                  14

Tube :               3/8”, copper

Pressure drop before and after postcoat systems

Face velocity ft/min

Pressure drop increase at standard face velocity (675 ft/min): 8% of initial pressure drop

Thermal resistance with and without post coat treatments

Face velocity in ft/min

Thermal resistance increase at standard face velocity (675 ft/min): 3% of initial resistance

Heat exchanger 4 (Tropic)

Type :                louvered fin (7 elements)

Thickness :         1 row, ± 2 cm

FPI :                  15

Tube :               3/8”, copper

Pressure drop before and after coating

Air velocity in m/s

Dp in mm H20

before Tropic treatment

after Tropic treatment

Pressure drop increase at standard face velocity (3 m/s): 9% of initial pressure drop

Thermal resistance before and after PoluAl treatment

Air velocity in m/s

Thermal resistance decrease at standard face velocity (3 m/s): 7 % of initial resistance

4 Conclusions

· Blygold coatings slightly affect heat exchanger capacity

· Average increase in pressure drop at 3 m/s face velocity is 5%

· Average increase of thermal resistance at 3 m/s velocity is 3%

· Blygold Plus Tropic has a greater influence than Blygold PoluAl

· Heat exchangers with poor copper aluminum joints may show an improved heat
  exchange capacity after coating with heat conductive Blygold coatings.

5 References

BIREF1.1 Charles. Manger., Corrosion prevention by protective coatings, National  Association of Corrosion engineers, 1986

BIREF1.3 Mines de Douai, Résultats d’essais d’échangeurs ailetés, 07-11-1996

BIREF1.6 Carrier SA, corrosion test results of coils at Kure Beach, June 1994

BIREF1.8 Hudson/Shell test of Blygold corrosion resistance and capacity influence

BIREF3.3 Lennox Industries inc. Post coat influence on heat exchange capacity, 27-03-2000, USA

BIREF3.4 Carrier SA ,dry performance comparison before and after coating. February 1994.