S. ZAHEER Akhtar, TAI Engineers, Owings Mills, Maryland, U.S.
Chillers are extensively used in industrial, manufacturing and commercial facilities. These complex machines consume a significant amount of energy. This article provides an overview of chiller operation and the factors to consider for potential energy savings.
Chillers generally utilize a basic refrigeration cycle in which a heat-absorbing refrigerant is circulated through a compressor, condenser, expansion valve and an evaporator. The refrigeration cycle can be represented on the pressure-enthalpy (P-H) diagram, as shown in FIG. 1.
The compressor takes in low-pressure/low-temperature refrigerant vapor and compresses it to a high-pressure/high-temperature vapor. This is the power input process, and the pressure difference at the compressor suction/discharge is the compressor “lift.” The hot refrigerant vapor leaving the compressor dissipates the heat in the condenser. The liquid formed in the condenser then passes through an expansion valve and enters the evaporator where it absorbs heat and turns back to vapor (refrigeration effect). The vapor then enters the suction of the compressor, and the cycle is repeated.
Refrigeration cycle efficiency. The efficiency of a refrigeration cycle is measured by its coefficient of performance (COP) or energy efficiency ratio (EER). The higher the COP or EER, the higher the efficiency.
The COP is defined as the ratio of useful energy transferred (refrigeration effect, kW) divided by the work input [power input, kilo-watt (kW)]. Another way of representing COP is in terms of kW/ton (t). For example, a COP of 5 is equivalent to 0.7 kW/t or COP = 3.516/(kW/t).
The EER is defined as the cooling output in Btu/hr divided by the power input in watts.
The COP or EER usually indicates the chiller efficiency at full load. This does not account for the fact that throughout most of the year, the chillers operate at partial loads when chiller efficiency is lower. To provide a more realistic value of chiller efficiency values, the industry uses integrated part load values (IPLV). The IPLV is calculated per the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) Standard 550/590 on the weighted percentage of operational hours at each operating condition. The % weighting of part-load points per ARI Standard is:
Based on the above, Eq. 1 calculates the IPLV:
IPLV = 1 ÷ [(0.01 / A) + (0.42 / B) + (0.45 / C) + (0.12 / D)] (1)
The values of A, B, C and D in Eq. 1 are the chiller efficiency values at 100% load, 75% load, 50% load and 25% load, respectively.
Note: The AHRI Standard has the evaporator leaving water temperature (LWT) at 44°F. If the chiller is run at conditions other than those specified in the Standard, then the efficiency value is considered as non-standard part load values (NPLVs).
A chiller with an efficiency of 0.7 kW/t–0.8 kW/t (or a COP between 4.4 and 5) is considered a “Good” value by the U.S. Department of Energy (FIG. 2).
The effect of increasing evaporator-leaving water temperature (LWT). The P-H diagram (FIG. 3) shows the effect of increasing the evaporator LWT.
By increasing the evaporator LWT, the evaporator pressure increases and, as a result, the refrigeration effect increases (the distance between E1a and E0 is greater than the distance between E1 and E0). At the same time, the compression process (power input) is decreased (the distance between E2 and E1a is less than the distance between E2 and E1). The combined effect of the increased refrigeration effect and lower power input enhances the chiller capacity and efficiency.
By the same analogy, if the evaporator LWT is decreased, the refrigeration effect is decreased and the power input increases. The net result is a decrease in chiller capacity and efficiency. As an example, if a 10-t unit rated for an evaporator LWT of 45°F is now required to operate at an evaporator LWT of 40°F, the 10-t unit will derate to a 9-t unit (an approximately 2% derating for every 1°F that the evaporator LWT is lowered).
The effect of reducing condenser-entering water temperature (EWT). The P-H diagram (FIG. 4) shows the effect of reducing the condenser EWT (or entering air temperature if air cooled). This in turn reduces the condenser pressure, which directly impacts the vapor compression cycle.
By decreasing the condenser pressure, the refrigeration effect increases and the power input decreases. This enhances chiller capacity and efficiency.
In a water-cooled condenser, the condenser is supplied with cold water from the cooling tower. The temperature of the cold water (and hence the condenser pressure) is dependent on the ambient wet bulb temperature, which is relatively high at humid locations. The higher the ambient wet-bulb temperature, the higher the temperature of cold water leaving the cooling tower. Therefore, the design approach temperature between the ambient wet bulb and the cold water leaving the tower should be as low as practically possible. This is the first of the two-step heat exchange. The second step is the approach temperature between the refrigerant in the condenser and the cold water.
In an air-cooled condenser, the temperature inefficiency due to the two-step approach temperatures mentioned above is eliminated. In this case, the only approach temperature is between the fluid temperature in the condenser and the ambient air dry-bulb temperature.
Common types of chillers. Chillers are based on the type of compressors and the means used for heat rejection in the circuit. Some of the common types include:
This article mainly deals with centrifugal compressors, which can be air-cooled or water-cooled.
COMPRESSOR CAPACITY CONTROL
All facilities experience some reduction in chiller demand occasionally. The chillers operating at reduced load can provide energy savings depending on the type of capacity control methods provided. Some of these methods are discussed below.
Dual-compressor chillers. The dual-compressor chiller provides energy savings at reduced load. For example, at 50% load point, only one of the two compressors is operational with full heat transfer surface areas available in the evaporator and condenser. This lowers the chiller’s energy usage while still maintaining the 50% load. As such, the dual-compressor chiller operates more efficiently at partial loads.
Variable frequency drives (VFDs). At reduced load points, VFDs change the compressor speed and this provides capacity control. For compressor speed control to work, the compressor “lift” must be reduced either by raising the evaporator LWT or by reducing the condenser supply water temperature. The combination of reduced speed and lower “lift” provides significant energy savings.
VFDs are often used in combination with inlet guide vanes to modulate the capacity of the chiller. Compressor speed is typically only reduced to about 60% of design speed. The VFDs act as a soft start and replace the compressor motor starter. With a VFD, the chiller starts more slowly and never draws more than 100% of full load amps. This reduces wear and tear on the motor and also provides for quick restarts (no cooldown time required between restarts) in case the chiller loses power during operation.
VFDs introduce drive losses and, consequently, the chiller is less efficient at full speed with a VFD compared to one without a VFD. Conversely, conventional single chillers (non-VFD driven) operate most efficiently at or near full load.
VFDs on cooling tower fans provide another opportunity to save on power consumption.
Use of two-stage compressor and refrigerant economizer. Chiller efficiency can be improved by the use of a two-stage compressor used in conjunction with a refrigerant economizer, as shown in FIG. 5. The refrigerant economizer is simply a flash gas separator installed after the first-stage expansion device.
The flash gas separator directs the flashed gas to the suction of the second stage of the compressor. The liquid collected in the separator is sent through a second expansion device before it enters the evaporator.
The flashed gas entering the second stage of the compressor cools the refrigerant gas exiting the first stage. This reduces the work of compression. The liquid refrigerant after the second-stage expansion is at a lower enthalpy than that in a one-step expansion, thus increasing the refrigeration effect.
The reduction in the work of compression and the increase in refrigeration effect in the above set-up enhances chiller efficiency. However, this set-up requires the installation of additional equipment (flash gas separator and an additional expansion device) in the vapor compression cycle.
Expansion device. The expansion device regulates the flow of refrigerant into the evaporator coil. The expansion device can be a fixed metering device or a thermal expansion valve (TXV or TEV) like the one shown in FIG. 6.
The fixed metering devices (fixed orifice or capillary tube) are preset and designed for a specific set of operating conditions. These devices are used on small refrigeration appliances. On the other hand, the thermal expansion valve is a modulating metering device. It is more efficient and flexible as it regulates refrigerant flow based on temperature and pressure at evaporator outlet. The thermal expansion valve senses and responds to the degree of vapor sub-cooling at the evaporator outlet for precise control of the liquid refrigerant entering the evaporator coil.
Hot gas bypass. A properly designed chiller is not expected to experience a compressor surge. However, if operating conditions deviate from design and the compressor “lift” is increased at low-load operation, then compressor surge can occur. The reason for installing a hot gas bypass is to maintain the minimum volumetric gas flow (FIG. 7) through the compressor to avoid surge or stall at low-load operation.
Another reason for the hot gas bypass is to provide a means for chiller capacity control since the work of compression on the recirculated refrigerant does not produce any refrigeration effect. As such, hot gas bypass is inefficient and should be avoided if possible.
Takeaway. Chiller plant efficiency is best understood by following the P-H diagram for the refrigerant in use. Chiller plants are major consumers of energy—a “good” energy utilization target value of a chiller plant (including chillers, chiller oil pump motor, chiller control system circuit, condenser pumps and cooling tower fans) is around 0.7 kW/t–0.8 kW/t. Facilities operating chiller plants should evaluate their energy utilization value and compare. HP
S. ZAHEER AKHTAR is a Senior Technical Process Engineer with TAI Engineers and has more than 35 yr of experience in the chemical, gas and power generation industries. Akhtar served for 9 yr with Exxon Chemicals at its ammonia-urea manufacturing plant and more than 20 yr with Bechtel Power Corp. and its affiliates, such as Bechtel Oil & Gas, Bechtel Nuclear and Bechtel Egypt. He also worked as a Plant Engineer at the Ghazlan Thermal Power Plant in Dammam, Saudi Arabia with Saudi Electric Co.
Akhtar has been a participant member of ASME Performance Test Code Committees 4.3 (Air Heaters), 19.2 (Pressure Measurement) and 19.3 (Temperature Measurement). He has also authored several papers covering various process/mechanical topics related to industry. Akhtar earned an MS degree in chemical engineering from the University of Manchester Institute of Science & Technology (UMIST) – England, and is a Professional Engineer registered in the state of New York.