Absorption Refrigeration Cycle

     Absorption - Refrigeration Cycle Descriptions

Absorption Chiller Refrigeration Cycle

                  

  

  The basic cooling cycle is the same for the absorption and electric chillers. Both systems use a low-temperature liquid refrigerant that absorbs heat from the water to be cooled and converts to a vapour phase (in the evaporator section). The refrigerant vapours are then compressed to a higher pressure (by a compressor or a generator), converted back into a liquid by rejecting heat to the external surroundings (in the condenser section), and then expanded to a low- pressure mixture of liquid and vapour (in the expander section) that goes back to the evaporator section and the cycle is repeated.

The basic difference between the electric chillers and absorption chillers is that an electric chiller uses an electric motor for operating a compressor used for raising the pressure of refrigerant vapours and an absorption chiller uses heat for compressing refrigerant vapours to a high-pressure. The rejected heat from the power-generation equipment (e.g. turbines, micro turbines, and engines) may be used with an absorption chiller to provide the cooling in a CHIP system.

The basic absorption cycle employs two fluids, the absorbate or refrigerant, and the absorbent. The most commonly fluids are water as the refrigerant and lithium bromide as the absorbent. These fluids are separated and recombined in the absorption cycle.

   In the absorption cycle the low-pressure refrigerant vapour is absorbed into the absorbent releasing a large amount of heat. The liquid refrigerant/absorbent solution is pumped to a high-operating pressure generator using significantly less electricity than that for compressing the refrigerant for an electric chiller. Heat is added at the high-pressure generator from a gas burner, steam, hot water or hot gases. The added heat causes the refrigerant to desorb from the absorbent and vaporise. The vapours flow to a condenser, where heat is rejected and condense to a high-pressure liquid. The liquid is then throttled though an expansion valve to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling. The remaining liquid absorbent, in the generator passes through a valve, where its pressure is reduced, and then is recombined with the low-pressure refrigerant vapours returning from the evaporator so the cycle can be repeated.

Absorption chillers are used to generate cold water (44°F) that is circulated to air handlers in the distribution system for air conditioning.

"Indirect-fired" absorption chillers use steam, hot water or hot gases steam from a boiler, turbine or engine generator, or fuel cell as their primary power input. Theses chillers can be well suited for integration into a CHIP system for buildings by utilising the rejected heat from the electric generation process, thereby providing high operating efficiencies through use of otherwise wasted energy.

"Direct-fired" systems contain natural gas burners; rejected heat from these chillers can be used to regenerate desiccant dehumidifiers or provide hot water.

Commercially absorption chillers can be single-effect or multiple-effect. The above schematic refers to a single-effect absorption chiller. Multiple-effect absorption chillers are more efficient and discussed below.

Multiple-Effect Absorption Chillers

                  

   In a single-effect absorption chiller, the heat released during the chemical process of absorbing refrigerant vapour into the liquid stream, rich in absorbent, is rejected to the environment. In a multiple-effect absorption chiller, some of this energy is used as the driving force to generate more refrigerant vapour. The more vapour generated per unit of heat or fuel input, the greater the cooling capacity and the higher the overall operating efficiency.

A double-effect chiller uses two generators paired with a single condenser, absorber, and evaporator. It requires a higher temperature heat input to operate and therefore they are limited in the type of electrical generation equipment they can be paired with when used in a CHP System.

Triple-effect chillers can achieve even higher efficiencies than the double-effect chillers. These chillers require still higher elevated operating temperatures that can limit choices in materials and refrigerant/absorbent pairs. Triple-effect chillers are under development by manufacturers working in cooperation with the U.S. Department of Energy.

Animation of a Direct-Fired Double-Effect Absorption Chiller

(Courtesy of Inter Energy Software)

Desiccant Dehumidification Cycle for Solid Desiccants A typical approach to using solid desiccants for dehumidifying air streams is by impregnating them into a light-weight honeycomb or corrugated matrix that is formed into a wheel. The desiccant-coated wheel is rotated through a "supply" or "process" air stream. The "active" section of the wheel removes moisture from the air and the dried air is routed to the building. By drying the air provided to a chiller, air-conditioning efficiencies are increased because a desiccant removes the moisture from the air more efficiently than a chiller or a direct-expansion (DX) evaporator does.

    

               

    The other section of the wheel rotates through a "reactivation" or "regeneration" air stream that dries the desiccant out and carries the moisture out of the building. The desiccant can be reactivated with air that is either hotter or drier than the process air.

"Passive" desiccant wheels that are used in total energy recovery ventilators (ERVs) and enthalpy exchangers use dry building exhaust air for regeneration. These simple enthalpy wheels are generally less expensive but also less effective than active desiccant units.

The "active" desiccant wheel can dry the supply air continuously, to any desired humidity level, in all weather, regardless of the moisture content of building exhaust air. They are regenerated with hot air from a burner or other heat source (such as rejected heat from a power generation equipment in a CHP system). This allows them to be used independently of or in combination with building exhaust air and thus, allows more operational/control flexibility. Enthalpy wheels or heat pipes can be added to transfer energy from the supply side to the exhaust side, reducing energy requirements and boosting efficiency.

The ability of a desiccant dehumidifier to use the heat rejected from a turbine, micro turbine, or engine-generator makes "active" desiccant systems well suited for integration into a CHP system for buildings providing dependable, low maintenance dehumidification performance at high operating efficiencies.

Animation of a typical Solid Desiccant Dehumidification Cycle.

(Courtesy of Inter Energy Software)

   Desiccant Dehumidification Cycle for Liquid Desiccants

   In a typical liquid desiccant system, shown below, the desiccant is distributed in one chamber (conditioner), using spray nozzles, where it contacts the passing process air stream to be dehumidified. Lithium chloride solution is the most common liquid desiccant used commercially. As the desiccant absorbs the moisture from the process air, heat is released. A cooling coil in the chamber (or chilled liquid desiccant itself) removes the heat of sorption, creating simultaneous desiccant dehumidification and after cooling, providing latent and sensible cooling.

      

(Courtesy of Munters Corporation)

   The moisture laden desiccant from the conditioning chamber is then pumped to the other chamber (regenerator), where heat is applied, using a heating coil. In the regenerator, heat drives off the water from the desiccant into an exhaust air stream. Heat to drive off the water could come from many sources, including exhaust gas streams from power generation and absorption cooling systems. The desiccant is now ready to be re-used in the conditioning chamber. It is pumped from the regeneration chamber, to be redistributed in the conditioning/dehumidification chamber.

   An inter changer is often used to cool the warmer desiccant leaving the regenerator by exchanging heat with the cooler desiccant from the conditioner. Additional process air sensible cooling may be required to provide process control or comfortable space dry bulb temperatures.

   One regenerator can handle desiccant from several conditioning chambers. Varying the concentration of desiccant in the solution controls humidity in the processed air.

Liquid desiccant systems not only control humidity in process air, but also scrub the air of particulates, killing bacteria and viruses.

Heating Fundamentals 2

Specific, Sensible, and Latent Heat

The specific heat of a substance is the ratio of the quantity of heat required to raise its temperature one degree Fahrenheit to the amount required to raise the temperature of the same weight of water one degree Fahrenheit (Figure 2-7). This may be expressed in the following formula:

    The standard used in water at approximately 62 to 63_F receives a rating of 1.00 on the specific heat scale. Simply stated, specific heat represents the Btu required to raise the temperature of one pound of a substance one degree Fahrenheit.
   Sensible heat is the part of heat that provides temperature change and that can be measured by a thermometer. It is referred to as such because it can be sensed by instruments or touch.
   Latent heat is the quantity of heat that disappears or becomes concealed in a body while producing some change in it other than a rise of temperature. Changing a liquid to a gas and a gas to a liquid are both activities involving latent heat. The two types of latent heat are: 

1. Internal latent heat

2. External latent heat

   These are explained in detail in the next section under Steam.

Heat-Conveying Mediums 

   As mentioned in Chapter 1, several methods are used to classify  heating systems. One method is based on the medium that conveys the heat from its source to the point being heated. When the majority of heating systems in use today are examined closely, it can be
seen that there are only four basic heat-conveying mediums involved:

  1. Air 

  2. Steam

  3. Water

  4. Electricity

Air

   Air is a gas consisting of a mechanical mixture of 23.2% oxygen (by weight), 75.5% nitrogen, and 1.3% argon with small amounts of other gases. It functions as the heat-conveying medium for warm-air heating systems.
   Atmospheric pressure may be defined as the force exerted by the weight of the atmosphere in every point with which it is in contact (Figure 2-8), and is measured in inches of mercury or the corresponding pressure in pounds per square-inch (psi).
   The pressure of the atmosphere is approximately 14.7 psi at sea level. The standard atmosphere is 29.921 inches of mercury (in Hg) at 14.696 psi. “Inches of mercury” refers to the height to which the column of mercury in a barometer will remain suspended to balance the pressure caused by the weight of the atmosphere.

Heating Fundamentals 1

      There is still considerable disagreement about the exact nature of heat, but most authorities agree that it is a particular form of energy. Specifically, heat is a form of energy not associated with matter and in transit between its source and destination point. Furthermore, heat energy exists as such only between these two points. In other words, it exists as heat energy only while flowing between the source and destination.
   So far this description of heat energy has been practically identical to that of work energy, the other form of energy in transit not associated with matter. The distinguishing difference between the two is that heat energy is energy in transit as a result of temperature differences between its source and destination point, whereas work energy in transit is due to other, non temperature factors.

British Thermal Unit
   Heat energy is measured by the British thermal unit (Btu). Each thermal unit is regarded as equivalent to one unit of heat (heat energy).
   Since 1929, British thermal units have been defined on the basis of 1 Btu being equal to 251.996 IT international Steam Table) calories, or 778.26 foot-pounds of mechanical energy units (work). Taking into consideration that one IT calorie equals 1⁄860 of a watt-hour, 1 Btu is then equivalent to about 1⁄3 watt hour.
    Prior to its 1929 redefinition, a Btu was defined as the amount of heat necessary to raise the temperature of one pound of water by one degree Fahrenheit. Because of the difficulty in determining the exact value of a Btu, it was later redefined in terms of the more fundamental physical unit.

Relationship Between Heat and Work 
  Energy is the ability to do work or move against a resistance.Conversely, work is the overcoming of resistance through a certain distance by the expenditure of energy.
   Work is measured by a standard unit called the foot-pound, which may be defined as the amount of work done in raising one

pound the distance of one foot, or in overcoming a pressure of one pound through a distance of one foot (Figure 2-1).The relationship between work and heat is referred to as the mechanical equivalent of heat; one unit of heat is equal to 778.26 ft-lb. This relationship (i.e., the mechanical equivalent of heat) was first established by experiments conducted in the nineteenth century. In 1843 Dr. James Prescott Joule (1818–1889) of Manchester, England, determined by numerous experiments that when 772 ft-lb of energy had been expended on 1 lb of water, the temperature of water had risen 1_F and the relationship between heat and mechanical work was found (Figure 2-2). The value 772 ft-lb is known as Joule’s equivalent. More recent experiments give higher figures and the value 778 (1 Btu _ 778.26 ft-lb). (See the preceding section.)

Heat Transfer
   When bodies of unequal temperatures are placed near each other, heat leaves the hotter body and is absorbed by the colder one until the temperatures are equal to each other. The rate by which the heat is absorbed by the colder body is proportional to the difference of temperature between the two bodies—the greater the difference in temperature, the greater the rate of flow of the heat.
    Heat is transferred from one body to another at lower temperature by any one of the following means (Figure 2-3):
1. Radiation
2. Conduction
3. Convection
    Radiation, insofar as heat loss is concerned, refers to the throwing out of heat in rays. The heat rays proceed in straight lines, and the intensity of the heat radiated from any one source becomes less as the distance from the source increases.
   The amount of heat loss from a body within a room or building through radiation depends upon the temperature of the floor, ceiling, and walls. The colder these surfaces are, the faster and greater will be the heat loss from a human body standing within the enclosure.If the wall, ceiling, and floor surfaces are warmer than the human body within the enclosure they form, heat will be radiated

from these surfaces to the body. In these situations a person may complain that the room is too hot.
   Knowledge of the mean radiant temperature of the surfaces of an enclosure is important when dealing with heat loss by radiation.
   The mean radiant temperature (MRT) is the weighted average temperature of the floor, ceiling, and walls. The significance of the mean radiant temperature is determined when compared with the clothed body of an adult (80_F, or 26.7_C). If the MRT is below 80_F, the human body will lose heat by radiation to the surfaces of  the enclosure. If the MRT is higher than 80_F, the opposite effect will occur.
   Conduction is the transfer of heat through substances, for instance, from a boiler plate to another substance in contact with it (Figure 2-4). Conductivity may be defined as the relative value of a material, compared with a standard, in affording a passage through itself or over its surface for heat. A poor conductor is usually referred to as a nonconductor OR insulator. Copper is an example of a good conductor. Figure 2-5 illustrates the comparative heat conductivity rates of three frequently used metals. The various materials used to insulate buildings are poor conductors. It should be

pointed out that any substance that is a good conductor of electricity is also a good conductor of heat.
   Convection is the transfer of heat by the motion of the heated matter itself. Because motion is a required aspect of the definition of convection, it can take place only in liquids and gases.
   Figure 2-4 illustrates how radiation, conduction, and convection are often interrelated. Heat from the burning fuel passes to the metal of the heating surface by radiation, passes through the metal by conduction, and is transferred to the water by convection (i.e., circulation). Circulation is caused by a variation in the weight of the water due to temperature differences. That is, the water next to the heating surface receives heat, expands (becomes lighter), and immediately rises as a result of displacement by the colder and heavier water above.
   Proper circulation is very important, because its absence will cause a liquid, such as water, to reach the spheroidal state. This, in turn, causes the metal of the boiler to become dangerously overheated. A liquid that has reached the spheroidal state is easy to recognise by its appearance. When liquid is dropped upon the surface of a highly heated metal, it rolls about in spheroidal drops

(Figure 2-6) or masses without actual contact with the heated metal. This phenomenon is caused by the repelling force of heat and the intervention of a cushion of steam.

Figure 2-6 Drop of water on a hot plate illustrating the spheroidal state.


                                                                                                                                   (continued)

Basic Refrigeration Cycle

   Basic Refrigeration Cycle

Principles of Refrigeration

• Liquids absorb heat when changed from liquid to gas

• Gases give off heat when changed from gas to liquid.
For an air conditioning system to operate with economy, the refrigerant must be used repeatedly. For this reason, all air conditioners use the same cycle of compression, condensation, expansion, and evaporation in a closed circuit. The same refrigerant is used to move the heat from one area, to cool this area, and to expel this heat in another area.

• The refrigerant comes into the compressor as a low-pressure gas, it is compressed and then moves out of the compressor as a high-pressure gas.
• The gas then flows to the condenser. Here the gas condenses to a liquid, and gives off its heat to the outside air.
• The liquid then moves to the expansion valve under high pressure. This valve restricts the flow of the fluid, and lowers its pressure as it leaves the expansion valve.
• The low-pressure liquid then moves to the evaporator, where heat from the inside air is absorbed and changes it from a liquid to a gas.
• As a hot low-pressure gas, the refrigerant moves to the compressor where the entire cycle is repeated.
Note that the four-part cycle is divided at the center into a high side and a low side This refers to the pressures of the refrigerant in each side of the system

The Basic Refrigeration Cycle

   Mechanical refrigeration is accomplished by continuously circulating, evaporating, and condensing a fixed supply of refrigerant in a closed system. Evaporation occurs at a low temperature and low pressure while condensation occurs at a high temperature and high pressure. Thus, it is possible to transfer heat from an area of low temperature (i.e., refrigerator cabinet) to an area of high temperature (i.e., kitchen).

   Referring to the illustration below, beginning the cycle at the evaporator inlet (1), the low-pressure liquid expands, absorbs heat, and evaporates, changing to a low-pressure gas at the evaporator outlet (2).

   The compressor (4) pumps this gas from the evaporator through the accumulator (3), increases its pressure, and discharges the high-pressure gas to the condenser (5). The accumulator is designed to protect the compressor by preventing slugs of liquid refrigerant from passing directly into the compressor. An accumulator should be included on all systems subjected to varying load conditions or frequent compressor cycling. In the condenser, heat is removed from the gas, which then condenses and becomes a high-pressure liquid. In some systems, this high-pressure liquid drains from the condenser into a liquid storage or receiver tank (6). On other systems, both the receiver and the liquid line valve (7) are omitted.

   A heat exchanger (8) between the liquid line and the suction line is also an optional item, which may or may not be included in a given system design.

                                                  Illustration of the basic refrigeration cycle.

   Between the condenser and the evaporator an expansion device (10) is located.

How to Evacuate a Refrigeration or Air Conditioning System

How long should you evacuate a system?

    Present day techniques of evacuation are meant to clean refrigeration and air conditioning systems to a degree never before reached in this trade. A good vacuum pump will eliminate 99.99% of the air and all of the moisture in a system if used properly. Moisture within a system will eventually react chemically with the compressor oil to form sludge. Further, the water will react with the refrigerant to form hydrochloric acid. Any of these combinations will seriously affect the performance and longevity of the system.

   The question of how long it will take a pump to accomplish this level of cleanliness is one being asked by every serviceman that ever picked up a vacuum pump. Unfortunately, there is no pat answer. "How long will evacuation take?" is like asking "How many pitches are there in a baseball game?" It depends on so many things.