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Absorption Chillers

General

The absorption cycle uses a heat-driven concentration difference to move refrigerant vapors (usually water) from the evaporator to the condenser. The high concentration side of the cycle absorbs refrigerant vapors (which, of course, dilutes that material). Heat is then used to drive off these refrigerant vapors thereby increasing the concentration again. Lithium bromide is the most common absorbent used in commercial cooling equipment, with water used as the refrigerant. Smaller absorption chillers sometimes use water as the absorbent and ammonia as the refrigerant. As you can probably guess, the absorption chiller must operate at very low pressures (about l/l00th of normal atmospheric pressure) for the water to vaporize at a cold enough temperature (e.g., at ~ 40°F) to produce 44°F chilled water.

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The simplified diagram here illustrates the overall flow path. Starting with the evaporator, water at about 40°F is evaporating off the chilled water tubes, thereby bringing the temperature down from the 54°F being returned from the air handlers to the required 44°F chilled water supply temperature. One ton of cooling evaporates about 12 pounds of water per hour in this step. This water vapor is absorbed by the concentrated lithium bromide solution due to its hygroscopic characteristics. The heat of vaporization and the heat of solution are removed using cooling water at this step. The solution is then pumped to the concentrator at a higher pressure where heat is applied (using steam or hot water) to drive off the water and thereby re-concentrate the lithium bromide. The water driven off by the heat input step is then condensed (using cooling tower water), collected, and then flashed to the required low temperature (40°F in our illustration) to complete the cycle. Since water is moving the heat from the evaporator to the condenser, it serves as the refrigerant in this cycle. There are also absorption chillers in use (e.g. in motor homes) that use ammonia as the refrigerant in the same cycle. The absorbent is the material that is used to maintain the concentration difference in the machine. Most commercial absorption chillers use lithium bromide. Lithium bromide has a very high affinity for water, is relatively inexpensive and non-toxic. However, it can be highly corrosive and disposal is closely controlled. Water of course is extremely low cost and safety simply isn't an issue.

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Absorption chillers are available in two types:

1. Single Effect (Stage) Units using low pressure (20 psig or less) as the driving force. These units typically have a COP of 0.7 and require about 18pph per ton of 9 psig steam at the generator flange (after control valve) at ARI standard rating conditions.

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2. Double Effect (2-Stage) Units are available as gas-fired (either direct gas firing, or hot exhaust gas from a gas-turbine or engine) or steam-driven with high pressure steam (40 to 140 psig). These units typically have a COP of 1.0 to 1.2. Steam driven units require about 9 to 10 pph per ton of 114 psig input steam at ARI standard rating conditions. Gas-fired units require an input of about 10,000 to 12,000 BTUH HHV per ton of cooling at ARI standard rating conditions. To achieve this improved performance they have a second generator in the cycle and require a higher temperature energy source.

Absorption chillers - maintenance considerations

General

Properly designed and installed absorption chillers can function without full time attendants. The machine can be started and brought on line with simple time clocks or energy management systems. Non-condensables are automatically purged and the operator can schedule normal routine maintenance. Obviously, local building codes may dictate that a full time operator is, or is not, required. This, in turn, is often a function of the size of the equipment, steam pressure, etc. Always consult local codes when considering these issues.

There are three primary maintenance areas: mechanical components, heat transfer components, and controls. The following segments discuss mechanical and heat transfer maintenance areas.

Mechanical components

One manufacturer's absorption chillers has a single motor/multiple pump configuration for refrigerant and solution flow and a purge unit. Other manufacturers use individual hermetic solution and refrigerant pumps cooled and lubricated by the pumped solution. Another uses open motors with a shaft seal.

Pictured here are two hermetic, refrigerant cooled and lubricated pump assemblies. The hermetically sealed motor drives the solution and refrigerant pump impellers. In this multiple pump arrangement, motor coolant and lubrication is by the fluid being pumped. Hermetic pump designs eliminate the need for external shaft seals Ð a maintenance item and potential source of air leakage.

Heat transfer components

The life, performance, and cooling capacity of absorption equipment hinges on keeping heat transfer surfaces free of scale and sludge. Even a thin coating of scale can significantly reduce capacity. Therefore, cooling tower water chemistry is critical, and failure to properly treat this water could void manufacturer warranties.

Scale deposits are best removed chemically. Sludge is best removed mechanically, usually by removing the headers and loosening the deposits with a stiff bristle brush. The loosened material is then flushed from the tubes with clear water.

Pump maintenance

When the electric motor and pump bearings fail, one design permits replacement of pump parts without removing the lithium bromide solution from the machine. The first step is closing the hand valves in the lubrication circuit, disconnecting the electrical supply, and removing the motor. The pump shaft seal maintains machine vacuum. Major pump repairs are accommodated by charging the machine with nitrogen to atmospheric pressure. Once complete, the machine is evacuated, and pump parts removed and repaired or replaced. Other designs require a more complicated replacement procedure.

Pump maintenance begins with the magnetic strainer which must be cleaned 2 weeks after the initial startup and at the mid-point in the cooling season. Shaft seals should be examined for wear at three year intervals.

Prolonged or seasonal shutdown

In the case of seasonal or prolonged shutdown, refrigerant may migrate from the evaporator to the absorption chiller causing a low refrigerant level in the evaporator pan and piping. Since refrigerant is used to lubricate pump and motor bearings, lubrication from an auxiliary source must be provided during the startup phase of operation. Once an operating charge of refrigerant has been recovered from the solution, the machine may be returned to normal operation.

This auxiliary circuit is usually established by connecting city water to the external connections of the pump lubrication piping. In all cases, follow the manufacturer's recommended procedures.

Purging non-condensable gases

All absorption chillers must be purged of non-condensable gases to maintain performance. The three methods used are steam jet, solution jet (or "motorless purge"), or a vacuum pump, with the vacuum pump being by far the most common.

Non-condensable gases migrate to the area of lowest pressure in the absorption chiller (the evaporator) where a small portion of the vapor is extracted and condensed in the purge unit using cooling water. Non-condensable are then evacuated by the vacuum pump. In normal operation, the purge system should operate about one hour a week. The vacuum pump oil level should be observed, maintained, and changed as necessary. Oil purge pump motor bearings should be inspected and replaced, and the belt adjusted as needed. In addition, the vacuum pump should be flooded with oil during seasonal shutdown to prevent internal corrosion.

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Purging of non-condensables can be accomplished using a "motorless purge" as shown here. Where motorless purging is used, an optional vacuum pump can also be used for evacuation.

In all cases, the operator should log purge operation and monitor purge operation trends. Increasing purge operation signals increasing in-leakage of air and moisture.

Applications

Single stage steam absorption chillers

Provide chilled water for cooling when low pressure steam, cooling tower (or other water for heat rejection), and electric power is available.

Two stage absorption chillers

Provide chilled water for cooling when high pressure steam, high temperature hot water (HTHW) or natural gas, as well as electric power and cooling tower (or other water for heat rejection) is available.

Waste heat fired absorption chillers

Provide chilled water for cooling when clean, hot exhaust gas, cooling tower (or other water for heat rejection), and electric power is available.

Best applications

Single stage steam absorption chillers

When steam in the 12 to 20 psig range from a process or other steam use is available at little or no cost (i.e. steam would otherwise be wasted).

Two stage absorption chillers

When steam in the 40 to 140 psig range from a process or other steam use is available at little or no cost (i.e. steam would otherwise be wasted), When natural gas is available at low cost relative to the cost of electric power,

When the heating load can not be readily served by an existing boiler and it can be served from this chiller/heater, thus avoiding adding a boiler or where space is not available for a boiler.

When adequate electric power is not readily available for added and needed cooling capacity,

When emergency cooling capacity is needed and stand-by generation capacity is not available to operate electric cooling. (Consider adding emergency generation capacity, which may be lower in cost than absorption cooling capacity).

Waste heat fired absorption chillers

Where exhaust from a gas turbine provides cooling for the intake air to improve turbine performance in hot weather,

Where cooling is required and clean exhaust gas is available, emitted from an industrial process such as those related to printing, drying or baking.

Possible applications

Single stage steam absorption chillers

When steam in the 12 to 20 psig range from a process or other steam use is available at a reasonable cost or where boilers must be operated for other reasons and the user is looking for other steam uses to adequately load the boiler.

Two stage absorption chillers

When steam in the 40 to 140 psig range from a process or other steam use is available at a reasonable cost or where boilers must be operated for other reasons and the user is looking for other steam uses to adequately load the boiler,

Replacement for existing inefficient single stage steam chiller without an electrical service upgrade.

Waste heat fired absorption chillers

Where clean exhaust gas is available and there are cooling requirements.

Waste heat fired absorption chillers

skilled operating personnel will not on duty during system operation,

operations are planned to use absorption chiller as a peak shaving unit. Absorption chillers require added to time and effort to bring on- and take off-line. Operators tend to end up using absorption as a base chiller and peak with the electric chiller, thereby defeating the purpose, and actually adding to, rather than saving, operating cost.

Extended operation at 30% and less of design capacity is likely.

Technology types (resource)

Two stage absorption chillers

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The energy efficiency of absorption can be improved by recovering some of the heat normally rejected to the cooling tower circuit. A two-stage or two-effect absorption chiller accomplishes this by taking vapors driven off by heating the first stage concentrator (or generator) to drive off more water in a second stage. Many absorption chiller manufacturers offer this higher efficiency alternative.

Notice that two separate shells are used. The smaller is the first stage concentrator. The second shell is essentially the single stage absorption chiller from before, containing the concentrator, condenser, evaporator, and absorption chiller. The temperatures, pressures, and solution concentrations within the larger shell are similar to the single-stage absorption chiller as well.

Steam at pressures typically in the l25 - 150 psig range is brought into the stainless steel tubes of the first stage concentrator causing the solution there to boil. The pressure at which boiling occurs and the pressure of the released refrigerant vapor is approximately 5 psig (20 psia). The partially concentrated solution from this first stage flows through the high temperature heat exchanger where it is cooled by the lower temperature dilute solution returning from the concentrator. This concentrate then passes into the lower pressure second stage concentrator where the vapors from the first stage take it to the final desired concentration levels. This second stage operates at a pressure of 0.1 atmosphere (1.5 psia).

The reuse of the vapors from the first stage generator makes this machine more efficient than single stage absorption chillers, typically by about 30%. Two-stage absorption chillers are typically driven by high-pressure (60 to 130 psig) steam, direct-fired with natural gas or #2 fuel oil, or using hot exhaust gas from combustion engines.

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Steam-fired 2-stage absorption chillers

Steam at pressures typically in the l25 - 150 psig range is brought into the stainless steel tubes of the first stage concentrator causing the solution there to boil. The pressure at which boiling occurs and the pressure of the released refrigerant vapor is approximately 5 psig (20 psia). The partially concentrated solution from this first stage flows through the high temperature heat exchanger where it is cooled by the lower temperature dilute solution returning from the concentrator. This concentrate then passes into the lower pressure second stage concentrator where the vapors from the first stage take it to the final desired concentration levels. This second stage operates at a pressure of 0.1 atmosphere (1.5 psia).

The reuse of the vapors from the first stage generator makes this machine more efficient than single stage absorption chillers, typically by about 30%.

Direct-fired absorption chillers

Direct-fired absorption chillers utilize a burner as the heat input for the absorption cooling cycle. Most operate either on natural gas or No. 2 fuel oil. Since the heat input is at a very high temperature, they achieve a very high efficiency for the absorption cycle...something approaching 12,000 Btu of fuel input for each ton hour of cooling output. The absorption cycle itself is virtually identical to that of the two-stage steam absorption chillers. However, unlike most steam absorption chillers, the direct-fired absorption chiller lends itself fairly readily to "chiller-heater" applications where both cooling and heating are achieved in the same unit. This can result in a smaller footprint for the boiler room in some situations.

Advantages

Where a boiler can be eliminated by the dual heating and cooling capability of this machine, the cost and space savings can be a significant. In addition, steam is not required, which can be important in situations where local codes require licensed boiler operators for steam-driven units but permit unmanned operation of direct-fired absorption chillers.

Disadvantages

Direct-fired absorption chillers require a stack to vent combustion products. This is not necessary in a steam-fired unit. In addition, the first cost of direct-fired units are higher than steam driven units. Maintenance costs on the heat rejection circuit tend to be higher due to more rapid scaling. Also be careful to check warranted life of absorption chiller heat transfer surfaces (especially the generator section) and the refrigerant and solution pumps. All absorption chillers use electric power to operate these pumps, the condenser water pumps and cooling tower fans. They also use more water as they must reject more heat and require larger cooling towers.

Absorption chillers are more difficult than electric chillers to put on-line (start up) and to take off-line (shut down) as they require a dilution cycle. All of these issues should be addressed in discussions with manufacturers, designers and mechanical contractors.

Waste heat fired absorption chillers

Most absorption chillers use either steam or fuel (natural gas, propane) for heat input. But, waste heat from process, reciprocating engine, gas turbine, or a cogeneration system can also be used in the absorption process. The exhaust should have a minimum temperature of about 550 F and a maximum of 1,500 F. The most common application is using the exhaust from a gas turbine to provide cooling for the intake air or other cooling requirements. The available cooling is a function of the exhaust gas temperature and mass flow rate, using this formula:

Chilling capacity in tons = m x (Tg - 375) / 40,950

Where m = mass flow rate in pound per hour

Tg = exhaust gas inlet temp (F) to absorption chiller

40,950 = conversion factor

More detail

Waste heat fired absorption chillers - steam

Waste steam from a cogeneration system obviously produces the same level of cooling as boiler generated steam, Low pressure waste steam sources (say 14 psig) typically require 18-20 pounds of steam per hour to produce one ton of cooling in a single-stage absorption chiller. That performance improves to 10-12 pounds per ton-hour of steam when steam pressures are in the 50 to 130 psig range and used in a 2-stage (double effect) absorption chiller.

Steam absorption chillers are nominally rated as follows:

  • Single stage: 9 psig at generator flange
  • Two stage: 114 psig steam input pressure.

Capacity ratings are decreased as steam pressure drops below nominal. For example, a nominal 100-ton unit's capacity will drop to 84 tons with 78.5 psig steam.

Waste heat fired absorption chillers - hot air

Direct-fired absorption chillers can often be modified to accept hot air or exhaust from a gas turbine or engine. Performance is almost totally dependent upon air temperature, For example, waste heat air temperatures °F or higher offer performance similar to direct-fired absorption chillers where every 13,000 Btu of heat recovered produces one ton of cooling. When calculating heat recovery, remember to assume waste heat leaving the absorption chiller at 375° to 400°F (this means the absorption chiller will not reclaim all of the waste heat potential).

For exhaust gas heat recovery

Chilling capacity (tons) = m x (Tg - 375)

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Heating capacity (BTUH) = m x (Tg - 375) x 0.257

where m = exhaust gas flow rate in pounds per hour

Tg = exhaust gas inlet temperature in °F

40,950 = cooling constant representing average gas specific heat, interconnect efficiency, cooling COP and the conversion from BTUH to tons

0.257 = heating constant representing average gas specific heat and the interconnect efficiency

375 = minimum temperature of exhaust gas leaving chiller in °F.