8. ANEXOS
8.1. Anexo de información estadística de interés
The two types of centralized cooling equipment we will discuss in this chapter are chill- ers and cooling towers.
Chillers
Chillers are commonly used in centralized cooling systems to extract heat from the work- ing fluid of the building cooling system and reject this heat to the outdoors. Electric chill- ers are the most common types of chillers used in centralized cooling systems. Chillers which utilize steam or natural gas as the fuel source are also used, but are less common.
Purpose
Most people are familiar with the air-conditioning system in their home, which uses a cooling coil containing refrigerant to cool the air that is blown across it by the supply fan. This type of cooling coil, which is referred to as a direct expansion (DX) coil,27 is
also used in HVAC systems for commercial buildings. However, DX coils, and their Figure 4-10 Crossover temperature for a typical water-source heat pump application.
100
C h a p t e r F o u r C e n t r a l p l a n t101
associated refrigeration systems, have certain limitations (particularly limits on the dis- tance between the cooling coil and the heat rejection equipment), which make them unsuitable for certain HVAC systems, particularly centralized HVAC systems. Chilled water cooling systems, on the other hand, do not have the limitations of DX cooling systems. These systems are utilized extensively in commercial HVAC systems because water can be cooled in a central location and distributed at great lengths to the various air systems and terminal equipment, which utilize this chilled water to cool the building.
At the heart of a chilled water system is the chiller. A chiller is a central piece of equipment whose sole purpose is to cool (or chill) the water returned from the building cooling equipment to a temperature usually between 42 and 45°F. This water is then pumped from the central chilled water plant to the building cooling equipment where it is used to cool the building.
If the chiller is used as part of a thermal storage system, it will produce approximately 18°F brine during off-peak (nighttime) hours. The brine is circulated through the thermal storage system to store thermal (cooling) energy in the thermal storage media, which is typically either ice or brine. During on-peak (afternoon) hours, the cooling energy stored in the thermal storage media is used to cool the building, which reduces the electrical demand of the facility and the associated energy cost during the cooling season.
Physical Characteristics
There are many different types of chillers. However, all chillers essentially consist of equipment that utilizes energy to extract heat from the chilled water loop and reject this heat to the outdoors. The energy source can be electricity, steam, or even natural gas. In this book, we will focus on chillers that utilize electricity as the source of energy to com- press a refrigerant in a closed vapor compression system to produce chilled water.
Chillers that utilize steam or natural gas as the energy source are called absorption chillers. Absorption chillers use an absorption process that occurs between a refrigerant (typically water) and an absorbent (typically lithium bromide) to produce chilled water. Absorption chillers are suitable for applications where there is an abundant source of steam or natural gas to drive the absorption process or where it is not desirable to utilize electric- ity for cooling. Although these types of chillers will not be discussed in this book, their application within a central chilled water plant is similar to that of an electric chiller.
Electric chillers contain the four main components that are a necessary part of a vapor compression system: the evaporator, the compressor, the condenser, and the expansion device.
The evaporator is contained within the cooler, which is one part of a chiller. The cooler is a shell and tube heat exchanger in which the refrigerant circulates through either the shell or the tubes of the cooler28 and is boiled (or evaporated) by the warmer
chilled water that is circulated through the other component of the cooler. The chilled water is cooled by the heat that is absorbed by the evaporation of the refrigerant.
The compressor draws the low-pressure gaseous refrigerant from the evaporator, compresses it, and discharges it at high pressure to the condenser. The most common types of compressors used for electric chillers are reciprocating, scroll, screw, and cen- trifugal compressors. Each of these compressors utilizes a different approach for com- pressing the refrigerant and each has characteristics that make it suitable for certain applications and size ranges of chillers.
The condenser rejects all of the heat from the vapor compression system, including the heat that is absorbed by the evaporator in the cooler and the heat that is added to
100
C h a p t e r F o u r C e n t r a l p l a n t101
the system by the compressor. Condensers can be water-cooled, air-cooled, or evapora- tive (air-cooled with water spray). A water-cooled condenser is usually part of a pack- aged indoor chiller and normally consists of a shell and tube heat exchanger in which the (warmer) refrigerant circulates through the shell of the heat exchanger and is con- densed by the (cooler) condenser water that is circulated through the tubes of the heat exchanger. The condenser water is warmed by the heat that is rejected by the condensa- tion of the refrigerant. The heat that is absorbed by the condenser water is commonly rejected to the outdoors through a cooling tower.
An air-cooled condenser is usually part of a packaged outdoor chiller, although it may also be the outdoor component of an indoor chiller. It consists of a finned-tube coil through which the refrigerant is circulated and condensed by outdoor air that is drawn across the coil by one or more condenser fans. An evaporative condenser is an air- cooled condenser which has an additional recirculating water spray system. The evapo- rative cooling effect of the water sprayed over the condenser coil increases the heat rejection capacity of the condenser.
A thermostatic expansion valve is installed in the refrigerant liquid line between the condenser and the evaporator, which throttles (or regulates) the liquid refrigerant flow to the evaporator. As a result, the pressure in the evaporator is below the saturation pressure of the liquid refrigerant, which causes the refrigerant to boil. The heat required to boil the refrigerant is absorbed from the chilled water circulating through the water- side of the cooler, thereby cooling the chilled water. The thermostatic expansion valve regulates the refrigerant flow through the evaporator to maintain a constant refrigerant suction temperature, ensuring that only refrigerant gas leaves the evaporator.
Connections
Connections for electric chillers consist of the following: 1. Water-cooled chiller (Figs. 4-11 and 4-12)
a. Chilled water supply and return piping connections to the cooler. b. Condenser water supply and return piping connections to the condenser. c. Refrigerant pressure relief piping connection.
(1) The refrigerant pressure relief piping will be connected to the refrigerant pressure relief device (a relief valve, rupture disk, or both) on the chiller and will be routed to the outdoors in accordance with the requirements of
ANSI/ASHRAE Standard 15-2010 —Safety Standard for Refrigeration Systems. d. Electrical power connection: The electrical power connection can be either
single-point or dual-point.
(1) Single-point electrical power connection
(a) In order to simplify the electrical power connection, a single-point electrical connection and unit-mounted disconnect switch are often available as optional accessories. In this case, the three-phase electrical power from the building electrical power distribution system would connect to the chiller at a single point: the unit- mounted disconnect switch. All wiring, unit controls, safeties, starters for the compressor(s) and oil pump, and other electrical components necessary for proper operation of the chiller would be factory-installed so the only field electrical connection required would be made at the unit-mounted disconnect switch. Also, a 120 volt (V)/1 Phase (Ø) control transformer is often available as an
102
C h a p t e r F o u r C e n t r a l p l a n t103
optional accessory so that the 120V/1Ø power required for the 120V/1Ø components on the chiller is derived through this trans- former from the three-phase power supply. This option avoids the requirement for a separate 120V/1Ø electrical power connection from the building electrical power distribution system.
(2) Dual-point electrical power connection
(a) If the chiller has two compressors, the standard offering is normally a dual-point electrical connection. In this case, two three-phase electrical power connections are required for the chiller, one for each compressor. (b) Unit-mounted disconnect switches are normally available for this
configuration as an optional accessory and so is the 120V/1Ø control transformer.
(3) Whether the electrical power connection to the chiller is single point or dual point should be coordinated with the project electrical engineer because the design of the building electrical power distribution system may favor one configuration over the other.
102
C h a p t e r F o u r C e n t r a l p l a n t103
e. Automatic temperature controls (ATC) connection.
(1) The ATC connection will be made at the chiller control panel and will integrate the operation of the chiller into the overall chilled water system. 2. Air-cooled chiller (Figs. 4-13 through 4-15)
a. Chilled water supply and return piping connections to the cooler. b. Electrical power connection.
(1) The electrical power connection is the same as is required for a water- cooled chiller, except additional factory-installed wiring and starters will be provided for the condenser fans.
c. ATC connection.
(1) The ATC connection is the same as is required for a water-cooled chiller.
Design Considerations
The following are design considerations for chillers. 1. Chiller selection
a. Type of compressor: Although it varies from one manufacturer to another, the type of compressors used and the associated range of chiller capacities are generally as follows:
(1) Reciprocating compressors: 25- to 75-ton chillers (2) Scroll compressors: 25- to 500-ton chillers Figure 4-12 Photograph of a water-cooled chiller.
C e n t r a l p l a n t
105
Figure 4-13 Floor plan representation of two air-cooled chillers.
Figure 4-14 Photograph of an air-cooled chiller.
C e n t r a l p l a n t
105
(3) Screw compressors: 75- to 800-ton chillers
(4) Centrifugal compressors: 100- to 4,000-ton chillers
A chiller manufacturer’s representative should be consulted to determine the appropriate type of chiller for each application.
b. Refrigerant: Refrigerants most commonly used in HVAC equipment fall into one of the following three categories: chlorofluorocarbon- (CFC), hydrochlorofluorocarbon- (HCFC), and hydrofluorocarbon- (HFC) based refrigerants.
In accordance with the Montreal Protocol, an international treaty administered by the United Nations Environment Programme (UNEP), which controls the consumption and production of ozone-depleting substances, CFC-based refrigerants, such as R-11 and R-12, are no longer produced due to their ozone-depletion potential (ODP). Today, CFC-based refrigerants are available only through stockpiled resources and are used to replenish refrigerant in existing equipment. HCFC-based refrigerants, such as R-22 and R-123, have a lesser ODP and are therefore not scheduled to cease production until the year 2030. HFC-based refrigerants, such as R-134a and R-410A, do not have any ODP and are therefore not regulated by the Montreal Protocol.
Chillers utilizing HFC-based refrigerants should be selected for use because they are not regulated by the Montreal Protocol. Chillers utilizing HCFC-based refrigerants should not be selected for use because, according to ASHRAE, the expected service life of chillers is 20 years or more, which would mean this equipment may still be operational after the year 2030 when HCFC-based refrigerants have ceased production.
Cost, availability in competing manufacturers’ equipment, equipment efficiency, and the owner’s requirements should also be given careful consideration when selecting the refrigerant for a chiller.
c. Capacity: Chiller capacity should equal the estimated peak cooling load of the areas served by the chiller. Some excess capacity can be allowed for future loads if these loads will be added within 5 years or so of the chiller installation. However, it is not advisable to oversize a chiller because of the Figure 4-15 Connection detail for an air-cooled chiller.
106
C h a p t e r F o u r C e n t r a l p l a n t107
added cost of the larger equipment and also because chillers typically operate most efficiently when they are fully loaded.
d. Multiple chillers: It is not common for there to be any redundancy in chiller capacity when the chilled water system serves cooling equipment providing comfort cooling only. Multiple chillers, each sized for the full load or some percentage thereof, are only necessary if the areas served have a critical requirement for cooling. Examples include certain areas of hospitals and data centers. Multiple chillers may also be desired for large-capacity cooling systems where the owner may want to limit each individual chiller to some maximum size. Finally, multiple chillers may be justified for a chilled water system that utilizes a thermal storage system. In this case, one chiller may be dedicated to producing chilled brine for the thermal storage system and another chiller may be dedicated to producing chilled water for the air systems and terminal equipment.
e. Capacity control: Because a chiller is seldom fully loaded, it is necessary for the chiller to be able to reduce its cooling capacity during part-load operation. The simplest way to reduce chiller capacity is to cycle the compressor. However, if this occurs too frequently, the life of the compressor will be shortened. If the only means of reducing the chiller capacity is through compressor cycling, multiple compressors for each chiller are recommended. This will reduce the frequency of cycling for each compressor and provide closer control of the chilled water supply temperature. Other means of controlling chiller capacity are available and depend upon the type of compressor used. Individual cylinders can be unloaded for reciprocating compressors, a slide valve can be used for screw compressors, and prerotation vanes can be used for centrifugal compressors. Although the HVAC system designer should know how the capacity is controlled, it is more important to know that it can be controlled and to what degree. Capacity control to 25% of a chiller’s full-load capacity is normally adequate for HVAC applications. For example, a 100-ton chiller should be capable of reducing its capacity down to 25 tons.
f. Efficiency: A chiller’s energy performance is rated in accordance with the guidelines established by the Air-Conditioning, Heating and Refrigeration
Institute (AHRI) Standard 550/590 (formerly ARI Standard 550/590). The two most common measures of a chiller’s full-load energy performance are:
(1) kW/ton: Electrical power input expressed in terms of kilowatts (kW) per cooling output expressed in terms of tons.29 A lower kW/ton
indicates better energy performance.
(2) Energy efficiency ratio (EER): Cooling output expressed in terms of British thermal units per hour (Btuh) per electrical power input expressed in terms of watts (W). A higher EER indicates better energy performance.
For example, a 100-ton air-cooled chiller that requires 110 kW input has a full-load energy performance of 1.10 kW/ton or 10.9 EER.30
Generally, water-cooled chillers have better energy performance than air- cooled chillers. However, one must be careful when comparing the energy performance ratings of water-cooled chillers to air-cooled chillers because
106
C h a p t e r F o u r C e n t r a l p l a n t107
the full-load electrical power input for water-cooled chillers is limited to the compressor power only. For air-cooled chillers, the full-load electrical power input includes the compressor power plus the condenser fan power. For example, a water-cooled chiller (unit) may be rated at 0.65 kW/ton compared to the same size air-cooled chiller (package) rated at 1.10 kW/ton. This is not a valid comparison because other equipment is associated with the water-cooled chiller system in addition to the chiller itself. When comparing the overall energy performance (kW/ton) of a water-cooled chiller (system) to an air-cooled chiller (package), the electrical power input (kW) for all of the equipment associated with the water-cooled chiller (system) must be included in the comparison. This includes the water-cooled chiller compressor(s), condenser water pump, and cooling tower fan(s). The combined electrical input (kW) of all of this equipment should be divided by the water-cooled chiller’s full-load cooling output in tons. This value can be used to compare the energy performance (kW/ton) of the water-cooled chiller (system) to the air-cooled chiller (package).
g. Voltage: Chillers used for HVAC applications will utilize three-phase electrical power. The highest secondary three-phase voltage available in the building will normally be used to serve the chiller in order to keep the wire sizes of the feeder serving the chiller as small as possible. For example, if a building has both 480V/3Ø and 208V/3Ø power available, the chiller will normally be selected to utilize 480V/3Ø power. The project electrical engineer should be consulted to determine the appropriate voltage for the chiller. The HVAC system designer should also ensure that the chiller can be furnished by the manufacturer at the desired voltage.
2. Freeze protection for outdoor chiller installations: For areas subject to freezing temperatures, chillers installed outdoors must have some means of protecting the water in the outdoor pipes and cooler from freezing. There are three ways to accomplish this:
a. Drain the outdoor piping and cooler if the chiller will not be used during freezing conditions.
(1) If the outdoor piping and cooler are empty, freezing is not a concern. Isolation valves for the outdoor components must be provided indoors and drains must be provided in the low points of the outdoor components. Draining of the outdoor components becomes a main- tenance responsibility for the owner and must be performed every year prior to the onset of freezing temperatures. Refilling of the outdoor components must also be performed prior to the onset of cooling operation in the spring of each year.
b. Provide electric heating for all components subject to freezing if water is used in the chilled water loop.
(1) Thermostatically controlled heat tape is required on all outdoor piping, valves, and specialties. The heat output of the heat tape, expressed in terms of watts/foot (W/ft), depends upon the size of the pipe, thickness of insulation, and the design winter outdoor temperature. The HVAC system designer should consult with the heat tape manufacturer’s representative or catalog data and specify the level of protection (W/ft) required for all exterior chilled water piping. Heat output of 5 W/ft is
108
C h a p t e r F o u r C e n t r a l p l a n t109
typical for heat tape applications. The manufacturer’s installation instructions must also be followed by the installing contractor because it is necessary to wrap the heat tape around the piping, valves, and specialties in an appropriate fashion in order to provide effective freeze protection. Furthermore, the chiller should be specified with an electric heater in the cooler in order to provide freeze protection down to 0°F. Freeze protection can also be provided down to −20°F if the chiller controls are able to start the chilled water pump and circulate chilled water through the cooler. The HVAC system designer should be aware that electric heat tape and an electric heater in the cooler do not provide any freeze protection in the event of a power outage unless these electric freeze protection components are connected to a backup electric power system. c. Utilize brine (typically a solution of propylene or ethylene glycol) in the
chilled water loop.
(1) The freezing point of the brine should be 15°F below the lowest expected ambient temperature. A solution of 40% propylene glycol is common because it provides freeze protection down to about −8°F.
(2) Although glycol solutions provide effective freeze protection, the brine has a higher specific gravity, higher viscosity, and lower specific heat than water. Therefore, glycol solutions are less-efficient fluids than water for the following reasons:
(a) The higher specific gravity means glycol solutions are more dense than water and therefore require more pumping power to circulate the same flow rate of water.