Overview - Chillers
Chillers
In a typical commercial application, a central chilled water plant, consisting of one or several chillers, produces chilled water. This chilled water is pumped to cooling coils within air handlers, fan coils, and/or unit ventilators, where the heat of the air is transferred to chilled water.
It is estimated that chillers consume 20 percent of the total electrical energy generated in North America. A typical chiller will be in operation for 12 to 15 years for a reciprocating compressor and 20- to 30 years for a rotary screw or centrifugal chiller.
A chiller is often run at less than optimal operating efficiencies during its useful life. Optimizing a chiller requires a thorough understanding of the operating parameters that affect the efficiency of a chilled water system and what steps can be taken to improve chiller operation.
Chillers come in two different forms:
Refrigeration equipment
Chillers are refrigeration devices. There are different forms of refrigeration, but the most common is the vapor-compression cycle. A typical vapor-compression cycle has four mechanical components that circulate refrigerant in a closed loop: a compressor, condenser, metering device, and evaporator.
Compressors
The compressor is the primary driver of the vapor-compression cycle. The function of the compressor is to compress the refrigerant from a low-pressure vapor to high-pressure vapor. The most common compressors used in chillers are reciprocating, rotary screw, centrifugal, and scroll compressors.
Reciprocating compressors are piston-style, positive displacement compressors. Reciprocating compressors have lost a large amount of market share over the past 20 years to the rotary screw and scroll type compressors because reciprocating compressors have a greater number of moving parts, higher initial cost, and typically higher maintenance costs.
Rotary screw compressors are also positive displacement compressors. Two meshing screw-rotors rotate in opposite directions, trapping refrigerant vapor, and reducing the volume of the refrigerant along the rotors to the discharge point.
Centrifugal compressors are dynamic compressors. These compressors raise the pressure of the refrigerant by imparting velocity or dynamic energy, using a rotating impeller, and converting it to pressure energy.
Scroll compressors are also positive displacement compressors. The refrigerant is compressed when one spiral orbits around a second stationary spiral, creating smaller and smaller pockets and higher pressures. By the time the refrigerant is discharged, it is fully pressurized.
Condensers
The next component in the vapor-compression cycle after the compressor is the condenser. The condenser is a heat exchanger that rejects heat from the chiller.
For water-cooled systems heat is typically rejected through a cooling tower sitting outside or to city water or well water. Heat can also be rejected to air-cooled condensers which sit outside.
The air-cooled condenser can be stand-alone or part of a packaged air-cooled chiller. Refrigerant leaves the condenser as a liquid.
Metering devices
The next component is the metering or expansion valve. The purpose of the expansion valve is to meter high pressure refrigerant from the condenser into the low pressure evaporator.
Evaporators
The evaporator provides the cooling effect to the chilled water being circulated throughout the facility. Refrigerant leaves the expansion valve at a low pressure and temperature and enters the evaporator as a cool liquid-vapor mixture.
The evaporator is a shell and tube heat exchanger, where the liquid refrigerant changes to a vapor when heat is absorbed from the secondary fluid or chilled water circulating through the facility.
Secondary equipment
There are also components of a chilled water system that handle the secondary fluids like the chilled water, and the condenser cooling water or air. The chilled water pumps circulate the chilled water to the cooling coils within the facility and then back to the chiller so it can be cooled again.
The condenser cooling water in a water-cooled system is also circulated by a pump through the condenser, picking up heat and sending it back to the cooling tower to reject heat.
An open loop cooling tower cools water through a combination of heat and mass transfer. The warm water from the condenser is circulated to the tower and distributed in the tower by spray nozzles, or splash bars. Outside air is circulated through the tower by a fan or natural draft as warm air rises through the tower.
As some water evaporates the remaining liquid water gives up heat to the evaporation process that effectively cools the condenser water. About five percent of the condenser water is lost to this airflow in a typical cooling tower requiring makeup water to be added to the cooling tower system.
The capacity of a cooling tower is typically controlled by means of a modulating the airflow through the tower. Airflow can be modulated by a simple on/off, two-speed on/off, or by using a variable speed drive on the fan motor to control airflow through a large range of fan speeds. The water is circulated through an open loop tower at a continuous flow.
Making the best choice
Designing, selecting, or improving a chilled water system to perform efficiently under a variety of loads or conditions is more involved than just picking the most efficient chiller available. Total energy consumption, including ancillary energy associated with cooling towers, pumps, and fans, needs to be considered. Appropriate sizing of a chiller or multi-chiller system can also play a role in the performance of a chilled water system.
The control of secondary equipment like fans can further optimize and increase the overall efficiency of a chilled water system.
Energy efficiency
The efficiency of a chiller is a measure of the cooling capacity versus the required input power into the chiller. Below shows typical efficiencies for both water-cooled and air-cooled chillers:
Air-cooled
(including condenser power >150 tons)
EER
COP
kW/ton
ASHRAE Standard 90.1 1999
9.6
2.8
1.26
Good
9.9
2.9
1.21
Best
10.6
3.1
1.13
Water-cooled
(>300 ton centrifugal compressor)
EER
COP
KW/ton
ASHRAE Code 90.1 1999
16.7
4.9
0.72
Good
18.5
5.4
0.65
Best
26.7
7.8
0.45
Most air-cooled system efficiencies are specified using a measure called the Energy Efficiency Ratio (EER), which is the Btu per hour of cooling capacity per watt of input power.
Water-cooled chiller efficiencies are often specified in terms of kW of input power per ton of cooling. One ton of cooling capacity is equal to 12,000 Btu/hr which is the cooling capacity made available by melting one ton of ice in an hour.
At first glance a water-cooled chiller appears to be a far more efficient solution, but one must factor in the additional energy associated with cooling tower fans and pumps.
A water-cooled chiller will also have a higher first cost than an air-cooled chiller. The additional maintenance costs associated with open cooling towers like water treatment to control bacterial and fungal growth as well as water softening to remove particulate and prevent scaling in the system should also be considered.
Size
Selecting the appropriate size of a chiller is critical to an efficient chilled water system. Sizing of the chiller or chillers should always be done using a thorough calculation of the maximum space cooling loads and process loads.
Loads for space cooling are calculated based on outdoor design conditions, solar loads, estimated cooling loads associated with internal loads from people and equipment, and infiltration and ventilation loads. Once the maximum cooling loads are determined the total size, in tons, of the chiller can be determined.
It is important to understand that cooling loads will fluctuate over time and the size or capacity of the chilled water system should be designed to accommodate these load changes.
A common practice to handle fluctuating loads is to install multiple chillers of different sizes that can be staged to operate so as to best accommodate the fluctuating load.
In addition, the system should be designed such that additional capacity can be added in the future at a minimal cost.
Cooling tower control
The amount of heat that needs to be rejected to the outdoors is a function of the cooling load. When the cooling load decreases for a facility there will be an associated decrease in required heat rejection of the cooling tower.
Normally, the heat rejection from a cooling tower is decreased by using on/off fan control. A more effective and efficient means of controlling cooling tower capacity is to vary the speed of the cooling tower fans to match the required heat rejection.
Two speed motors will help save energy and can be an inexpensive solution, but not as effective as fans with variable speed drives which will typically provide fan modulation from 30 to 100 percent.
Variable speed drives can provide a high return on investment and a payback of two years or less for facilities operating continuously throughout the year like plastic injection molding plants, hospitals, or industrial facilities with process cooling loads.
Secondary loop chilled water flow control
Variable speed drives can be used on the secondary chilled water loop to save energy. Typically, when the demand for cooling decreases, a three-way valve will begin to bypass chilled water around the cooling coil.
A more efficient method is to use two-way valves combined with variable speed drives on the pump motors to adjust the flow of water through the cooling coils to provide the required cooling.
An existing secondary loop with three-way valves can be retrofit for operation with a variable speed drive by plugging the bypass port in the three-way valve, but valve operators must be checked to verify that they have sufficient power to operate in this mode.
Free cooling
Instead of operating energy intensive chillers during cool, dry outdoor conditions, “free cooling” uses outside air to provide direct cooling to the facility or process using the cooling tower.
Chilled water plants with required cooling, such as plastics molding and extrusion plants, food processing plants, hospitals, and industrial facilities with large heat-rejection loads are good candidates for free cooling.
Three-way valves can be used so free cooling and mechanical cooling, using the chiller, are provided at the same time.
Rejection heat through the cooling tower should be high enough during below 32F outdoor temperatures to minimize ice build-up in the cooling tower. Typically a heaters coil is placed in the sump of a cooling tower as a backup to avoid freezing.
In addition, fan reversal controls to change the flow of air through the cooling tower are sometimes used to de-ice internal components within the cooling tower.
On the horizon
Most chillers operate below their design capacity for the majority of their life. A tremendous amount of resources are still being invested in designing compressors that operate at high efficiencies during partial loading.
Some manufacturers are offering high-speed centrifugal compressors with magnetic bearings and built in variable speed control making them ideally suited for part loads.
In addition, with the tighter control provided by increased sensor accuracy and the ability to create more sophisticate control solutions, some chiller manufacturers are allowing for variable chilled water flow through the chiller to provide a simple pump energy savings solution with low initial investment.
Energy management systems (
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With increasing pressure to decrease demand or kW usage during peak demand periods, thermal storage, or off-peak ice making, is becoming increasingly popular.
Thermal storage uses the chiller to produce ice or cool down a thermal storage system during off-peak hours and then consumes the cooling capacity of this thermal storage during peak demand periods during on-peak periods to decrease or eliminate demand contributed by the chiller.
A water\glycol mixture around 20-25F is typically used to make ice. Although this can reduce billed demand, it may actually use more energy than running the chiller at normal chilled water temperatures.
Material courtesy of Alliant Energy