Retrofitting to improve performance
Traditionally, air cooled chillers were used for smaller applications with lower heat loads, but in the last 10 -15 years, with the advances in centrifugal compressor technology, air cooled chillers are being used in larger...
Air cooled chillers are a preferred choice in designs where availability of water is scarce and the weather is hot and dry for a majority of the year. Air cooled chillers offer many advantages such as the lesser number of system components (lowering maintenance costs), compact design, effective use of open space, lower operating costs etc. Traditionally, Air cooled chillers were used for smaller applications with lower heat loads, but in the last 10 -15 years, with the advances in centrifugal compressor technology, Air cooled chillers are being used in larger, more critical installations. Multirole compressor configurations have allowed the designers to offer designs for a wide range of loads thus further optimizing on the cots. The Air cooled chiller also scores better on the overall life cycle costs since the operating costs are lower due to absence of the condenser water systems and cooling towers.
Since condensation of the refrigerant is undertaken using the ambient air, the heat exchanger is a key component of the Air cooled chiller and the performance of the plant is directly affected by the performance of the heat exchanger. Deterioration of the heat exchanger effectiveness will have a direct impact on the capacity of the chiller to provide desired cooling to the work space. Unfortunately, the performance of the heat exchanger is dependent on a number of factors which all contribute to lowering heat transfer rates as the chiller operates over time, thus lowering the system efficiency as the plant ages. System performance can be dramatically improved by replacement of the condenser cooling coils when system performance has deteriorated beyond acceptable levels. This article showcases a case study of the retrofit of the condensers coils of an air cooled chiller and the benefits that accrued to the owners
The air cooled chiller that this case study is based on is part of 3 chillers system for air conditioning a large office space which has 24x7 operations. The building is located in the northern part of India, which experiences long hot and dry summers and a short monsoon season. The plants are located on the terrace of the building and the location of the building is such that there is adequate air flow across the terrace, without any large buildings obstructing the flow patterns. The key characteristics of the chiller are listed in table 1. The operations and maintenance of the chillers is undertaken by a dedicated onsite team and the systems are maintained to a high standard.
Need for retrofit
The chillers were installed in 2007 when the building became operational. As part of the M&M teams annual maintenance shutdown activities in late 2015, performance assessment of the 3 chillers was undertaken. The chillers physical condition, efficiency, operating parameters etc. were analyzed from the system logs as well as onsite visual assessment. While the typical life of Air cooled chiller is 12 – 15 years, the performance of the chillers was observed to be below the design parameters, with higher condenser pressures and lower EER (Energy Efficiency Ratios) or IKw based on calculations. The reasons for the deterioration of the performance was analyzed and the problem identified as the ineffective heat exchange across the condenser, leading to higher system pressures and consequent lower efficiency. The heat exchanger fins were seen to have a high level of deterioration as well. The cause of the higher level of deterioration of the heat exchange surface was attributed to the ambient conditions. The building was located in a zone where the last 4 – 5 years had seen a large number of building construction activities, leading to higher dust levels and increase size of suspended particulate matter in the air.
Since the condensers performance effects the overall chiller performance, a decision was taken to replace the condenser coils for one of the chillers due to the limited time available for shut down provided by the Maintenance team.
Business case for the retrofit
Condenser coils replacement involves both time and financial outlays. While the performance of the chiller was not as per design, the overall air conditioning system was delivering required thermal comfort to the occupants and workspace due to the redundancy and design of the system. However, the energy costs of the system had been increasing steadily over the past 4 – 5 years. There was thus a case to undertake corrective action to lower operating costs.
Chiller energy performance was reviewed from the BMS system data and annual energy consumption and correlated costs documented to create the baseline. With the help of the chiller manufactures, cost estimates were developed for the heat exchanger replacement and the projected efficiency gains calculated. The new heat exchanger coils evaluated enabled higher heat exchange rates due to the advances in fin design that had taken place since the chiller was installed. The key costs associated with the chiller heat exchanger replacement were identified as
• Cost of the heat exchanger
• Import costs as the Heat exchanger coils had to be procured from the OEM’s international locations
• Labor and material costs during the replacement phase
• Refrigerant top up costs
• Testing and commissioning costs post replacement
The potential efficiency gains and subsequent lowering of energy consumption as a comparison to the cost of the replacement were analyzed. The resulting Return on Investment (RoI), based on a simple Net Present Value (NPV) approach was assessed to be approximately 18 months. The relatively short payback period and high efficiency gains helped get a sign off from the management for the replacement.
Why heat exchanger performance fails
A Heat exchanger (HE) as the name suggest, is used to transfer heat across a surface. When the transfer is between two liquids, sensible heat is transferred, while if it there is a phase change, latent heat transfer takes place. Most HVAC applications involve sensible heat exchanger.
The rate of heat transfer across a heat exchanger is given by the formula Q = UAΔtm where
• Q is the rate of heat transfer
• U is the Overall heat transfer coefficient
• A is the surface area across which the heat transfer takes place
• Δtm is the temperature difference between the two liquids
In the above equation, U depends on the properties of the material and the distribution of the heat exchange surface such as cross flow or counter flow arrangements. The larger the areas A, the more the heat transfer that can occur and hence, HE design focuses a great deal on how to increase transfer area without increasing overall size. The temperature difference is a function of the system design as it depends on the inlet and out let temperatures required for the application in mind. Another important parameter in HE operations and design is the “approach Temperature. Refer to figure 1. The approach is the difference between T2 and t1. A smaller approach will need a larger surface area to get the desired heat transfer.
Figure 1: Heat Exchanger Temperatures...
With the above overview of HE theory, we can now correlate why a heat exchanger’s performance can deteriorate during its life. The key reasons are
- Lowering of U: The heat transfer coefficient changes due to various factors but mostly due to fouling of the surfaces. The scales reduce the velocity of flow and hence the heat transfer rate in addition to the actual heat transfer. Scaling occurs due to poor treatment of the water or due to high temperatures in the system than designed. Other causes of increase in U dust and microbiological growth over the fins in air cooled chiller and algae growth in the headers and condenser plates of water cooled chillers.
- Reduction of surface area: In air cooled chillers, the fins across which heat transfer occurs are fragile and tend to get damaged over a period of time and use. The damaged sections, due to the reduced surface area are not able to contribute to the full extent to the heat exchange and the overall transfer rates reduce.
- Temperature difference: When the HE is selected, the plant load is used as the basis. If during the life of the plant, there is a significant change in the heat load, the HE will be operating at higher temperatures which will result in lowering of the efficiency.
Results of retrofit
The retrofit was carried out in the month of Dec which is a typical lean month for HVAC operations. Post commissioning and operations, the plant’s functioning was compared with the other air cooled chiller in the system during the summer months. This helped the O&M team to quantify the actual gains due to the change in the heat exchanger. Table 1 lists the comparison of the operating parameters:
The replacement of the chiller resulted in a savings of approx. 8 – 9 % on the energy consumption. In addition, the overall system efficiency was improved as the time taken to cool the workspace reduced. Indicated Kw, which is a direct measure of the energy output to the energy input for chiller was brought down from 0.998 to 0.58 due to the change of the heat exchanger coil.
As air conditioning systems age, the performance of the plant starts to deteriorate from the design point, leading to higher powerconsumption and operating costs. The failure rates also increase due to the plant operating at off design values. While replacement may not always be an option, component level replacements that can increase the efficiency of the plant can be evaluated to keep costs low. There is an investment requirement for such changes as replacement of the HE coils, but with advances in technology, the new components tend to be significantly more efficient and hence, the returnof Investments period reduces. O&M team should thus identify opportunities such as the case study shared above to improve system performance as well as reduce operating costs.
AUTHORS CREDIT & PHOTOGRAPH
Director - Operations
CBRE South Asia Pvt Ltd.
Asset Services - India
Working as DGM -
(For a leading real
estate services firm)