Vam

LDO requirement 1. Annual LDO requirement is based on CERC 1ml/kwhr = 123 m3 Monthly requirement is = 10.26 m3 2. LDO requirement during precommissioning activities Duration of precommissioning activities for steam blows = 30days Number of Steam blows = 60 Duration of Each steam blows = 4hrs LDO requirement is = The distance from the plant to the nearest LDO depot (Bhilai) is 290 KMs. From the above, it may be noted LDO storage will have to be sized for receipt of LDO by road tankers, which favours governing criteria.

LDO requirement during precommissioning activities = Buffer stock required to ordering next tankers = HFO requirement 1. Annual HFO requirement is based on CERC 1ml/kwhr = 4344.5 m3 Monthly requirement is = 362 m3 2. HFO requirement during operation •

Hot Startup

Number of startup = 35/year Duration of each startup = 4hrs Annual HFO requirement is = 5262.4 m3 Monthly requirement is = 439 m3 •

Low load Flame stabilization

Duration of Low load flame stabilization is = 12hrs Annual HFO requirement is = 451.06 m3 Monthly requirement is = 37.6 m3 For evaluating the HFO requirement, the Hot startup requirement as well as the Low load Flame Stabilization requirement is considered. Total Annual HFO requirement is = 5713.45 m3 Monthly requirement is = 476.12 m3 3. HFO receipt at site – by rail wagons Number of rail wagons in the rake =48 Capacity of each rail wagons = 61 m3 Total HFO capacity of rail wagons = 2928 m3 From this above, it may be noted HFO storage will have to be sized by rail wagons, which favours governing criteria. Storage required for unloading all wagons

= 2928 m3

Buffer stock required to ordering next rake

= 1500 m3

for receipt of HFO

Free board

= 500 m3

Storage tank capacity

= 5000 m3

Inside this Article 1. 2. 3. 4. 5. 6. 7.

Introduction to How Air Conditioners Work Air-conditioning Basics Window and Split-system AC Units Chilled-water and Cooling-tower AC Units BTU and EER Energy Efficient Cooling Systems See more » How Air Conditioners Work

• • • •

More Home Videos »

Home Temperature Home Thermostat Maintain an Air Conditioner PlanetGreen.com: Home Heating When the temperature outside begins to climb, many people seek the cool comfort of indoor air conditioning. Like water towers and power lines, air conditioners are one of those things that we see every day but seldom pay much attention to. Wouldn't it be nice to know how these indispensable machines work their magic? Air conditioners come in various sizes, cooling capacities and prices. One type that we see all the time is the window air conditioner, an easy and economical way to cool a small area:

People who live in suburban areas usually have a condenser unit in the backyard:

If you live in an apartment complex, you'll probably see multiple condensers for each dwelling:

Most businesses and office buildings have condensing units on their roofs, and as you fly into any airport you notice that warehouses and malls may have 10 or 20 condensing units hidden on their roofs:

At office complexes, you'll find large cooling towers that are connected to the air conditioning system:

Even though each of these machines has a pretty distinct look, they all work on the same principles. In this article, we'll examine air conditioners -- from small to huge -- so you know more about what you're seeing. We'll also look at some new, energy-efficient cooling methods.

Air-conditioning Basics Most people think that air conditioners lower the temperature in their homes simply by pumping cool air in. What's really happening is the warm air from your house is being removed and cycled back in as cooler air. This cycle continues until your thermostat reaches the desired temperature. An air conditioner is basically a refrigerator without the insulated box. It uses the evaporation of a refrigerant, like Freon, to provide cooling. The mechanics of the Freon evaporation cycle are the same in a refrigerator as in an air conditioner. According to the Merriam-Webster Dictionary Online, the term Freon is generically "used for any of various nonflammable fluorocarbons used as refrigerants and as propellants for aerosols."

Diagram of a typical air conditioner. This is how the evaporation cycle in an air conditioner works (See How Refrigerators Work for complete details on this cycle): 1. 2. 3. 4.

The compressor compresses cool Freon gas, causing it to become hot, high-pressure Freon gas (red in the diagram above). This hot gas runs through a set of coils so it can dissipate its heat, and it condenses into a liquid. The Freon liquid runs through an expansion valve, and in the process it evaporates to become cold, lowpressure Freon gas (light blue in the diagram above). This cold gas runs through a set of coils that allow the gas to absorb heat and cool down the air inside the building. Mixed in with the Freon is a small amount of lightweight oil. This oil lubricates the compressor. Air conditioners help clean your home's air as well. Most indoor units have filters that catch dust, pollen, mold spores and other allergens as well as smoke and everyday dirt found in the air. Most air conditioners also function as dehumidifiers. They take excess water from the air and use it to help cool the unit before getting rid of the water through a hose to the outside. Other units use the condensed moisture to improve efficiency by routing the cooled water back into the system to be reused. So this is the general concept involved in air conditioning. In the next section, we'll take a look at window and split-system units.

Window and Split-system AC Units A window air conditioner unit implements a complete air conditioner in a small space. The units are made small enough to fit into a standard window frame. You close the window down on the unit, plug it in and turn it on to get cool air. If you take the cover off of an unplugged window unit, you'll find that it contains:

• • • • • •

A compressor An expansion valve A hot coil (on the outside) A chilled coil (on the inside) Two fans A control unit The fans blow air over the coils to improve their ability to dissipate heat (to the outside air) and cold (to the room being cooled).

When you get into larger air-conditioning applications, its time to start looking at split-system units. A splitsystem air conditioner splits the hot side from the cold side of the system, like this:

The cold side, consisting of the expansion valve and the cold coil, is generally placed into a furnace or some other air handler. The air handler blows air through the coil and routes the air throughout the building using a series of ducts. The hot side, known as the condensing unit, lives outside the building. The unit consists of a long, spiral coil shaped like a cylinder. Inside the coil is a fan, to blow air through the coil, along with a weather-resistant compressor and some control logic. This approach has evolved over the years because it's low-cost, and also because it normally results in reduced noise inside the house (at the expense of increased noise outside the house). Other than the fact that the hot and cold sides are split apart and the capacity is higher (making the coils and compressor larger), there's no difference between a splitsystem and a window air conditioner. In warehouses, large business offices, malls, big department stores and other sizeable buildings, the condensing unit normally lives on the roof and can be quite massive. Alternatively, there may be many smaller units on the roof, each attached inside to a small air handler that cools a specific zone in the building. In larger buildings and particularly in multi-story buildings, the split-system approach begins to run into problems. Either running the pipe between the condenser and the air handler exceeds distance limitations (runs that are too long start to cause lubrication difficulties in the compressor), or the amount of duct work and the length of ducts becomes unmanageable. At this point, it's time to think about a chilled-water system.

Chilled-water and Cooling-tower AC Units In a chilled-water system, the entire air conditioner lives on the roof or behind the building. It cools water to between 40 and 45 degrees Fahrenheit (4.4 and 7.2 degrees Celsius). This chilled water is then piped throughout the building and connected to air handlers as needed. There's no practical limit to the length of a chilled-water pipe if it's well-insulated.

You can see in this diagram that the air conditioner (on the left) is completely standard. The heat exchanger lets the cold Freon chill the water that runs throughout the building. In all of the systems described earlier, air is used to dissipate the heat from the outside coil. In large systems, the efficiency can be improved significantly by using a cooling tower. The cooling tower creates a stream of lower-temperature water. This water runs through a heat exchanger and cools the hot coils of the air conditioner unit. It costs more to buy the system initially, but the energy savings can be significant over time (especially in areas with low humidity), so the system pays for itself fairly quickly. 1. 2. 3. 4. 5. 6.

Cooling towers come in all shapes and sizes. They all work on the same principle: A cooling tower blows air through a stream of water so that some of the water evaporates. Generally, the water trickles through a thick sheet of open plastic mesh. Air blows through the mesh at right angles to the water flow. The evaporation cools the stream of water. Because some of the water is lost to evaporation, the cooling tower constantly adds water to the system to make up the difference.

Cooling Towers

The amount of cooling that you get from a cooling tower depends on the relative humidity of the air and the barometric pressure. For example, assuming a 95-degree Fahrenheit (35-degree Celsius) day, barometric pressure of 29.92 inches (sea-level normal pressure) and 80-percent humidity, the temperature of the water in the cooling tower will drop about 6 degrees to 89 degrees Fahrenheit (3.36 degrees to 31.7 degrees Celsius). If the humidity is 50 percent, then the water temperature will drop perhaps 15 degrees to 80 degrees Fahrenheit (8.4 degrees to 26.7 degrees Celsius). And, if the humidity is 20 percent, then the water temperature will drop about 28 degrees to 67 degrees Fahrenheit (15.7 degrees to 19.4 degrees Celsius). Even small temperature drops can have a significant effect on energy consumption. Whenever you walk behind a building and find a unit that has large quantities of water running through a thick sheet of plastic mesh, you will know you have found a cooling tower! In many office complexes and college campuses, cooling towers and air conditioning equipment are centralized, and chilled water is routed to all of the buildings through miles of underground pipes. In the next section, we'll look at how much all this cooling power costs.

Absorption Chilling A chiller is a machine that removes heat from a liquid via a vapor-compression or

absorption

refrigeration cycle. A vapor-compression water chiller is comprised of the 4 major components of the vapor-compression refrigeration cycle (compressor, evaporator, condenser, and some form of metering device). These machines can implement a variety of refrigerants. Absorption chillers utilize water as the refrigerant and rely on the strong affinity between the water and a lithium bromide solution to achieve a refrigeration effect. Most often, pure water is chilled, but this water may also contain a percentage of chilled

glycol

and/or

corrosion inhibitors; as

other fluids such as thin oils can be well.

Chilled water is used to cool and dehumidify air in mid- to large-size commercial, industrial, and institutional (CII) facilities. Water chillers can be either water cooled, air-cooled, or evaporatively cooled. Water-cooled chillers incorporate the use of cooling towers which improve the chillers' thermodynamic effectiveness as compared to air-cooled chillers. This is due to heat rejection at or near the air's wet-bulb temperature rather than the higher, sometimes much higher, dry-bulb temperature. Evaporatively cooled chillers offer efficiencies better than air cooled, but lower than water cooled. Water cooled chillers are typically intended for indoor installation and operation, and are cooled by a separate condenser water loop and connected to outdoor cooling towers to expel heat to the atmosphere. Air Cooled and Evaporatively Cooled chillers are usually intended for outdoor installation and operation. Air cooled machines are directly cooled by ambient air being mechanically circulated directly through the machine's condenser coil to expel heat to the atmosphere. Evaporatively cooled machines are similar, except they implement a mist of water over the condenser coil to aid in condenser cooling, making the machine more efficient than a traditional air cooled machine. No remote cooling tower is typically required with either of these types of packaged air cooled or evaporatively cooled chillers. Where available, cold water readily available in nearby water bodies might be used directly for cooling, or to replace or supplement cooling towers. The

Deep Lake Water Cooling System

in

Toronto, Canada, is an example. It dispensed with the need for cooling towers, with a significant cut in carbon emissions and energy consumption. It uses cold lake water to cool the chillers, which in turn are used to cool city buildings via a district cooling system. The return water is used to warm the city's drinking water supply which is desirable in this cold climate. Whenever a chiller's heat rejection can be used for a productive purpose, in addition to the cooling function, very high thermal effectiveness’s are possible.

Use in air conditioning air conditioning systems, chilled water is typically distributed to heat exchangers, or coils, in air handling units, or other type of terminal devices which cool the air in its respective space(s), In

and then the chilled water is re-circulated back to the chiller to be cooled again. These cooling coils transfer sensible heat and latent heat from the air to the chilled water, thus cooling and usually dehumidifying the air stream. A typical chiller for air conditioning applications is rated between 15 to 1500 tons (180,000 to 18,000,000 BTU/h or 53 to 5,300 kW) in cooling capacity. Chilled water temperatures can range from 35 to 45 degrees, depending upon application requirements.

Use in industry In industrial application, chilled water or other liquid from the chiller is pumped through process or laboratory equipment. Industrial chillers are used for controlled cooling of products, mechanisms and factory machinery in a wide range of industries. They are often used in the plastic industry in injection and blow molding, metal working cutting oils, welding equipment, die-casting and machine tooling, chemical processing, pharmaceutical formulation, food and beverage processing, vacuum systems, X-ray diffraction, power supplies and power generation stations, analytical equipment, semiconductors, compressed air and gas cooling. They are also used to cool high-heat specialized items such as MRI machines and lasers. The chillers for industrial applications can be centralized, where each chiller serves multiple cooling needs, or decentralized where each application or machine has its own chiller. Each approach has its advantages. It is also possible to have a combination of both central and decentralized chillers, especially if the cooling requirements are the same for some applications or points of use, but not all.

Decentral chillers are usually small in size (cooling capacity), usually from 0.2 tons to 10 tons. Central chillers generally have capacities ranging from ten tons to hundreds or thousands of tons.

Vapor-Compression Chiller Technology There are basically five different types of compressors used in vapor compression chillers:

Reciprocating

screw-driven compression, and centrifugal electric motors, steam, or gas turbines. They produce their cooling effect via the "reverse-Rankin" cycle, also known as 'vaporcompression'. With evaporative cooling heat rejection, their coefficients-of-performance (COPs) compression,

scroll

compression,

compression are all mechanical machines that can be powered by

are very high and typically 4.0 or more. In recent years, application of Variable Speed Drive (VSD) technology has increased efficiencies of vapor compression chillers. The first VSD was applied to centrifugal compressor chillers in the late 1970s and has become the norm as the cost of energy has increased. Now, VSDs are being applied to rotary screw and scroll technology compressors.

How Absorption Technology Works Absorption chillers' thermodynamic cycle are driven by heat source; this heat is usually delivered to the chiller via steam, hot water, or combustion. Compared to electrically powered chillers, they have very low electrical power requirements - very rarely above 15 kW combined consumption for both the solution pump and the refrigerant pump. However, their heat input requirements are large, and their COPs are often 0.5 (single-effect) to 1.0 (double-effect). For the same tonnage capacity, they require much larger cooling towers than vapor-compression chillers. However, absorption chillers, from an energy-efficiency point-of-view, excel where cheap, high grade heat or waste heat is readily available. In extremely sunny climates, solar energy has been used to operate absorption chillers. The single effect absorption cycle uses water as the refrigerant and lithium bromide as the absorbent. It is the strong affinity that these two substances have for one another that makes the cycle work. The entire process occurs in almost a complete vacuum. 1. Solution Pump – A dilute lithium bromide solution is collected in the bottom of the absorber shell. From here, a hermetic solution pump moves the solution through a shell and tube heat exchanger for preheating. 2. Generator – After exiting the heat exchanger, the dilute solution moves into the upper shell. The solution surrounds a bundle of tubes which carries either steam or hot water. The steam or hot water transfers heat into the pool of dilute lithium bromide solution. The solution boils, sending refrigerant vapor upward into the condenser and leaving behind concentrated lithium bromide. The concentrated lithium bromide solution moves down to the heat exchanger, where it is cooled by the weak solution being pumped up to the generator. 3. Condenser – The refrigerant vapor migrates through mist eliminators to the condenser tube bundle. The refrigerant vapor condenses on the tubes. The heat is removed by the cooling water which moves through the inside of the tubes. As the refrigerant condenses, it collects in a trough at the bottom of the condenser. 4. Evaporator – The refrigerant liquid moves from the condenser in the upper shell down to the evaporator in the lower shell and is sprayed over the evaporator tube bundle. Due to the extreme vacuum of the lower shell [6 mm Hg (0.8 kPa) absolute pressure], the refrigerant liquid boils at approximately 39°F (3.9°C), creating the refrigerant effect. (This vacuum is created by hygroscopic action - the strong affinity lithium bromide has for water - in the Absorber directly below.) 5. Absorber – As the refrigerant vapor migrates to the absorber from the evaporator, the strong lithium bromide solution from the generator is sprayed over the top of the absorber tube bundle. The strong lithium bromide solution actually pulls the refrigerant vapor into solution, creating the extreme vacuum in the evaporator. The absorption of the refrigerant vapor into the lithium bromide solution also generates heat which is removed by the cooling water. The now dilute lithium bromide solution collects in the bottom of the lower shell, where it flows down to the solution pump. The chilling cycle is now completed and the process begins once again.

Absorption Cooling – The History The Absorption Cycle was invented in 1846 by Ferdinand Carré for the purpose of producing ice with heat input. Commercial production began in 1923. Since then it has grown into a major market,

with

5,000

new

chillers

shipped

in

the

United

States

every

year.

The Ammonia Absorption Process The Absorption Cycle is based on the principle that absorbing ammonia in water causes the vapor pressure to decrease. Absorption cycles produce cooling and/or heating with thermal input and minimal electric input, by using heat and mass exchangers, pumps and valves. An absorption cycle can be viewed as a mechanical vapor-compression cycle, with the compressor replaced by a generator, absorber and liquid pump. The absorption cycle enjoys the benefits of requiring a fraction of the electrical input, plus uses the natural substances ammonia and water, instead of ozone depleting halocarbons. The absorption cycle enjoyed widespread use from the 1920’s as gas powered refrigerators/ice-makers.

The basic operation of an ammonia-water absorption cycle is as follows. Heat is applied to the generator, which contains a solution of ammonia water, rich in ammonia. The heat causes high pressure ammonia vapor to desorb the solution. Heat can either be from combustion of a fuel such as clean-burning natural gas, or waste heat from engine exhaust, other industrial processes, solar heat, or any other heat source. The high pressure ammonia vapor flows to a condenser, typically cooled by outdoor air. The ammonia vapor condenses into a high pressure liquid, releasing heat which can be used for product heat, such as space heating. The high pressure ammonia liquid goes through a restriction, to the low pressure side of the cycle. This liquid, at low pressures, boils or evaporates in the evaporator. This provides the cooling or refrigeration product. The low pressure vapor flows to the absorber, which contains a water-rich solution obtained from the generator. This solution absorbs the ammonia while releasing the heat of absorption. This heat can be used as product heat or for internal heat recovery in other parts of the cycle, thus unloading the burner and increasing cycle efficiency. The solution in the absorber, now once again rich in ammonia, is pumped to the generator, where it is ready to repeat the cycle. An absorption cycle can use a variety of working pairs. The working pair is made up of a refrigerant, typically ammonia or water; and a solution which absorbs the refrigerant. Other working pairs include lithium-bromide-water; TriDroxide-water; and Alkitrate-water. Tridroxide and Alkitrate are Energy Concepts patented working pairs with specialty applications in industry. Absorption cycles can operate at high efficiency by utilizing advanced cycles, using generatorabsorber heat exchange, multiple pressures, and multiple effects. These cycles use extensive internal heat recovery to require less prime fuel input to produce the same thermal output. High efficiency operation, plus benefits of environmentally friendly refrigerants, clean-burning fuels, and

few moving parts requiring maintenance make absorption a very good choice for consumers. Absorption cycles can produce a variety of thermal outputs. In common commercial uses today there are gas-fired absorption chillers, which produce chilled water for space cooling applications. The absorption cycle can produce low temperature cooling for ice production or cold storage. Turbine inlet cooling is a very efficient use of absorption cooling, boosting turbine efficiency by up to 15%. Many other applications exist in industry, where waste heat is available and cooling is required. Advanced cycles can also produce electrical or shaft power by producing steam or high pressure vapor to power a turbine/generator pair. How Absorption Cooling Works Like the compressor in an electric vapor compression cycle, the absorption system uses its "thermal" compressor (consisting of the generator, absorber, pump and heat exchanger) to boil water vapor (refrigerant) out of a lithium bromide/water solution and compress the refrigerant vapor to a higher pressure. Increasing the refrigerant pressure also increases its condensing temperature. The refrigerant vapor condenses to a liquid at this higher pressure and temperature. Because this condensing temperature is hotter than the ambient temperature, heat moves from the condenser to the ambient air and is rejected. The high-pressure liquid then passes through a throttling valve that reduces its pressure. Reducing its pressure also reduces its boiling point temperature. The low-pressure liquid then passes into the evaporator and is boiled at this lower temperature and pressure. Because the boiling temperature is now lower than the temperature of the conditioned air, heat moves from the conditioned air stream into the evaporator and causes this liquid to boil. Removing heat from the air in this manner causes the air to be cooled. The refrigerant vapor then passes into the absorber where it returns to a liquid state as it is pulled into the lithium bromide solution (the absorption process). The diluted lithium bromide solution is pumped back to the generator. Because lithium bromide (the absorbent) does not boil, water (the refrigerant) is easily separated by adding heat. The resultant water vapor passes into the condenser, the absorbent solution returns to the absorber, and the process repeats. Simplified

diagram

of

a

single

effect

absorption

cycle

Although the process is similar to conventional electric vapor compression systems, absorption cooling substitutes a generator and absorber, called a thermal compressor, for an electric compressor. Efficiency and lower operating costs are achieved through the use of a pump rather than a compressor and a heat exchanger to recover and supply heat to the generator. Double-effect absorption cooling adds a second generator and condenser to increase the refrigerant flow, and therefore the cooling effect, for a fraction of the heat input of a single-effect system.

Industrial chiller technology Industrial chillers typically come as complete packaged closed-loop systems, including the chiller unit, condenser, and pump station with recirculation pump, expansion valve, no-flow shutdown, internal cold water tank, and temperature control. The internal tank helps maintain cold water temperature and prevents temperature spikes from occurring. Closed loop industrial chillers recirculate a clean coolant or clean water with condition additives at a constant temperature and pressure to increase the stability and reproducibility of water-cooled machines and instruments. The water flows from the chiller to the application's point of use and back. If the water temperature differentials between inlet and outlet are high, then a large external water tank would be used to store the cold water. In this case the chilled water is not going directly from the chiller to the application, but goes to the external water tank which acts as a sort of "temperature buffer." The cold water tank is much larger than the internal water tank. The cold water goes from the external tank to the application and the return hot water from the application goes back to the external tank, not to the chiller.

The less common open loop industrial chillers control the temperature of a liquid in an open tank or sump by constantly recirculating it. The liquid is drawn from the tank, pumped through the chiller and back to the tank. An adjustable thermostat senses the makeup liquid temperature, cycling the chiller to maintain a constant temperature in the tank. One of the newer developments in industrial water chillers is the use of water cooling instead of air cooling. In this case the condenser does not cool the hot refrigerant with ambient air, but uses water cooled by a cooling tower. This development allows a reduction in energy requirements by more than 15% and also allows a significant reduction in the size of the chiller due to the small surface area of the water based condenser and the absence of fans. Additionally, the absence of fans allows for significantly reduced noise levels. Most industrial chillers use refrigeration as the media for cooling, but some rely on simpler techniques such as air or water flowing over coils containing the coolant to regulate temperature. Water is the most commonly used coolant within process chillers, although coolant mixtures (mostly water with a coolant additive to enhance heat dissipation) are frequently employed.

Industrial chiller selection Important specifications to consider when searching for industrial chillers include the power source, chiller IP rating, chiller cooling capacity, evaporator capacity, evaporator material, evaporator type, condenser material, condenser capacity, ambient temperature, motor fan type, noise level, internal piping materials, number of compressors, type of compressor, number of fridge circuits, coolant requirements, fluid discharge temperature, and COP (the ratio between the cooling capacity in KW to the energy consumed by the whole chiller in KW). For medium to large chillers this should range from 3.5-4.8 with higher values meaning higher efficiency. Chiller efficiency is often specified in kilowatts per refrigeration ton (kW/RT). Process pump specifications that are important to consider include the process flow, process pressure, pump material, elastomer and mechanical shaft seal material, motor voltage, motor electrical class, motor IP rating and pump rating. If the cold water temperature is lower than -5ºC, then a special pump needs to be used to be able to pump the high concentrations of ethylene glycol. Other important specifications include the internal water tank size and materials and full load amperage. Control panel features that should be considered when selecting between industrial chillers include the local control panel, remote control panel, fault indicators, temperature indicators, and pressure indicators. Additional features include emergency alarms, hot gas bypass, city water switchover, and casters.

Refrigerants A vapor-compression chiller uses a refrigerant internally as its working fluid. Many refrigerants options are available; when selecting a chiller, the application cooling temperature requirements and refrigerant's cooling characteristics need to be matched. Important parameters to consider are the operating temperatures and pressures. There are several environmental factors that concern refrigerants, and also affect the future availability for chiller applications. This is a key consideration in intermittent applications where a large chiller may last for 25 years or more.

warming potential

Ozone depletion potential

(ODP) and

global

(GWP) of the refrigerant need to be considered. ODP and GWP data for some of the more common vapor-compression refrigerants:

Refrigerant R-134a 0

ODP

GWP 1300

R-123

0.012 120

R-22

0.05

R401a

0.027 970

R404a

0

3260

R407a

0

???

R407c

0

1525

R408a

0.016 3020

1700

R409a

0.039 1290

R410a

0

R500

0.7

R502

0.18

1725 ??? 5600

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 vapor phase (in the evaporator section). The refrigerant vapors 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 vapor (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 vapors and an absorption chiller uses heat for compressing refrigerant vapors to a high-pressure. The rejected heat from the power-generation equipment (e.g. turbines, microturbines, and engines) may be used with an absorption chiller to provide the cooling in a CHP 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 vapor 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 vaporize. The vapors 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 vapors 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. to Top of Page

"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 CHP system for buildings by utilizing 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 vapor into the liquid stream, rich in absorbent, rejected to the environment. In

is a

multiple-effect absorption chiller, some of this energy is used as the driving force to generate more refrigerant vapor. The more vapor 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

Absorpton

Chiller

(Courtesy of InterEnergy Software) to Top of Page

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 desiccantcoated 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, microturbine, 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 InterEnergy Software) to Top of Page

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 aftercooling, 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 interchanger 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. ABSORPTION REFRIGERATOR

The absorption refrigerator is a refrigerator that uses a heat source (e.g. solar, kerosenefueled flame) to provide the energy needed to drive the cooling system. Absorption refrigerators are a popular alternative to regular compressor refrigerators where electricity is unreliable, costly, or unavailable, where noise from the compressor is problematic, or where surplus heat is available (e.g. from turbine exhausts or industrial processes). Absorption refrigerators powered by heat from the combustion of liquefied petroleum gas are often used for food storage in recreational vehicles. Both absorption refrigerators and compressor refrigerators use a refrigerant with a very low (sub-zero Fahrenheit) boiling point. In both types, when this refrigerant evaporates or boils, it takes some heat away with it, providing the cooling effect. The main difference between the two types is the way the refrigerant is changed from a gas back into a liquid so that the cycle can repeat. A compressor refrigerator uses an electrically-powered compressor to increase the pressure on the gas, and then condenses the hot high pressure gas back to a liquid by heat exchange with a coolant (usually air). Once the high pressure gas has cooled, it passes through a pressure release valve which drops the refrigerant temperature to below freezing. An absorption refrigerator changes the gas back into a liquid using a different method that needs only heat, and has no moving parts. The other difference between the two types is the refrigerant used. Compressor refrigerators typically use an HCFC, while absorption refrigerators typically use ammonia.

Principles Absorptive refrigeration uses a source of heat to provide the energy needed to drive the cooling process. The most common use is in commercial climate control and cooling of machinery. Absorptive refrigeration is also used to air-condition buildings using the waste heat from a gas turbine or water heater. The process is very efficient, since the gas turbine produces electricity, hot water and air-conditioning (see Trigeneration). The basic thermodynamic process is not a conventional thermodynamic cooling process based on Charles' Law. Instead, it is based on evaporation, carrying heat, in the form of faster-moving (hotter) molecules from one material to another material that preferentially absorbs hot molecules. A familiar example is human sweating. The water in sweat evaporates and is "absorbed" into air, carrying away heat from the body. However, absorptive refrigerators differ in that they regenerate their coolants in a closed cycle, while people need to keep replacing their lost water (evaporated sweat) through drinking.

The classic gas absorption refrigerator sends liquid ammonia into a hydrogen gas. The liquid ammonia evaporates in the presence of hydrogen gas, providing the cooling. The now-gaseous ammonia is sent into a container holding water, which absorbs the ammonia. The water-ammonia solution is then directed past a heater, which boils ammonia gas out of the water-ammonia solution. The ammonia gas is then condensed into a liquid. The liquid ammonia is then sent back through the hydrogen gas, completing the cycle. A similar system, common in large commercial plants, uses a solution of lithium bromide salt and water. Water under low pressure is evaporated from the coils that are being chilled. The water is absorbed by a lithium bromide/water solution. The water is driven off the lithium bromide solution using heat. Another variant uses air, water, and a salt water solution. As shown in the figure below, warm moist air is passed through a sprayed solution of salt water. The spray lowers the humidity. The less humid warm air is then passed through an evaporative cooler which cools and rehumidifies. Humidity is removed from the cooled air with another spray of salt solution. The salt solution is regenerated by heating it under low pressure, causing water to evaporate. The water evaporated from the salt solution is recondensed, and rerouted back to the evaporative cooler.

Process A single-pressure absorption refrigerator uses three substances: ammonia, hydrogen gas, and water, whereas large industrial units generally use only two, a refrigerant such as ammonia, and an absorbent such as water (with an expansion valve and pump, not described here). Normally, ammonia is a gas at room temperature (with a boiling point of -33 °C), but the system is pressurized to the point that the ammonia is a liquid at room temperature. The cooling cycle starts at the evaporator, where liquefied anhydrous ammonia enters. (Anhydrous means there is no water in the ammonia, which is critical for exploiting its sub-zero boiling point.) The "evaporator" contains another gas (in this case, hydrogen), whose presence lowers the partial pressure of the ammonia in that part of the system. The total pressure in the system is still the same, but now not all of the pressure is being exerted by ammonia, as much of it is due to the pressure of the hydrogen. Ammonia doesn't react with hydrogen - the hydrogen is there solely to take up space - creating a void that still has the same pressure as the rest of the system, but not in the form of ammonia. Per Dalton's law, the ammonia behaves only in response to the proportion of the pressure represented by the ammonia, as if there was a vacuum and the hydrogen

wasn't there. Because a substance's boiling point changes with pressure, the lowered partial pressure of ammonia changes the ammonia's boiling point, bringing it low enough that it can now boil below room temperature, as though it wasn't under the pressure of the system in the first place. When it boils, it takes some heat away with it from the evaporator - which produces the "cold" desired in the refrigerator. The next step is separating the gaseous ammonia-hydrogen mixture. Separation from the hydrogen is simple, and this is where the "absorber" comes in. Ammonia readily mixes with water, and hydrogen does not. The absorber is simply a downhill run of tubes in which the mixture of gases counterflows upwards in contact with water trickling down. At the top of the flow, with most of the ammonia gone into solution there, the gas is mainly hydrogen, free to return to the evaporator, while at the bottom of the flow, the dissolved AND gaseous ammonia concentrations are highest respectively, which is in accordance with the principle of a counterflow absorber, and now the solution is ready for entry to the boiler. At this point, the ammonia-water solution is not usable for refrigeration, as the mixture won't boil at a low enough temperature to be a worthwhile refrigerant. It's now necessary to separate the ammonia from the water. This is where the heat from the flame comes in. When the right amount of heat is applied to the mixture, the ammonia bubbles out. This phase is called the "generator". The ammonia isn't quite dry yet - the bubbles contain gas but they're made of water, so the pipe twists and turns and contains a few minor obstacles that pop the bubbles so the gas can move on. The water that results from the bubbles isn't bad - it takes care of another need, and that is the circulation of water through the previous absorption step. Because that water has risen a bit while it was bubbling upwards, the flow of that water falling back down due to gravity can be used for this purpose. The maze that makes the ammonia gas go one way and the bubble water go the other is called the "separator". The next step is the condenser. The condenser is a sort of heat sink or heat exchanger that cools the hot ammonia gas back down to room temperature. Because of the pressure and the purity of the gas (there is no hydrogen or water here), the ammonia condenses back into a liquid, and at that point, it's suitable as a refrigerant and the cycle starts over again.