Chillers typically account for approximately 35-50% of an entire buildings power consumption, making them the single largest power consumer for most properties. As power prices continue to increase, the need for vigilance in regards to the chillers health must also become a high priority. Whilst the chiller may be providing adequate cooling for your current needs, and may be running without any visible alarms or faults, these factors alone are not a true or adequate reflection of the health of the chiller. The chillers health may be continuously degrading over time, due to a wide variety of causes, which, if left unchecked, result in significant decreases in the chillers efficiency, and very costly increases in the total power consumption required to providing the cooling needed. Even when preventative maintenance programs have been adhered to, many things that will directly affect the chiller’s efficiency may escape unnoticed.
It has been found, that many chillers lose up to about 30% of their capacity and efficiency, without having been identified as an issue, meaning that in a typical building, the buildings power bill may have increased by up to 15%, without anyone identifying the cause, or even realising that this could have been prevented, or at least minimised.
Whilst it may not be possible to fully restore the chillers performance back to its design levels, once you have identified that your chillers health has suffered, it may be possible to identify the nature and scale of the losses, and schedule various works that can help to reduce these losses to the lowest point possible.
A Check My Chiller Health Report provides you with an up to date, accurate reflection of the chillers status at that point in time. The report will provide the user with some possible causes for any health impact on the chiller, which may then be further investigated on site by the user, or by a trained professional, in order to hopefully remedy any of the issues identified. My taking prompt action, you can maximise the energy efficiency possible for the chiller in its current state, resulting typically in significant reductions in power consumption, providing ongoing saving benefits. This report may also highlight any capacity related issues that may be preventing the chiller from achieving the required capacity that your building requires, and provide indicative steps about how to increase the capacity of the chiller, closer to the original design capacity.
In addition, the overall reliability and efficiency of a chiller, is directly linked to how effectively it has been maintained, how well are all its components working, and how appropriately it has been set up. Issues that have been highlighted, then proactively addressed, before they cause a critical shut down, will result in less down time, more consistent operation, and potential avoidance of costly chiller repair bills, or unscheduled repair works.
The Report provided also provides information that can be utilised to make more informed decisions about the operational state of the chiller, and a clearer indication of the chillers performance, in both efficiency and capacity, and therefore the ongoing cost to the owner. As such, this would provide the means for more informed decision making, and more accurate Return on Investment calculations to be conducted when looking into potential chiller replacement options.
Chiller Capacity is calculated by multiplying the flow rate, with the Chilled Water Delta T (the difference between the entering and leaving chilled water temperatures), along with the specific heat of water (assumed to be 4.186 kJ/kg/°C)
There are many potential causes of chiller efficiency and capacity decreases, so here are a list of some of the more common issues:
For more information on the above, please click on the specific item to get additional information.
Evaporator Approach: If the evaporator approach (Evaporator Approach = Leaving Chilled Water Temperature – Saturated Refrigerant Evaporating Temperature) is too high, the capacity of the chiller will be negatively affected, as the compressor may not be able to deliver as much capacity, or must work much harder to achieve the same capacity as at the design approach temperatures as the suction pressure is lower than normal. A high Evaporator Approach could be an indication of a loss of refrigerant, or an unbalanced refrigerant distribution in the chiller due to a faulty level sensor, or expansion valve, or a controls issue. It could also be due to fouling within the water circuit, however this should be relatively unlikely, due to most chilled water circuits being a closed loop, and treated at the initial stages to prevent fouling occurring. It some cases, this can also be due to the presence of additives in the water, such as glycol, which changes the properties of the water that give it the ability to absorb or transport the heat. The properties that would affect this would be the Specific Heat (the ability of a fluid to absorb and transport heat), the Specific Gravity (the density of the water as compared to water), the Viscosity (the ability of a fluid to flow), and the Thermal Conductivity (the rate of heat transfer possible).
Condenser Approach: If the condenser approach (Condenser Approach = Saturated Refrigerant Condensing Temperature – Leaving Condenser Water Temperature) is too high, the capacity of the chiller will be negatively affected, as the compressor may not be able to deliver as much capacity, or must work much harder to achieve the same capacity as at the design approach temperatures as the discharge pressure would be higher than normal. A high Condenser Approach is most commonly due to fouling within the water circuit, however, it may also be due to a loss of refrigerant, or an unbalanced refrigerant distribution in the chiller due to a faulty level sensor, or expansion valve, or a controls issue. It some cases, this can also be due to the presence of additives in the water, such as brine, which changes the properties of the water that give it the ability to absorb or transport the heat. The properties that would affect this would be the Specific Heat (the ability of a fluid to absorb and transport heat), the Specific Gravity (the density of the water as compared to water), the Viscosity (the ability of a fluid to flow), and the Thermal Conductivity (the rate of heat transfer possible).
Fouling: Fouling is often found to account for about 30% of the energy losses in a chiller. Fouling is typically caused by sludge, mud, scale, biological growth or contaminants found in the water, that accumulate, or build up over time on the water side of the heat transfer surface of the tube. As this build up occurs in the tubes, the energy consumption also increases as the fouling acts as an insulator, and negatively impacts the thermal conductivity of the tube, and therefore the rate at which the heat exchange between the water and refrigerant can occur. As this causes the chillers efficiency to decrease, and the power consumption to increase, it often causes the compressor to reach its maximum power available much sooner than it should, resulting in a significant loss of capacity.
Approach is the difference in temperature between the leaving water temperature, and the saturated (liquid) refrigerant temperature. In the Evaporator, the refrigerant must be colder than the leaving chilled water temperature in order to cool the warmer entering chilled water down to the chilled water set point. In the Water Cooled Condenser, the refrigerant must be warmer than the leaving condenser water temperature in order to transfer heat out of the high temperature discharge gas, into the condenser water circuit, in order to condense the refrigerant back into a liquid state. In an Air Cooled Condenser, the refrigerant must be much warmer than the ambient air temperature, in order to transfer the heat from the high temperature discharge gas, into the air, in order to condenser the refrigerant back into a liquid state.
Evaporator Approach = Leaving Chilled Water Temperature – Saturated Refrigerant Evaporating Temperature. A Normal Evaporator Approach temperature on a flooded chiller is between 0-2°C, and on a DX chiller between 3-5°C, although these may be slightly higher on a single pass machine.
WC Condenser Approach = Saturated Refrigerant Condensing Temperature – Leaving Condenser Water Temperature. A normal water cooled Condenser Approach Temperature is between 0.5-2.5°C, although these may be slightly higher on a single pass machine.
AC Condenser Approach = Saturated Refrigerant Condensing Temperature – Ambient Air Temperature. An Air-cooled Condenser would typically have an approach at full load of between 10-15°C.
High Condenser Approach: Mechanical Tube Cleanings can be performed, which uses a brush, and fresh water to thoroughly, manually, and mechanically clean each individual tube. These should typically be carried out approximately yearly.
High Evaporator Approach: The refrigerant distribution system should be inspected, and a refrigerant leak detection should be carried out to identify if this is currently a leak resulting in an insufficient refrigerant charge. Mechanical Tube Cleanings can also be performed, which uses a brush, and fresh water to thoroughly, manually, and mechanically clean each individual tube. These should typically be carried out approximately every 5 years.
Incorrect Pressure/Temperature Refrigerant Relationship: This indicates the presence of contaminants in the refrigerant, various non-condensibles, most typically due to the presence of air. Refrigerant Analysis can be performed at an off-site laboratory, to identify the purity of the refrigerant, the presence of non-condensables, and the presence of oil, or other contaminants. If these are found, it may be possible to purge the air, or you may need to replace the entire refrigerant charge with new refrigerant.
High compressor superheat: High compressor superheat is typically the result of the evaporator being starved of a sufficient refrigerant volume. This may be due to the refrigerant distribution system, such as an issue with the refrigerant level sensor, a refrigerant pressure/temperature sensor, the expansion valve, a blocked filter/dryer, a restriction in the liquid line (for example a valve not fully open), the control settings, or due to an insufficient refrigerant charge due to a refrigerant leak.