SECTION 10 CHILLED WATER AIR CONDITIONING SYSTEMS UNIT 48

SECTION 10 CHILLED WATER AIR CONDITIONING SYSTEMS UNIT 48 HIGH-PRESSURE, LOW-PRESSURE, AND ABSORPTION CHILLED-WATER SYSTEMS

UNIT OBJECTIVES • List various types of chilled-water air conditioning systems • Describe the operation of a typical chilled-water system • Describe the compressors typically used on chilled-water systems • Describe the difference between direct expansion and flooded chillers • Explain the concept of approach temperature in chiller systems • Explain the application and purpose of the purge unit • Describe the absorption cooling system process • Describe the motor types used in chiller systems After studying this unit, the reader should be able to

CHILLERS • Refrigerate circulating water • Chilled water is circulated and used to absorb heat from the building • 45° water is supplied to the building (design) • 55° water is returned to the chiller (design) • Water is cooled from 55° to 45° in the chiller • Compression type and absorption type

55° air to occupied space 75° air from occupied space 55° water from coil (to chiller) 45° water to coil (from chiller)

Chilled water coils Centrifugal pump Chiller barrel Metering device Condenser

COMPRESSION CYCLE CHILLERS • Compression (vapor pumps): reciprocating, scroll, screw and centrifugal • Normal boiling point of the refrigerant is 38° • Normal condensing temperature of the refrigerant is 105° • Classified as high-pressure or low-pressure systems

RECIPROCATING COMPRESSOR CHILLERS • Multiple small compressors are commonly used instead of one large compressor • If the large compressor fails, the entire system is off line • If one small compressor fails, the others continue to operate, providing some backup • Large chillers must have capacity control • Prevents compressor short cycling • Reduces compressor wear

CYLINDER UNLOADING • Provides means for controlling compressor capacity • As the capacity is reduced, the power needed to operate the compressor is also reduced • Compressors operate with lower compression ratios when unloaded • Blocked suction • Utilizes a solenoid valve • When the valve closes, no refrigerant enters the cylinder • Suction valve lift unloading • Accomplished by lifting the suction valve from its seat • Prevents the cylinder from pumping

Solenoid valve (open) Common suction line Common discharge line Compressor cylinders and pistons (both cylinders are pumping)

Solenoid valve (closed) Common suction line Common discharge line Compressor cylinders and pistons (the left cylinder is not pumping)

SCROLL COMPRESSOR CHILLERS • Positive displacement compressor • Efficient, low noise levels, fewer moving parts can pump small amounts of liquid refrigerant without compressor damage • Compressor uses two nested scrolls • Equipped with check valves to prevent backward flow in the off cycle

ROTARY SCREW COMPRESSOR CHILLERS • Can handle large volumes of refrigerant with few moving parts • Positive displacement compressor • Can handle small amounts of liquid refrigerant without compressor damage • Compressors range from 50 to 700 tons • Capacity control is accomplished with a slide valve • Usually equipped with oil separators

CENTRIFUGAL COMPRESSOR CHILLERS (HIGH-PRESSURE) • Refrigerant is moved from the low side of the system to the high side by centrifugal force • Gear boxes are used to enable the compressor to reach speeds of about 30,000 rpm • When the head pressure becomes too high or the evaporator pressure becomes too low, the compressor stops pumping

Impeller Suction (inlet) Discharge (outlet) Centrifugal compressor housing

CENTRIFUGAL COMPRESSOR CHILLERS (HIGH-PRESSURE) • Compressor is lubricated by a separate motor and pump • Capacity control is accomplished by the use of guide vanes • Load limiters are used to prevent compressor overload • Centrifugal compressors can be hermetically sealed or can have open drives

EVAPORATORS FOR HIGH-PRESSURE CHILLERS • Liquid refrigerant boils when it absorbs heat from the circulating water • Most commonly made of copper • Chillers have a water-to-liquid heat exchange in the evaporator • Can be direct expansion type or flooded type

DIRECT EXPANSION EVAPORATORS • Also known as dry-type evaporators • Operate with a predetermined superheat • Thermostatic expansion valves are normally used to control refrigerant flow to the evaporator • Water is piped to the shell of the chiller barrel • Refrigerant enters the chiller barrel from the end

One-pass chiller barrel Suction line Water to remote cooling coils Chiller barrel Liquid line Centrifugal pump

Two-pass chiller barrel Suction line Chiller barrel Water to remote cooling coils Liquid line Centrifugal pump

Three-pass chiller barrel Suction line Water to remote cooling coils Chiller barrel Liquid line Centrifugal pump

Four-pass chiller barrel Suction line Chiller barrel Water to remote cooling coils Liquid line Centrifugal pump

FLOODED EVAPORATOR CHILLERS • Refrigerant enters the barrel at the bottom • Water boxes are used to direct water flow through the tubes • By design, water enters the chiller at 55° and leaves at 45° • The refrigerant is usually about 7° cooler than the leaving water (approach temperature) • Flooded chillers usually have permanently mounted thermometers and pressure gages • Freeze protection may be required

Suction line Refrigerant boiling temperature: 38°F 45°F Liquid line Water to remote cooling coils 55°F Approach temperature = 45°F - 38°F = 7°F

CONDENSERS FOR HIGH-PRESSURE CHILLERS • Used to transfer heat from the system • Can be air cooled or water cooled • Air-cooled condensers require less maintenance • Heat can be recovered for use in other applications • Can be used to heat domestic water

WATER-COOLED CONDENSERS • Usually shell and tube type (for high-pressure chillers) • Water circulates in the tubes • Refrigerant is piped into the shell • Bottom of the shell acts as a receiver • Can be equipped with water boxes or marine water boxes

Hot discharge gas from compressor Subcooled liquid from condenser Water Tubes Shell Warm water out Cool water in Bottom of the condenser acts as the receiver

Directs water through the tubes Access to tubes for cleaning Water box Hot discharge gas from compressor Subcooled liquid from condenser

CONDENSER SUBCOOLING • Design condensing temperature is about 105° when 85° water is supplied to the condenser • Subcooling the refrigerant adds to the system capacity • One degree for subcooling can increase capacity by 1% • Normal approach temperature is about 10° • Head pressure must be controlled • Head pressure can be maintained by a bypass valve • Bypasses water during a startup when the condenser water is too cold

Hot gas from compressor Refrigerant condensing temperature: 105°F 95°F Liquid line Water to cooling tower 85°F Approach temperature = 105°F - 95°F = 10°F Water from cooling tower

AIR-COOLED CONDENSERS • Usually constructed of copper tubes and aluminum fins • Multiple tans are used for head pressure control purposes • Head pressures are typically higher on air-cooled systems • Air-cooled condensers require less maintenance than water-cooled condensers

METERING DEVICES FOR HIGH-PRESSURE CHILLERS • Thermostatic expansion valve • Maintains constant evaporator superheat • The more evaporator superheat, the slower the heat exchange • Orifice • Fixed bore metering device • Flow rate is determined by the pressure drop across it

METERING DEVICES FOR HIGH-PRESSURE CHILLERS • Float-type metering devices • Low-side float • Located at the inlet of the chiller barrel • Maintains a constant liquid level in the barrel • High-side float • Located in the liquid line before the evaporator • Opens when the level of liquid refrigerant is higher in the liquid line than the evaporator • Electronic expansion valves

LOW SIDE FLOAT Suction gas to compressor Water box Liquid level in the evaporator Water box 55°F water 45°F water Float ball Liquid refrigerant from condenser Float valve seat

HIGH SIDE FLOAT From condenser Suction gas to compressor Water box Liquid level in the evaporator Float ball 55°F water 45°F water

LOW-PRESSURE CHILLERS • Typically use R-11, R-113, or R-123 • CFC refrigerants are no longer available • Manufacture of CFC refrigerants has been completely halted • Equipped with the same components as high-pressure chillers • Newer chillers use R-123

COMPRESSORS • Low-pressure chillers use centrifugal compressors • Centrifugal compressors run at speeds up to 30,000 rpm • Suction line fastened to the housing in the center • Compressed refrigerant is trapped in the volute and guided to the condenser • Can be operated in series with each other • Low-pressure chillers can have refrigerant working pressure as low as 15 psig

CONDENSERS FOR LOW-PRESSURE CHILLERS • Low-pressure chillers have water-cooled condensers • Usually are shell and tube type • Water is circulated through the tubes • Refrigerant is piped into the shell • Located above the evaporator • The liquid leaving the condenser flows to the evaporator by gravity • Can also have a subcooling loop

METERING DEVICES FOR LOW-PRESSURE CHILLERS • Controls the flow of refrigerant to the evaporator • Orifice and the float type are typically used • Same as those used on high-pressure chillers • High side float • Low side float

PURGE UNITS • Low-pressure chillers operate with the suction pressure in a vacuum • R-113 systems operate with both high- and low-pressure sides in a vacuum • If a leak occurs, air will enter the system • Air can cause system problems • Air can removed by the purge unit

ABSORPTION AIR-CONDITIONING CHILLERS • Very different from compression process • Uses heat instead of a compressor • Has many piping connections • Chilled water piping • Condenser water piping • Steam or hot water piping • Equipped with oil or gas burners • Usually range from 100 to 1,700 tons

ABSORPTION AIR-CONDITIONING CHILLERS • Water is impractical as a refrigerant in a compression system • For water to boil at 40°, the pressure must be 0.122 psia • Volume of rising vapor from boiling water is excessive • 2,444 cubic feet of water vapor would have to be removed for each pound of water that boils at 40°

ABSORPTION AIR-CONDITIONING CHILLERS • Water is used as the refrigerant in absorption systems • Does not use a compressor • Uses salt solutions • Lithium Bromide (LiBr) is commonly used to attract the water (called absorbent) • Lithium Bromide is usually mixed with distilled water to create a 60% LiBr/40% water solution • Absorption means to attract moisture

BASIC ABSORPTION CYCLE • Evaporator section • Water (refrigerant) metered into the evaporator • Water experiences a pressure drop to 0.122 psia • This cold water is sprayed over the evaporator tube bundle system circulating water • The cold water then evaporates, absorbing heat from the system water

BASIC ABSORPTION CYCLE • Absorber section • The LiBr solution attracts the water vapor • The LiBr solution then becomes diluted by the vapor • The LiBr has the water vapor removed in the concentrator (condenser) • The diluted LiBr solution is called the weak solution

BASIC ABSORPTION CYCLE • Concentrator and condenser section • The diluted weak solution is boiled • The heat used to boil the solution is either steam or hot water • The water vapor then condenses to a liquid • It is gathered and metered back to the evaporator • The concentrated solution is drained back to the absorber

SOLUTION STRENGTH • The greater the difference between the weak and strong solutions, the greater the system capacity • An over-concentrated strong solution can become rock salt • The start-up technician is responsible for the trim (proper adjustment of the charge) • When the system is initially started up, samples of the strong and weak solutions are taken and compared

SOLUTIONS INSIDE THE ABSORPTION SYSTEM • Corrosion occurs when air is introduced to the system • Systems must be kept as clean as possible • Filters are used to stop solid particles • Magnetic devices are used to remove steel particles • Solutions may appear to be rusted, but this is normal

CIRCULATING PUMPS FOR ABSORPTION SYSTEMS • Centrifugal type • Shaft and impellers are made of non-corrosive materials • Motors • Hermetically sealed • Operate within the system atmosphere • Cooled with cold refrigerant water from the evaporator (closed loop) • Manufacturer’s recommendations should be followed when servicing the pump motors

CAPACITY CONTROL • Can be accomplished by throttling the heat supply in the concentrator • 12 to 14 pounds of steam at full capacity • 6 pounds of steam pressure at half capacity • Can be accomplished by controlling the flow of the weak solution to the concentrator

SECTION 10 CHILLED WATER AIR CONDITIONING SYSTEMS UNIT 48

SECTION 10 CHILLED WATER AIR CONDITIONING SYSTEMS UNIT 48

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