Technical Study Part 1

CORROSION AND POLLUTION’S
INFLUENCE ON HEAT EXCHANGE CAPACITY

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 potentially the joint between the copper and aluminum. As long as the fins are tightly joined to the copper tube, without gaps or interference of organic layers or corrosive products, the heat transfer will be optimal. Pollution on the fin material will also influence the heat transfer of a heat exchanger.

Corrosion

The joint between the copper tubes and aluminum fins is one of the more corrosion sensitive parts of an air-conditioning unit. Aluminum and copper are incompatible metals. What we mean by this is that the metals have a different potential. When these metals touch each other and there is an electrolyte in the form of a conducting fluid present, a current (flow of electrons) will flow from the less noble metal (aluminum) to the more noble metal (copper). Once aluminum starts loosing electrons it begins to dissolve easily and reacts into an aluminum corrosion product. The joint that existed between copper and aluminum is now replaced by a copper aluminum oxide joint. Heat conducting capacity of aluminum oxide is much lower than that of un-corroded aluminum. Therefore, the heat transfer between copper tubes to aluminum fins is significantly decreased.

Pollution

If pollution on the fins is limiting the airflow through the heat exchanger, the temperature of the air that is passing over the aluminum fins will increase (the same kW in less kg of air). This will cause the temperature difference between the liquid/gas in the copper tube and the air passing over the fins to decrease. A smaller temperature difference will result in reduced heat transfer.

To describe what this reduced heat transfer in heat exchangers will do to the cooling capacity of an air-conditioning unit, the basic principle of refrigeration is presented here:

The basic idea behind a refrigerator is very simple: It uses the evaporation of a liquid to absorb heat. The absorbed heat is then released at a higher pressure/temperature to the environment.

There are five basic parts to any refrigeration (or air-conditioning) system:

• Compressor
• Heat-exchanging pipes outside the unit
• Expansion valve
• Heat-exchanging pipes inside the unit
• Refrigerant - liquid that evaporates inside the refrigerator to create the cold temperatures

How the basic mechanism of a refrigeration unit works:

• The compressor squeezes or compresses the refrigerant gas. This raises the refrigerant's pressure and temperature (orange), so the heat-exchanging coils outside the refrigerator allow the refrigerant to dissipate the heat of pressurization.
• As it cools, the refrigerant condenses into liquid form (purple) and flows through the expansion valve.
• When it flows through the expansion valve, the liquid refrigerant is allowed to move from a high-pressure zone to a low-pressure zone, so it expands and evaporates (light blue). In evaporating, it absorbs heat, making it cold.
• The coils inside the refrigerator allow the refrigerant to absorb heat, making the inside of the refrigerator cold. The cycle then repeats.

This is a fairly standard, although somewhat unsatisfying, explanation of how a refrigerator works. In a standard pressure enthalpy diagram this refrigeration cycle will follow the blue lines in Diagram 1:

As previously mentioned, corrosion and pollution appear to cause poor conductivity of copper/aluminum joints and result in a lower temperature difference between liquid/gas and outside air. Because of this, the heat transfer is insufficient for proper air-conditioning. To compensate for this problem the condensing temperature must be raised. An elevated condensing temperature will create a greater temperature difference between the liquid/gas in the tubes and the air passing over the fins. This increased temperature difference will return the heat transfer capacity back to normal.

In order to correct corrosion and pollution, air-conditioning systems must operate at elevated condensing temperatures. This can only be accomplished by increasing the condensing pressure (discharge pressure of the compressor). Accordingly, the refrigeration cycle will now follow the red lines in Diagram 2.

The elevated condensing temperature/pressure results in reduced cooling capacity and increased power demand (energy consumption) from the compressor. To what extent the condensing temperature/pressure is elevated depends on outside temperature, type of refrigerant, type of compressor, etc. It can easily be determined by looking at the manufacturer’s data regarding cooling capacity and energy consumption at different outside temperatures.

For example, a Carrier unit Model # 38 RA 040:

Effect of corrosion and pollution on cooling heat exchangers

When corrosion is present on the cooling heat exchanger the effect on the air-conditioning unit may be slightly different. The occurring heat transfer problem will be the same as described above but the effect on the refrigeration cycle will be a bit different. Less heat is absorbed in the cooling coil, which will directly influence the cooling capacity. The compressor will run only partial load. To achieve the desired cooling capacity the compressor will have to run much longer under these unfavorable conditions. Another option is that the suction pressure is reduced. This will result into the same effect as an increased condensing pressure.

What we can conclude from this information is that it is clear that the heat exchangers in a refrigeration cycle will determine the efficiency and capacity to a great extent. We can also conclude that the heat exchangers are most vulnerable to corrosion and pollution. Therefore, corrosion and pollution will significantly affect the capacity and energy consumption of an air-conditioning system.

2 Test Description

To determine how much influence corrosion and pollution will have on the condensing temperature/pressure and thus on the capacity of a heat exchanger (and the complete air conditioning unit), several tests are applied. Three test cases are presented in this study.

Case 1. Heat exchanger capacity loss in an extremely aggressive environment

Several heat exchangers were exposed to extremely aggressive conditions. Before and after this exposure the thermal resistance and pressure drop are measured. To measure the influence of geometry, coating and corrosion on the heat exchange capacity, the following heat exchangers were tested:

• Bare aluminum plain fin
• Pre-coated plane fin
• Bare aluminum louvered fin
• Pre-coated louvered fin
• Bare aluminum louvered fin 17 FPI
• Bare aluminum louvered fin 22 FPI

Case 2. Pilot plant for energy consumption control

Two split unit air-conditioning systems run simultaneously. Both systems are equipped with suction pressure sensors, discharge pressure sensors and energy consumption sensors. One unit is protected with a Blygold PoluAl coating to prevent corrosion; the other unit is corroded and polluted. The units are running under exactly the same conditions and show no difference in pressures at the start of the exposure time.

After 500 hours the units are inspected and pressures are measured.

Case 3 Chillers in Hong Kong, a practical case

Four air-conditioning chillers had been in operation for 6 years in Hong Kong. The condensing coils were completely deteriorated due to corrosion and required replacement. Two replacement units were equipped with untreated copper/aluminum condensing coils, two other replacement units were equipped with Blygold PoluAl treated heat exchangers.

After 4 years, with yearly maintenance being performed, the units were checked for efficiency.

3 Test Cases

In this section three cases are presented where the influence of corrosion and pollution to the heat exchanger capacity is determined. (In these three charts the 80 ft refers to the distance the unit is from the sea.)

Case 1 Heat Exchanger Capacity Loss in an Extremely Aggressive Environment

After 6 months the 17 FPI coil has lost more than 40 % of its initial capacity. The coil with 22 FPI lost more than 70 % of its initial capacity. The fin density appears to be an important parameter for capacity loss in corrosive conditions

In this graph the influence of the amount of air passing a heat exchanger is determined. Both coils are of the same dimensions and plain fin type. At 400 fpm air velocity the capacity loss is more than 40% while at 600 fpm the loss is over 60%. The air velocity (air quantity) appears to be an important parameter. More air will bring more aggressive media to the metals of the heat exchangers.

In this graph the capacity loss of 4 heat exchangers is determined. The influence of the fin type and the influence of a pre-coat can be determined from this graph. It appears those louvered fins are much more vulnerable to pollution and corrosion than plain fins. A pre-coat system slows down but does not stop the corrosion process. Capacity loss with pre-coat is still huge after six months.

Case 2. Pilot Plant for Energy Consumption Measurements

After 500 operating hours the units already show remarkable differences.

The difference in discharge pressure of 2.9 bar (for R-410A refrigerant this equals 4.9°C) results in a reduced cooling capacity of approximately 5% and an increased energy consumption of 6%.

Case 3. Chillers in Hong Kong, a practical case:

The Blygold treated units showed no pollution or corrosion. The untreated units showed no pollution but moderate corrosion. The effect of the corrosion on the capacity of the unit may be checked by comparing the discharge pressure (or condensing temperatures).

Difference in unit with and without corrosion is 4.1 bar discharge pressure. For R22 refrigerant this equals 9°C difference. All this not only results into a difference in cooling capacity (9%) but also in reduction in efficiency. The COP (Coefficient Of Performance) of the corroded chiller is 3.0 compared to 3.4 of the chiller without corrosion.

Energy consumption is approx 12% higher for the corroded unit, even though it is generating 9% less cooling capacity.

4 Conclusions

• Corrosion and pollution will affect the capacity of air-conditioning systems. Corrosion and pollution will affect the energy consumption of air-conditioning systems.
• The environment, coil geometry, air-conditioner type, airflow, refrigerant etc will determine how much the influence of corrosion and pollution will be.
• Cooling capacity losses are found ranging from 5% after 500 hours in a moderate environment to 70% after 4400 hours in an aggressive environment.
• Increase in energy consumption is found ranging from 6% after 500 hours in moderate environments, (no maintenance) to 12% after 7300 hours in a moderate environment (with good maintenance).
• High quality methods to prevent corrosion on heat exchangers will prevent capacity loss of air-conditioning.
• Periodic maintenance to remove pollution from heat exchangers prevents unnecessary capacity loss and an increase of energy consumption by air-conditioning systems.

5 References

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

BIREF1.2 Energy Saving, Blygold Info Sheet, February 2000

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

BIREF1.4 Cost saving analysis of Chiller with Blygold and Chiller without Treatment, Carrier Hong Kong, October 1999

BIREF1.5 Blygold Heat Conducting News, page 3, July 1999

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

BIREF1.7 Energy saving effect of Blygold coating on chillers of Twin tower Israel

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