An industrial process chiller is a system that removes heat from a liquid via vapor compression. The vapor-compression refrigeration cycle, in turn, is a process that relies on a phase change of the refrigerant in order to absorb heat from the fluid.
The refrigeration cycle depends on four major components for this process to work:
- The evaporator.
- The compressor.
- The condenser.
- The expansion valve.
The evaporator is where the heat transfer from the liquid to the refrigerant occurs. In a chiller, this liquid typically is water or another heat transfer fluid like ethylene or propylene glycol. Much like the evaporation of a puddle of water to a vapor in the surrounding air, a phase change occurs that absorbs heat in the process. The liquid refrigerant goes through a similar phase change in the evaporator by absorbing the heat from the circulating chiller fluid, changing it to a gas. Once the evaporated refrigerant gas leaves the evaporator, it enters the compressor and is compressed to a high pressure, high temperature gas.
The compressor consumes electrical energy and converts it to heat during this process. This is called the heat of compression. The purpose of the compressor is to raise the refrigerant pressure and temperature to a point where either the cooler ambient air or a water source can remove both the heat absorbed from the process fluid and the heat of compression. The combination of the heat absorbed from the process fluid and the heat of compression is called the total heat of rejection. The discharge gas exiting the compressor then is sent to the condenser, where the total heat of rejection is removed from the refrigerant gas, changing it back to a liquid.
In an air-cooled chiller, this is done via cooler air passing over the condenser coil using a fan. In a water-cooled chiller, this is done by using a separate water source —for example, a cooling tower or city water — that is cooler than the condensing temperature of the refrigerant. Once the liquid refrigerant leaves the condenser, it goes to the expansion valve, which drops the high pressure, high temperature liquid to a low pressure, low temperature liquid before it enters the evaporator again. The valve is regulated by monitoring the state of the refrigerant leaving the evaporator using a temperature and pressure sensor to actuate a small pin in an orifice that meters the right amount of refrigerant. The cycle then is repeated.
Chiller efficiency can be affected by all of the major components. First, the compressor efficiency will affect the overall chiller efficiency. The use of variable-speed compressors in chillers makes them highly efficient. A variable-speed compressor regulates its speed to meet the exact demand of the process by monitoring the fluid temperature.
As the process load decreases, the leaving fluid temperature from the chiller will decrease, causing the compressor to slow down to avoid wasting unnecessary energy during load changes. The compressor has a DC motor that is controlled by an inverter drive. DC motors have the ability to control the speed over a wide range and have a low power demand on startup. The drive monitors the voltage and amperage along with other parameters. It works with the chiller control to keep the compressor operating at its most efficient point. The drive will even adjust the compressor speed to keep it operating safely (within its required parameters) when challenging conditions — for instance, a higher or lower ambient, or process temperatures that typically cause a chiller to shut down — exist.
These efficient chillers also use variable-speed condenser fans that in turn adjust speed to the most efficient operating point. This point is determined by an algorithm in the control that monitors all of the sensors included with the chiller. In conditions where the ambient temperature is low, the chiller will take advantage of this. It can automatically adjust the fan speed to keep the condensing pressure at its lowest to reduce the energy needed to compress the refrigerant.
Along with the variable-speed compressor and condenser fan, highly efficient chillers can incorporate an electronic expansion valve (EEV) rather than a mechanical valve. Mechanical valves operate on the system pressure, which can fluctuate with ambient and process temperature fluctuations, causing a loss in efficiency and reliability. The EEV does not need pressure to operate; instead, it is electronic. It works with the control algorithm to meter the refrigerant to the evaporator in order to make it operate as efficiently as possible. Also, the chillers can have more surface area on the evaporators than required for the full load in order to further improve efficiency. With some chillers, the control used can employ up to 10 pressure and temperature sensors along with other parameters from the compressor drive to control the compressor, condenser fan and EEV to an efficient, reliable operating point.
Two Commonly Used Efficiency Ratios
Chiller efficiency is a calculated ratio of the energy consumed over the amount of heat absorbed from the process fluid. Two commonly used efficiency ratios are:
- Coefficient of performance.
- Energy-efficiency ratio.
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Coefficient of Performance (COP). COP is the ratio between the refrigeration capacity, or heat removed, in BTU per hour or watts, and the electrical power input, in BTU per hour or watts. Therefore, the COP has no dimension. In simple terms, the higher the COP, the more efficient the system is.
Energy-Efficiency Ratio (EER). EER is the ratio between the refrigeration capacity, or heat removed, in BTU per hour, and the electric power input, in watts. The units of measure of EER are BTU per watt in an hour. In simple terms, the higher the EER, the more efficient the system is.
Most companies publish their COP or EER at 100 percent load capacity of the chiller. Because a chiller does not always run at 100 percent capacity, however, this may not be an ideal representation of the typical equipment performance. Drawbacks include:
- The performance is calculated at a single operating point.
- It does not include how a chiller will perform during partial-load conditions.
- Equipment with an excellent full-load efficiency may have less than satisfactory partial-load efficiency.
Using an Integrated Partial-Load Value
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The Air-Conditioning, Heating and Refrigeration Institute (AHRI) developed AHRI 550/590, a standard that defines the integrated partial-load value (IPLV) as a performance characteristic for chillers.
For nearly every chiller, over the lifetime of the chiller, it is running at 100 percent capacity only 1 percent of the time. In other words, 99 percent of the time, it is at partial-load conditions.
The IPLV is calculated using the efficiency of the equipment while operating at four capacities.
- A, which is an EER at 100 percent load.
- B, which is an EER at 75 percent load.
- C, which is an EER at 50 percent load.
- D, which is an EER at 25 percent load.
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The allocation of time running at these four loads is defined by AHRI as follows:
- A is 1 percent.
- B is 42 percent.
- C is 45 percent.
- D is 12 percent.
Given the above, IPLV is calculated:
IPLV = (1 percent x A) + (42 percent x B) + (45 percent x C) + (12 percent x D)
Why is IPLV important? The IPLV can affect energy usage and operating costs throughout the lifetime of the process chiller equipment. In fact, electrical consumption is the largest operating cost in almost all chiller-based cooling operations.
Chillers that provide the highest IPLV ratings use variable-speed compressors, electronic expansion valves and variable-speed fan motors. These components, along with a proprietary control algorithm, can produce better efficiency.
In my testing, the IPLV ratings of some highly efficient process chillers can be up to 60 percent higher than comparable products with IPLV ratings of approximately 15 (EER). For instance, one high efficiency 10-ton chiller that was tested — operating 16 hours a day (two shifts), five days a week, 52 weeks a year, in a location with a $0.14 per KW-hr electric rate equates to about $450 per IPLV (EER) point per year. In other words, a difference of seven points in the IPLV rating could increase or decrease energy costs by $3,150 per year.
The IPLV values for a sample line of chillers are shown in table 1. Chiller manufacturers can provide this data during the RFQ process to aid in the equipment-buying decision.