Getting The Most Out Of Your Heat Exchangers
Heat exchangers are critical for the efficient operation of the chiller plant. Any deterioration in the condenser heat transfer will directly impact the efficiency of the chiller and lower the cooling capacity...
Buildings require to be cooled in summers and in some cases, heating of the building space is also required (cold climates as experienced in northern regions of India). Transfer of heat from or to a building environment is undertaken by the use of heat exchanger. A Heat Exchanger (HE) is thus essentially a mechanical system to transfer heat from one medium to another. Heat exchangers are thus a critical component of a building HVAC system as their design is fundamental to the operation of the whole plant.
Heat exchangers are simple in theory but complex in design as well as operation. A minor change in design parameters can alter a HE’s operating parameters, and in turn, the plant’s performance characteristics. Heat exchanger design is a continuously evolving field, with the main focus being on improvement in HE exchange. The HE is a static device, and hence leads to the assumption that not much maintenance would be needed for such a component of a HVAC system, the truth is, in fact, very different. HE as critical to the efficient operation of the HVAC plant and require as much – if not more – attention in operations as well as maintenance to ensure that the overall plant runs as per design.
Overview of heat exchangers
There are many types of HEs in the market depending on the type of application and design requirement. Heat exchange typically occurs between two fluids and usually across a medium. The most common classification methods of HEs are as follows:
Nature of heat transfer: This classification is based on the mechanism of heat transfer between the two fluids:
Direct type – where the two liquids physically mix and heat is transferred. An example of such a heat transfer is between water and air in HVAC system cooling towers.
Recuperators – Where the hot and cold fluids flow simultaneously across a separating medium and heat is transferred across this medium. The HE in the HVAC system – condensers and evaporators are of this type.
Type of flow: The direction of flow is a commonly used classification approach. Counter flow HE have the hot and cold fluids flowing opposite to each other, while parallel flow HE have fluids moving in the same direction.
Number of passes: Another common classification method is the number of passes the hot and cold fluids make over the passage of the HE. Single pass systems are not seen these days as HE design has evolved to allow multiple passes to increase heat transfer.
HE components: The two HE in an HVAC system are the condenser and evaporator. Both of these are of the Shell and Tube type, where one of the fluids is in the shell and the other passes through the shell through tubes. The tubes are held together by the HE covers at either end and inlet and outlet points are provide for the hot and cold fluids. The other components of a HE are the instruments such as pressure gauges and temperature gauges, as well as any instrumentation that is fitted onto the condenser or evaporator. Figure 1 shows a typical heat exchanger.
Key HE terminologies: For the plant operator, detailed knowledge of HE design terminologies such as thermal conductivity of the material, expansion coefficients, etc. are not needed. What is essential is to understand the purpose of the HE, and accordingly observe the parameters so that any deviation are spotted immediately and corrective action can be taken Thus, for routine operations, the key parameters that the operator and maintainer are required to know are the temperature difference between the fluids entering and leaving the HE and the pressure at different points in the unit.
Condensers: As the name suggests, the function of a condenser is to condense the hot gases coming out from the chiller compressor and reduce the pressure and temperature. In water cooled system, water is circulated across the condenser to condense the refrigerant gas. Water takes away heat form the hot gases, which results in condensing of the gas (lowering of temperature, and hence, pressure). The water is then passed through a cooling tower where it gives up its heat to the atmospheric air in another heat exchange process and again pumped into the condenser for the next cycle of heat removal. In air cooled systems, air is passed over the condenser coils and air takes away the heat from the refrigerant.
Evaporators: In the evaporator, the cold refrigerant liquid and water is the medium that gets cooled. Essentially, the cold refrigerant takes away the heat from the water that is circulating in the chilled water lines of a HVAC system. After evaporation, the heated refrigerant flows into the compressor, where it is heated and then condensed to repeat the vapour compression cycle of the HVAC plant.
Heat exchanger operations:
As mentioned earlier in the article, a HE is designed to operate within a narrow range of system parameters. Thus, it is essential to operate the plant as per the design recommendations to ensure that the correct heat transfer processes occur and the system operates to its best efficiency. In case the cooling water in a condenser fails, the hot gases will damage the condenser tubes. Similarly, if the flow of water stops in the evaporator, the cold refrigerant will cause the water in the tubes to freeze, resulting in the tubes cracking as ice occupies a larger volume than water.
While most modern HVAC systems have automatic control of the HE system, the operator should know how the control is affected and the implication of an out of design situation. The key parameter that an operator needs to monitor during
HE operations is:
Inlet / outlet temperatures: The temperature difference between the inlet and outlet temperatures of the hot and cold fluid as well as between the inlet of hot fluid and outlet of cold fluid are the basic parameters that should be observed. Any variation of these from design is an indication of a problem in the HE operation.
Pressure drop across the HE: Since the diameter of the tubes is small to allow for multiple passes and a greater amount of fluid, the flow of water encounters resistance from the walls of the tubes. This is seen as pressure drop across the inlet and outlet of the tube headers. An increase in the pressure drop is an indication of a malfunctioning HE.
Flow rates: While no instruments are usually installed to measure flow rates on HE in the HVAC systems in buildings, the operator should check that the pumps of the HVAC system feeding the condensers are operating at the correct rpm and pressures. Evaporators are fitted with anti-freeze sensors that trip the plant in case the cold fluid temperature reaches a certain temperature, usually 4-5 degrees Celsius. Modern systems also have flow sensors in the piping of the condensers and evaporators.
Measuring HE performance:
The performance of a heat exchanger deteriorates over a period of time on account of many factors such a quality of water and operating methods. Water is the most common HE fluid in HVAC systems as it is easily available, cheap and has good heat transfer characteristics. However, water quality is always suspect in the building services environment and these impacts the HE operations. The main problems that are encountered in HE operations are given below.
Fouling: This is the build of contaminants on the surface of the tubes leading to a reduction of heat transfer between the hot and cold fluids. In addition, since the contaminants reduce the pipe flow areas, the pressure drop increases, which leads to higher pumping power and consequently higher energy costs. Fouling is caused by biological contaminants in the water or mineral deposits, which attach themselves to the tube surfaces. Fouled tubes are usually cleared by mechanical means or by caustic leaning processes.
Scaling: This occurs when certain salts in the water precipitate and form a film around the walls of the tube. Since the entire surface of the tube is covered, the heat transfer is adversely affected, impacting HE efficiency. The salts form the film at elevated temperatures and cannot be removed by mechanical means. Scaling is countered by using acidic solutions that dissolve the scales and is a time intensive process.
Measuring HE performance:
The effect of fouling and scaling is to reduce the heat exchange between the two fluids. There is no direct way to measure the drop in HE efficiency, although a lowering of the temperature difference between the inlet and outlet is an indication of a defective HE. The recommended way to assess the performance of the HE is to carry out a simple heat exchange calculation.
The overall heat transfer co-efficient ‘U’ is used to measure the performance of a HE. The formula used is shown in
In this equation: U is the overall heat transfer coefficient. Q is the heat duty measured for sensible and latent heat separately. A is the area of heat exchange and LMTD is the Logarithmic Mean Temperature Difference, which is a function if the temperature difference between the fluids. W is the mass of the fluid entering the system.
The steps involved in finding out U are shown in figure 3. From data obtained through the calculations, the performance of the HE can be assessed by comparing the values obtained with the design values of the HE. The key inferences that can be obtained from the performance test are:
- Pressure drop across the HE: If this is more, it could be an indication of fouling. If it is less, it could be due to increased average bulk temperature of the HE due to lower performance.
- Temperature gradient: This gives an indication of the effectiveness of the heat transfer.
- Heat Transfer Co-efficient: This is the overall parameter, which gives an indication of the condition of the HE. A lower U value is on account of fouling of the HE tubes.
Heat exchanger maintenance:
As heat exchangers are in continuous operation, and there is usually not standby HE for a particular plant, maintenance of HE should be undertaken as per the OEM guidelines at a minimum. As there are no moving parts, preventive approach to maintenance is sufficient to keep the HE in a good condition. The key maintenance activities that need to be undertaken for the HE in a chiller plant are:
- Visual observation: Daily checks on the pressure and temperature parameters will help identify early on if any fouling has started.
- Maintaining water quality: This is a very important maintenance activity as poor quality of water will result in sludge formation, fouling and scaling.
- Avoid water stagnation: In case the HE is not in use for an extended period of time, the water in the system should be circulated at regular intervals to avoid rust formation in the tubes.
- Use of Anti Fouling/Scaling Additives: Where possible, the water should be dosed with anti-fouling/scaling additives that help in inhibiting the formation of scales.
- Annual tube cleaning: Minor amount of fouling and scaling will occur during operation. This is a progressive activity and if the scale layer is not removed in time, the impact on heat transfers increases substantially. Thus, the condensers should be cleaned at least once a year by shutting down the plant.
Heat exchangers are critical for the efficient operation of the chiller plant. Any deterioration in the condenser heat transfer will directly impact the efficiency of the chiller and lower the cooling capacity. A fouling factor of 0.001 (a measure of thickness of scale) can lead to an energy loss of 10 to 111 %. For a 500 TR plant, this translates to an annual increase in operating cost of approx. Rs 6,40,000. Thus, not only is regular HE maintenance a good practice, it also saves energy for the owners. The reality is however different, and HE maintenance is often the last priority in most maintenance activities, either due to ignorance or due to lack of priority when the budgets are drawn up for the maintenance activity. The best way to get the maximum out of the heat exchangers in the system is thus to have an effective maintenance program and regular measurement of the performance HE.
Aneesh Kadyan Director - Operations CBRE South Asia Pvt Ltd., Asset Services - India
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