Refrigeration and Air conditioning

UNIT-7

Refrigeration System Operating Characteristics

General

Refrigeration systems must operate at all hours of the year, even when the building is unoccupied. Warmer weather tends to push refrigeration equipment to its capacity limit, thus creating a maximum operating kW and kWh. Evaporators - must be selected to provide the required cooling at all expected ambient conditions even with the maximum frost on the coils (i.e., just prior to defrosting). Evaporator coils used include two types of refrigeration systems: flooded evaporator and direct expansion. For direct expansion systems, two of the most commonly used refrigerant liquid metering devices are the capillary tube and the thermostatic expansion valve. In addition, proper provisions must be made for periodic defrosting of evaporator air-side surfaces. Defrosting may be accomplished using refrigerant compressor discharge hot-gas, water spray, or manually as selected to meet the user's objectives. Suitable drain connections should be provided to carry off the water resulting from defrost operations. Condensers - must be selected to operate at all outdoor weather conditions in the area. Air-cooled condensers must be supplied with the proper controls to permit operation at low outdoor ambient conditions. Water-cooled condensers may require water regulating valves to keep condensing pressure high enough to enable the thermal expansion valves to function. The type of condenser selected depends largely on the size of the cooling load, refrigerant used, quality and temperature of available cooling water (if any), and noise considerations. Water-cooled condensers require cooling water from an external cooling tower, or from a lake, well, river or other similar source. Once-through use of city water for condensing purposes is prohibited in most locations. Air-cooled condensers are the most popular since they avoid other problems of water acquisition, treatment and disposal. The trade-off may be higher electrical consumption. As seen here, the evaporative condenser is a combination of a water cooled condenser and an air-cooled condenser that rejects heat through the evaporation of water into an airstream traveling across a condenser coil. Compressors - must be sized to meet the varying needs of each application. Provision must be made to protect the compressor from liquid carry over from the evaporator, in addition to the normal safety controls (high and low pressure cutout. oil pressure, etc.). The most common type of compressor used for commercial refrigeration systems is the reciprocating compressor. Reciprocating compressor types include single-stage (booster or high state), internally compounded, and open, hermetic or semi-hermetic.

 

HEAT TRANSFER

The second important law of thermodynamics is that heat always travels from a warm object to a colder one. The rate of heat travel is in direct proportion to the temperature difference between the two bodies.

Assume that two steel halls are side by side in a perfectly insulated box. One ball weighs one pound and has a temperature of 400° F., while the second ball weighs 1,000 pounds and has a temperature of 390° F. The heat content of the larger ball is tremendously greater than the small one, but because of the temperature difference, heat will travel from the small ball to the large one until the temperatures equalize.

Heat can travel in any of three ways: radiation, conduction, or convection.

Radiation is the transfer of heat by waves similar to light waves or radio waves. For example, the sun's energy is transferred to the Earth by radiation. One need only step from the shade into direct sunlight to feel the impact of the heat waves, even though the temperature of the surrounding air is identical in both places. There is little radiation at low temperatures, and at small temperature differences, so radiation is of little importance in the actual refrigeration process. However, radiation to the refrigerated space or product from the outside environment, particularly the sun, may be a major factor in the refrigeration load.

Conduction is the flow of heat through a substance. Actual physical' contact is required for heat transfer to take place between two bodies by this means. Conduction is a highly efficient means of heat transfer as any service-man who has touched a piece of hot metal can testify.

Convection is the flow of heat by means of a fluid medium, either gas or liquid, normally air or water. Air may be heated by a furnace, and then discharged into a room to heat objects in the room by convection.

In a typical refrigeration application, heat normally will travel by a combination of processes, and the ability of a piece of equipment to transfer heat is referred to as the overall rate of heat transfer. While heat transfer cannot take place without a temperature difference, different materials vary in their ability to con-duct heat. Metal is a very good heat conductor, while asbestos has so much resistance to heat flow it can be used as insulation.

What is refrigerant:-

At any hardware store or electronics store you can buy a can of electronics duster. This is just a spray can that blasts a stream of gas when you pull the trigger. You use it to blow the dust out of tiny crevices in electronic circuits. Your teacher is going to bring some cans of electronics duster to class and you can feel for yourself what happens when you spray this stuff. If you spray it long enough, the can will get very cold. It can even get cold enough to give you frostbite.

If you're inquisitive, you may be asking "what gas does the electronics duster shoot out?" It so happens that the gas is a hydrofluorocarbon, or an HFC. Remember that HFCs are the family of compounds used to replace chlorofluorocarbons as refrigerants. It's obvious from feeling the can that HFCs can make things cold, but just how do they do that?

Heat and Changes of State

This is a change of state, of course. As you may remember, for every change of state there is a heat of transition. When a solid becomes a liquid, it absorbs heat in the process of melting. This is called the heat of melting. When a liquid becomes a gas, it absorbs heat in the process. This is called the heat of vaporization. This works backward, too. When gases condense to become liquids they give off heat, and when liquids freeze to become solids, they give off heat as well.

Heat of Vaporization and Your Refrigerator

A refrigerator works in the same way. In a refrigerator, an HFC is pumped through a tube called a coil, like you see in the animation below. In the coil, there is a plug with a small hole in it called a throttle valve. Because this opening is so small, pressure builds up behind the throttle valve, enough for the HFC to become a liquid. Slowly, the HFC passes through the throttle valve. On the other side of the throttle valve, the pressure is not as high. So the boiling point of the HFC drops low enough for the HFC to evaporate. As it evaporates, the HFC absorbs heat from its surroundings, specifically the inside of the refrigerator. The inside of the refrigerator then gets cold.

But there's more to this story. The HFC keeps moving through the coil. The coil passes to the outside of the refrigerator and to the compressor. The compressor puts pressure on the HFC, which condenses back into a liquid, and the whole process can start all over again.

What Makes a Good Refrigerant?

Why is it so hard to find a good refrigerant? To be a good refrigerant, a compound has to live up to a few requirements. Obviously, we want something that is nontoxic. We also want something that is unreactive. The refrigerant has to be stable for the lifetime of the refrigerator. In addition, we want a compound that is ozone-safe. But on top of all these criteria, we need a compound that has a low boiling point. So why not use nitrogen (N₂)? It's nontoxic (we breathe it all the time), the atmosphere is already full of it, and its boiling point is way down at -196°C. That's a little too low, as we'll soon see. We want a refrigerant to a have a low boiling point but not too low, because when a refrigerator is running, the refrigerant is constantly being boiled from a liquid to a gas, and then being condensed back into a liquid again. If the boiling point is too low, it will be hard to condense back into a liquid.

VAPOUR COMPRESSOR SYSTEM

Vapor compression refrigeration is the primary method used to provide mechanical cooling. All vapor compression systems consist of four basic components (plus the interconnecting piping): evaporator, compressor, condenser, and an expansion device. The evaporator and condenser are heat exchangers that evaporate and condense the refrigerant while absorbing and rejecting heat. The compressor takes the refrigerant vapors from the evaporator and raises the pressure sufficiently for the vapor to condense in the condenser. The expansion device controls the flow of condensed refrigerant at this higher pressure back into the evaporator.

Historically, the common refrigerants were R-11, R-12, R-22, and compounds in the R-500 series. With the CFC phaseout, new refrigerants have been developed to replace R-11 and R-12 in new equipment. These new refrigerants can also be used to retrofit existing equipment in many cases. However, these retrofits are not "drop-ins" and should be done by trained technicians.

Food processors often use ammonia (R-717). While potentially hazardous, ammonia is inexpensive and environmentally benign. Experts anticipate wider use of ammonia due to concerns over CFC phase-out. Interestingly, R-22 was developed as a safe alternative for cooling systems that would perform best at ammonia refrigerant characteristics.

The manufacturer selects the specific refrigerant used in any equipment to best match the cooling system design and size. The availability and cost of these refrigerants and the consequences of refrigerant leaks and disposal have become very serious concerns for today's building owners and the design community. Each of these issues is addressed in other areas of this interactive knowledge program.

Vapor Compression Systems - The Evaporator

The evaporator and condenser are both heat exchangers. Whether they move heat to or from air or water or refrigerant is merely a matter of design. On the design day the evaporator typically cools either:

1. Air returning from the building space (or outside air) to ~ 55 - 60°F

2. Water from about ~ 54°F as it returns from building air handlers to ~ 44°F.

In both cases the evaporator boils the selected refrigerant to provide this cooling. The pressure at which the refrigerant boils is exactly that which satisfies the energy balance of heat-in equals heat-out.

The refrigerant is circulated through numerous parallel paths. As the refrigerant flows and evaporates along these paths the pressure will drop as well. This in turn drops the temperature of the refrigerant as it evaporates. Consequently, properly designed direct expansion coils operate with the coldest refrigerant temperatures closest to the coil exit. However, the refrigerant temperature coming out of this coil is usually a little warmer than this to provide some level of superheat to be sure liquid refrigerant isn't leaving the coil and entering compressor (where it could cause mechanical failure in some designs).

Shell and tube heat exchangers commonly have water circulated through the tubes and refrigerant boiling around the tubes. There are also designs where refrigerant flows within the tubes and water flows over the tubes. Baffles are normally used in this case to direct water flow in a serpentine fashion to optimize heat transfer. Almost all large chillers use shell and tube evaporators with water flowing through the tubes.

 

 

Vapor Compression Systems - Evaporator Control

In comfort cooling applications, actual cooling loads are seldom at full load conditions. Capacity control is achieved in finned coil evaporators that directly chill air by splitting the coil into independent sections. The principal reason is to permit coil sections to be activated and deactivated to better match coil cooling capacity with compressor loading. The combination of smaller coil sections controlled by correspondingly sized expansion valves improves valve performance and part load humidity control.

Capacity control in shell and tube evaporators is usually handled using the return water temperature. For example, if the full-load temperature range for chilled water is from 44°F to 54°F, water returning at 50°F indicates the cooling load is about 60%. Liquid refrigerant is metered to the evaporator to match the load using an orifice plate system or an expansion valve. On large chillers, the expansion valve is pilot operated.

 

 

 

 

 

 

 

 

 

 

Vapor Compression Systems -
The Condenser

The refrigerant is recovered by condensing it in a heat exchanger using air or water to reject the heat. Air cooled condensers are most common in smaller sizes, up to about 200 ton capacity. Technically, there is no upper limit on the size of an air cooled condenser, but operating cost issues usually dictate water cooled units for applications over about 100 tons.

There are two water cooled designs: cooling towers and evaporative condensers. Both work on the principal of cooling by evaporating water into a moving air stream. The effectiveness of this evaporative cooling process depends upon the wet bulb temperature of the air entering the unit, the volume of air flow and the efficiency of the air/water interface.

Evaporative condensers use water sprays and air flow to condense refrigerant vapors inside the tubes. The condensed refrigerant drains into a tank called a liquid receiver. Refrigerant subcooling can be accomplished by piping the liquid from the receiver back through the water sump where additional cooling reduces the liquid temperature even further.

Cooling towers are essentially large evaporative coolers where the cooled water is circulated to a remote shell and tube refrigerant condenser. Notice the cooling water is circulating through the tubes while refrigerant vapor condenses and gathers in the lower region of the heat exchanger. Notice also that this area "subcools" the refrigerant below the temperature of condensation by bringing the coldest cooling tower water into this area of the condenser. The warmed cooling water is sprayed over a fill material in the tower. Some of it evaporates in the moving air stream. The evaporative process cools the remaining water.

The volume of water used by both evaporative condensers and cooling towers is significant. Not only does water evaporate just to reject the heat, but water must be added to avoid the buildup of dissolved solids in the basins of the evaporative condensers or cooling towers. If these solids build up to the point that they foul the condenser surfaces, the performance of the unit can be greatly reduced.

 

Defrosting

Defrosting is a procedure, performed periodically on refrigerators and freezers to maintain their operating efficiency. Over time water vapour in the air condenses on the cooling elements within the cabinet. It also refers to leaving frozen food at a higher temperature prior to cooking.

Defrosting a freezer

The resulting ice inhibits heat transfer out of the cabinet increasing running costs. Furthermore as the ice builds up it takes increasing space from within the cabinet - reducing the space available for food storage. Defrosting the unit is achieved by:-

• Temporarily removing all food from the cabinet.
• Turning off power to the unit.
• Leaving the doors to the unit open
• Waiting for the ice to melt and draining it appropriately. Using a towel is advisable when completing this step.

The process may be sped up by mechanical removal of ice, or the introduction of gentle heat into the cabinet. Placing a pan of hot water in the cabinet and closing it is an effective method. Using a fan to blow in room temperature air will also greatly speed up the melting process as well as help to evaporate the damp surfaces. Note that the fastest manual way is to use a vacuum cleaner: simply insert the hose into the exhaust port (nearly all are designed for this), and use the wand to blow on the coils; this method is much faster than any other.

Any mechanical removal of ice should be done gently so that the equipment is not damaged.

It is generally recommended that defrosting should be done annually.

Many newer units employ automatic defrosting (often called "frost-free" or "no frost") and do not require manual defrosting in normal use.

 

AC Compressor

Air conditioning is the cooling and air for comfort, the term can refer to any form of cooling, heating or ventilation that modifies the condition of air. An air conditioner is an instrument, system, or machinery designed to calm down the air temperature and humidity within an region, typically using a refrigeration cycle but sometimes using evaporation, commonly for comfort cooling in buildings and motor vehicles.

Humidity control
Air conditioning tools usually reduces the humidity of the air. From the processed air the coil evaporator condenses the water vapor, (much like an ice-cold drink will condense water on the outside of a glass), sending the water to a drain and removing water vapor from the cooled space and reducing the relative humidity. Since the human perspire to make himself cool by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space.

Relative Humidity

The amount of water vapor in the air at any given time is usually less than that required to saturate the air. The relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapor density.

The most common units for vapor density are gm/m3. For example, if the actual vapor density is 10 g/m3 at 20°C compared to the saturation vapor density at that temperature of 17.3 g/m3 , then the relative humidity is

Calculation

What is Humidification?

It is the artificial regulation of humidity in home environments, industrial environments, and health care applications such as artificial respiration. To be comfortable, people require a certain amount of ambient humidity -- not too high, and not too low. Adequate humidification in a manufacturing environment stabilizes moisture in wood, paper, and textiles, while preventing warping in glue joints. In all environments, humidification reduces fire risk and static electricity while making the area feel comfortable.

In humidification, two quantities are commonly used. Absolute humidification is expressed in grams of moisture per cubic volume of air, while the more commonly used relative humidification is expressed as a ratio between the amount of moisture currently in the air and the maximum moisture the air could hold before condensation occurs. A typical comfortable level of relative humidification is between 35% and 50%. Excess humidity can cause the growth of mold or fungus. Too little humidity can cause static discharge or the accumulation of unwanted dust, contributing to allergies.

Many humidifiers are cheap and require little maintenance. In industrial settings, they are often hung from the ceiling among duct work. Humidification is intimately tied to heating and cooling systems. The level of humidity in the air is also a function of the temperature. Therefore, humidity control systems are often integrated with cooling systems.

Dehumidifier

A dehumidifier is mostly a household appliance that reduces the level of humidity in the air, usually for health reasons, as humid air can cause mold and mildew to grow inside homes, which has various health risks. Relative humidity is preferably 30 to 50%.[1] Very high humidity levels are also unpleasant for human beings, can cause condensation and can make it hard to dry laundry or sleep. Higher humidity is also preferred by most insects, including clothes moths, fleas and cockroaches. Dehumidifiers are used in industrial climatic chambers for keeping certain level of humidity.

 

 

 

 

 

Dew point Control

Dew Point Control, LLC (DPC) is an equipment leasing company that provides an array of choices for hydrocarbon dew point control. Working within an industry that is in constant flux, DPC strives to match each customer's needs with the most cost efficient technology.

·         Meet transporting pipeline hydrocarbon dew point specifications.

·         Capture NGL liquid upgrade income.

·         Capture additional income by removing the "crude" component prior to NGL processing to avoid processing, transportation, fractionation and marketing fees.

·         Controlling gathering system liquid drip for efficiency and safety reasons.

·         Improve measurement volumes recorded by orifice custody transfer meters by removing the liquid buildup on the orifice plates.

·         Prove the potential of new gathering systems prior to the capital commitment of the deep liquid NGL recovery facility.

·         Short term lease to move gas to market during the construction phase of a deep liquid recovery NGL facility.

·         To provide additional short term capacity to a deep NGL liquid recovery facility by conditioning gas that bypasses the existing facility or leaning the NGL component of the gas entering the NGL liquid recovery facility.

 

Types of air conditioner equipment

Window and through-wall units

Room air conditioners come in two forms: unitary and packaged terminal PTAC systems. Unitary systems, the common one room air conditioners, sit in a window or wall opening, with interior controls. Interior air is cooled as a fan blows it over the evaporator. On the exterior the air is heated as a second fan blows it over the condenser. In this process, heat is drawn from the room and discharged to the environment. A large house or building may have several such units, permitting each room be cooled separately. PTAC systems are also known as wall split air conditioning systems or ductless systems.[5] These PTAC systems which are frequently used in hotels have two separate units (terminal packages), the evaportive unit on the exterior and the condensing unit on the interior, with tubing passing through the wall and connecting them. This minimizes the interior system footprint and allows each room to be adjusted independently. PTAC systems may be adapted to provide heating in cold weather, either directly by using an electric strip, gas or other heater, or by reversing the refrigerant flow to heat the interior and draw heat from the exterior air, converting the air conditioner into a heat pump. While room air conditioning provides maximum flexibility, when cooling many rooms it is generally more expensive than central air conditioning.

Evaporative coolers

In very dry climates, evaporative coolers are popular for improving comfort during hot weather. This type of cooler is the dominant cooler used in Iran, which has the largest number of these units of any country in the world, causing some to referring to these units as "Persian coolers." An evaporative cooler is a device that draws outside air through a wet pad, such as a large sponge soaked with water. The sensible heat of the incoming air, as measured by a dry bulb thermometer, is reduced. The total heat (sensible heat plus latent heat) of the entering air is unchanged. Some of the sensible heat of the entering air is converted to latent heat by the evaporation of water in the wet cooler pads. If the entering air is dry enough, the results can be quite comfortable; evaporative coolers tend to feel as if they are not working during times of high humidity, when there is not much dry air with which the coolers can work to make the air as cool as possible for dwelling occupants. Unlike air conditioners, evaporative coolers rely on the outside air to be channeled through cooler pads that cool the air before it reaches the inside of a house through its air duct system; this cooled outside air must be allowed to push the warmer air within the house out through an exhaust opening such as a open door or window.[7]

These coolers cost less and are mechanically simple to understand and maintain.

Portable air conditioners

Portable air conditioners (or PACs) are moveable units that can be used to cool a specific room in a home and do not require permanent installation.^[8] Warm air in the room is drawn in through inlets on the portable air conditioner. The air is circulated through the unit and is cooled by evaporator coils with refrigerant running through them and then blown out through the front. Remaining hot air in the unit is expelled and vented through the back with an exhaust hose.^[9] All portable air conditioners require exhaust hoses for venting.

Single Hosed Units

A single hosed unit has one hose that runs from the back of the portable air conditioner to the vent kit where hot air can be released. A single hosed portable air conditioner can cool a room that is 475 sq. ft. or smaller and has at most a cooling power of 12,000 BTUs.^[10]

Dual Hosed Units

Dual hosed units are typically used in larger rooms. One hose is used as the exhaust hose to vent hot air and the other as the intake hose to draw in additional air (usually from the outside). These units generally have a cooler power of 12,000-14,000 BTUs and cool rooms that are around 500 sq. ft.^[10] The reason an intake hose is needed to draw in extra air is because with higher BTU units, air is cycled in large amounts and hot air is expelled at a faster rate. This creates negative air pressure in the room, and the intake hose stabilizes the room's air pressure.^[9]

Split Units

Portable units are also available in split configuration, with the compressor and evaporator located in a separate external package and the two units connected via two detachable refrigerant pipes, as is the case with fixed split systems. Split portable units are superior to both single and dual hosed mono-portable units in that interior noise and size of the internal unit is greatly reduced due to the external location of the compressor, and no water needs to be drained from the internal unit due to the exterior location of the evaporator.

A drawback of split portable units compared with mono-portables is that a surface exterior to the building, such as a balcony must be provided for the external compressor unit to be located.

Heat and Cool Units

Some portable air conditioner units are also able to provide heat by reversing the cooling process so that cool air is collected from a room and warm air is released. These units are not meant to replace actual heaters though and should not be used to cool rooms lower than 50 °F (10 °C).

Central air conditioning

Central air conditioning, commonly referred to as central air (U.S.) or air-con (UK), is an air conditioning system which uses ducts to distribute cooled and/or dehumidified air to more than one room, or uses pipes to distribute chilled water to heat exchangers in more than one room, and which is not plugged into a standard electrical outlet.

With a typical split system, the condenser and compressor are located in an outdoor unit; the evaporator is mounted in the air handler unit. With a package system, all components are located in a single outdoor unit that may be located on the ground or roof.

Central air conditioning performs like a regular air conditioner but has several added benefits:

• When the air handling unit turns on, room air is drawn in from various parts of the building through return-air ducts. This air is pulled through a filter where airborne particles such as dust and lint are removed. Sophisticated filters may remove microscopic pollutants as well. The filtered air is routed to air supply ductwork that carries it back to rooms. Whenever the air conditioner is running, this cycle repeats continually.

• Because the condenser unit (with its fan and the compressor) is located outside the home, it offers a lower level of indoor noise than a free-standing air conditioning unit.

Mini (Small) Duct, High Velocity

A central air conditioning system using high velocity air forced through small ducts (also called mini-ducts), typically round, flexible hoses about 2 inches in diameter. Using the principle of aspiration, the higher velocity air mixes more effectively with the room air, eliminating temperature discrepancies and drafts. A high velocity system can be louder than a conventional system if sound attenuators are not used, though they come standard on most, if not all, systems.^[11]

The smaller, flexible tubing used for a mini-duct system allows it to be more easily installed in historic buildings, and structures with solid walls, such as log homes. These small ducts are also typically longer contiguous pieces, and therefore less prone to leakage. Another added benefit of this type of ducting is the prevention of foreign particle buildup within the ducts, due to a combination of the higher velocity air, as well as the lack of hard corners.^[12]