The Principles of Basic Refrigeration
A chiller is simply a device that used to remove heat from something. For industrial purposes, chillers can be thought of as a component within a complex mechanical system that is used to remove heat from a process or substance. To really understand what a chiller is, a fundamental knowledge of the principles of basic refrigeration is required. Welcome to Berg’s School of Cool.
Before getting into the fundamentals of refrigeration, a few basic definitions should be considered:
A). Heat is a form of energy transferred by virtue of a difference in temperature. Heat exists everywhere to a greater or lesser degree. As a form of energy it can be neither created or destroyed, although other forms of energy may be converted into heat, and vice versa. It is important to remember that heat energy travels in only one direction; from a warmer to a cooler object, substance, or area.
latent heat – the heat required to convert a solid into a liquid or vapor, or a liquid into a vapor, without change of temperature. Latent heat, energy absorbed or released by a substance during a change in its physical state (phase) that occurs without changing its temperature. The latent heat associated with melting a solid or freezing a liquid is called the heat of fusion; that associated with vaporizing a liquid or a solid or condensing vapor is called the heat of vaporization. The latent heat is normally expressed as the amount of heat (in units of joules or calories) per mole or unit mass of the substance undergoing a change of state.
B). Cold is a relative term referring to the lack of heat in an object, substance, or area. Another definition describes it as the absence of heat, no process yet has been devised of achieving “absolute zero,” the state in which all heat has been removed from any object, substance, or area. Theoretically, this zero point would be 459.69 degrees below zero on the Fahrenheit thermometer scale or 273.16 degrees below zero on the Celsius thermometer scale.
C). Refrigeration, or cooling process, is the removal of unwanted heat from a selected object, substance, or space and its transfer to another object, substance, or space. Removal of heat lowers the temperature and may be accomplished by use of ice, snow, chilled water or mechanical refrigeration.
D). Mechanical refrigeration is the utilization of mechanical components arranged in a “refrigeration system“ for the purpose of transferring heat.
E). Refrigerants are chemical compounds that are alternately compressed and condensed into a liquid and then permitted to expand into a vapor or gas as they are pumped through the mechanical refrigeration system to cycle.
The refrigeration cycle is based on the long-known physical principle that a liquid expanding into a gas extracts heat from the surrounding substance or area. (You can test this principle by simply wetting your finger and holding it up. It immediately begins to feel cooler than the others, particularly if exposed to some air movement. That’s because the liquid in which you dipped it is evaporating, and as it does, it extracts heat from the skin of the finger and air around it).
Refrigerants evaporate or “boil” at much lower temperatures than water, which permits them to extract heat at a more rapid rate than the water on your finger.
F). Refrigeration system fundamental components
The job of the refrigeration cycle is to remove unwanted heat from one place and discharge it into another. To accomplish this, the refrigerant is pumped through a closed refrigeration system. If the system was not closed, it would be using up the refrigerant by dissipating it into the surrounding media; because it is closed, the same refrigerant is used over and over again, as it passes through the cycle removing some heat and discharging it. The closed cycle serves other purposes as well; it keeps the refrigerant from becoming contaminated and controls its flow, for it is a liquid in some parts of the cycle and a gas or vapor in other phases.
The metering device is a point where we will start the trip through the cycle. This may be a thermal expansion valve, a capillary tube, or any other device to control the flow of refrigerant into the evaporator, or cooling coil, as a low-pressure, low-temperature refrigerant. The expanding refrigerant evaporates (changes state) as it goes through the evaporator, where it removes the heat from the substance or space in which the evaporator is located.
The heat will travel from the warmer substance to the evaporator cooled by the evaporation of the refrigerant within the system, causing the refrigerant to “boil” and evaporate, changing it to a vapor. This is similar to the change that occurs when a pail of water is boiled on the stove and the water changes to steam, except that the refrigerant boils at a much lower temperature.
Now, this low-pressure, low-temperature vapor is drawn to the compressor where it is compressed into a high-temperature, high-pressure vapor. The compressor discharges it to the condenser so that it can give up the heat that it picked up in the evaporator. The refrigerant vapor is at a higher temperature than the air passing across the condenser (air-cooled type), or water passing through the condenser (water-cooled type); therefore that is transferred from the warmer refrigerant vapor to the cooler air or water.
In this process, as heat is removed from the vapor, a change of state takes place and the vapor is condensed back into a liquid, at a high-pressure and high-temperature.
The liquid refrigerant travels now to the metering device where it passes through a small opening or orifice where a drop in pressure and temperature occurs, and then it enters into the evaporator or cooling coil. As the refrigerant makes its way into the large opening of the evaporator tubing or coil, it vaporizes, ready to start another cycle through the system.
The refrigeration system requires some means of connecting the basic major components – evaporator, compressor, condenser, and metering device – just as roads connect communities. Tubing or “lines” make the system completely so that the refrigerant will not leak out into the atmosphere. The suction line connects the evaporator or cooling coil to the compressor, the hot gas or discharge line connects the compressor to the condenser, and the liquid line is the connecting tubing between the condenser and the metering device (Thermal expansion valve). Some systems will have a receiver immediately after the condenser and before the metering device, where the refrigerant is stored until it is needed for heat removal in the evaporator.
There are many different kinds and variations of the refrigeration cycle components. For example, there are at least a half dozen different types of compressor, from the reciprocating, piston through a screw, scroll and centrifugal impeller design, but the function is the same in all cases – that of compressing the heat laden vapor into a high-temperature vapor.
There are a number of different types of metering devices to regulate the liquid refrigerant into the evaporator, depending on the size of equipment, a refrigerant used, and its application.
The mechanical refrigeration system described above is essentially the same whether the system is a domestic refrigerator, a low-temperature freezer, comfort air conditioning system, industrial chiller, or commercial cooling equipment. Refrigerants will be different and the size of the equipment will vary greatly, but the principle of operation and the refrigeration cycle remains the same. Thus, once you understand the simple actions that are taking place within the refrigeration mechanical cycle you should have a good understanding of how a refrigeration system works.
Vapour Absorption Refrigeration Systems
Vapour Absorption Refrigeration Systems (VARS) belong to the class of vapor cycles similar to vapor compression refrigeration systems. However, unlike vapor compression refrigeration systems, the required input to absorption systems is in the form of heat. Hence these systems are also called heat operated or thermal energy-driven systems. Since conventional absorption systems use liquids for the absorption of refrigerant, these are also called as wet absorption systems. Since these systems run on low-grade thermal energy, they are preferred when low-grade energy such as waste heat or solar energy is available. As absorption systems use natural refrigerants such as water or ammonia they are environment-friendly. In the absorption refrigeration system, the refrigeration effect is produced mainly by the use of energy as heat. In such a system, the refrigerant is usually dissolved in a liquid. A concentrated solution of ammonia is boiled in a vapor generator producing ammonia vapor at high pressure. The high-pressure ammonia vapor is fed to a condenser where it is condensed to liquid ammonia by rejecting energy as heat to the surroundings. Then, the liquid ammonia is throttled through a valve to low pressure. During throttling, ammonia is partially vaporized and its temperature decreases.
This low-temperature ammonia is fed to an evaporator where it is vaporized removing energy from the evaporator. Then this low-pressure ammonia vapor is absorbed in the weak solution of ammonia. The resulting strong ammonia solution is pumped back to the vapor generator and the cycle is completed. The COP of the absorption system can be evaluated by considering it as a combination of a heat pump and a heat engine.
CARE AND MAINTENANCE OF REFRIGERATING UNITS
Regular cleaning and maintenance not only makes your equipment work more efficiently, but it also helps it last longer. When parts are worn and dirty, the unit runs more frequently causing more wear and tear on the parts. Eventually, the unit breaks down for good long before it should have.Preventing these issues is simpler than you think. Call in a qualified refrigeration service company at least twice a year and up to once a month, depending on the load and usage of your equipment, to do the following tasks:
Clean evaporator and condenser coils
Check lines for condensation
Clean fan blades and inspect the fan motor
Check integrity of insulation
Check for air leaks through cracks, holes, and worn parts like gaskets and seals
Lubricate door hinges and handles
Check for loose electrical connections
Clear drain lines of debris
Check temperature and defrost settings and calibrate thermometers
Check filters on ice makers
Thoroughly clean all ice maker components
TYPES OF REFRIGERATING UNITS IN HOTEL INDUSTRY
The usual categories of refrigerating units employed in the hotel industry are:
Reach – in Units
Counter – top or table – top Units
Walk – in Units
Special – purpose Units
Each of the above-mentioned units can have both cooler and freezer sections in them. Some refrigerators are now divided into four zones to store different types of food. They are as follows:Subzero (up to –18 degree Celsius) – Freezer Section0 degree Celsius – Meat preservation Section5 degree Celsius – Refrigerator10 degree Celsius – Vegetables Section
Reach–in refrigerators
Coolers & Freezers – Capacities of such units are usually expressed in terms of the internal space volume (cubic metre) or in mass (Kg) of material being stored in it.Typical interior temperature range for cooler should be 4.4 degree Celsius to 7 degree Celsius and for a freezer should be -15 degree Celsius to -23 degree Celsius.
Counter top units
Small counter top refrigerators are frequently used to facilitate service.They are usually coolers and have features similar to those of reach-in coolers but of much smaller size usually up to a size of 100 litres only.
Walk – in refrigeration system
As the name suggests, food operators can directly move into such units.They are much bigger in size and height than walk – in refrigerators.A typical size would be 4 X 5 X 3 m, or even more, or as lass as having a floor area of 2 sq. m..They are normally custom built.
Special purpose units
Ice Cream Conservator and frozen food conservator: They are usually “well type” and open vertically through a lid. This minimizes heat loss as cool and heavy air inside cannot escape upward. They are usually maintained at temperatures between -23 to -18 degree Celsius.
Wet Fish Cabinets: They contain deep galvanized drawers in which fish can be embedded in ice. They keep moist storage conditions necessary for maintaining the quality of fish. They are usually maintained at temperatures between 0 to -2 degree Celsius.
Display Cabinets: In many hotels, there is a refrigerated display cabinet kept in the dinning for the customers to choose varieties of meats for grilling. They are also used to display bakery products and desserts.
Bottle Coolers: They are used in dinning and are usually in bars. They are constructed in two ways. One is open refrigerated through type and the other is glass front cabinet type with hinged doors or sliding doors.
Ice makers: Modern ice making machines need only to be connected to a cold water supply line. Water flows over a refrigerated plate and gets converted into a slab of ice. This slab of ice passes over a wire net which is electrically heated and the slab melts along the wire mesh and falls through the square holes as separate small cubes of ice.
Desirable properties of refrigerants
It should be non poisonous and non irritant.
It should be non flammable.
It should be chemically stable.
It should be non corrosive and should not react with the parts / components of the system.
It should have no objectionable odour or smell.
It should be easily and reliably detectable in case of leakage.
Its LH of vapourisation should be high so that quantity of high refrigerant can be minimized.
Its cost should be low.
It should be easily available.
Air Conditioning
Any type of conditioning of air which includes circulating air in a room, filtering air, cooling or heating air, humidifying or dehumidifying the air.
Complete air conditioning would mean all such processes being carried on the air.
The refrigeration plant is necessary for such an air – conditioning system.
Terminologies used in Air Conditioning
Psychrometry – The study and measurement of the properties of air and water vapor mixture are known as psychrometry.
Dry Air – Atmospheric air consisting of a mixture of gases excluding its water vapor content is known as dry air.
Moisture – The water vapor contained in air is called moisture.
Moist Air – Atmospheric air along with gases and water vapor is known as moist air.
Unsaturated Air – The moist air which does not contain the maximum amount of water vapor that it can hold at a given temperature is known as unsaturated air.
Saturated Air – The mist air which contains the maximum amount of water vapor that it can hold at a given temperature, is known as saturated air.
Dry Bulb Temperature (DBT) – The reading of the temperature of air given by an ordinary mercury thermometer, having its bulb dry and not under the sun or other heat-radiating objects.
Dew Point Temperature (DPT) – At atmospheric temperature atmospheric air always contains water in the form of vapors. If this air is cooled down, the water vapor is turned into water droplets. This temperature at which the water vapors turns into water droplets is called dew point temperature.
Wet Bulb Temperature (WBT) – Temperature of air indicated by a mercury thermometer whose bulb is covered by a piece of wet muslin is known as wet-bulb temperature.
Specific Humidity (SH) – It is defined as the total quantity of water vapor in the moist air.
Relative Humidity (RH) – it is defined as the ratio of actual moist air content of a given volume at a particular temperature to the maximum amount of water vapor if the air is saturated at the same temperature.
PRINCIPLES OF AIR-CONDITIONING
The goal is to keep it more comfortable inside the house than it is outside.
Types of Air Conditioning Plants
1.Centralized air–conditioning System
2.Unit air Conditioners
Window type units
Split-type units
Centralized Air Conditioning Plant
The central air conditioning plants or the systems are used when large buildings, hotels, theaters, airports, shopping malls, etc. are to be air-conditioned completely.
The air conditioning system operates essentially in a closed cycle.
Return air from different spaces in the property such as lodging rooms, dining areas, stores, banquet halls, kitchen, etc. comes back through the return duct is drawn by what is known as an extraction fan or exhausted.
This return air although a little stale is still cold/hot and relatively free from dirt and other contaminants present in the fresh air.
Instead of exhausting the entire return gas out and sucking in fresh air only, it is returned to the circuit to minimize energy requirement for cooling/heating and filtration.
Components of Centralised Air Conditioning Plant
FRESH AIR INLET – It admits fresh air through a louver/grille situated on an outside wall very much away from the kitchen, boiler house, and water closet (WC) outlets.
FILTERS – It is employed to arrest suspended particulate matter such as dirt and dust from inlet air.
AIR WASHERS – This is the air – conditioning section which comprises of the pre-heater coil, refrigerating coil and banks of water sprays. The functions of air washers are to control humidity and clean air.
SCRUBBER PLATES – These are a series of zigzag plates with a stream of water running down them and are used to catch the dirt-laden water droplets coming out from the water spray section in the air washer.
WATER ELIMINATOR PLATES – These are a series of dry zigzag plates to remove any remaining free moisture in the air stream.
MIST ELIMINATOR – It is a metallic / non-metallic bed through which air is flown upwards. Fine mist is trapped in the bed and fall down as they coalesce (combine) to bigger size.
AIR HEATER – This is composed of heating coils to bring the air up to the desirable room temperature, if necessary.
INLET FAN – It is a fan for drawing air through the inlet, ducting and forcing it into the various spaces in the building.
FRESH AIR INLET/LOUVERS – They are fitted in the inlet of the fresh air for uniform entry of fresh air and hence smooth mixing with return air before entry to the air – conditioning section.
ROOM INLET GRILLES – One or more grilles in each room for the controlled inflow of air.
ROOM EXTRACTION GRILLES – One or more grilles in each room for the outflow of air. One to discharge outside into the atmosphere and another leading back to the AC plant inlet for the re-circulation.
EXTRACTION FAN – It is the fan for extracting air in the return path of the air ducting coming back from rooms and spaces.
CENTRAL EXTRACTION DAMPER – In case where complete central control is exercised a part of the return air from all spaces is discharged to the atmosphere through a control damper valve in the delivery side of the extractor fan.
DUCTING – Ducting is a term used to denote the passage for the flow of air from the AC plant through various passages in the building opening into the closed space to be cooled and return to the inlet ducting in the AC plant. This also includes the ducting for the exhaust to the atmosphere.
Unit Air Conditioners – Window Type AC
This is a completely self-contained unit with the compressor, condenser, evaporator, refrigerant piping, and air filter all assembled in a very compact manner.
The window AC is usually 0.5 to 5 tons incapacity.
The latest practice is to use sealed type compressor units so that the possibility of leakage of refrigerant is eliminated.
There is a provision to control the fresh air intake.
It is easy to install, operate and maintain.
Its running cost is high.
It requires at least one wall of the room free and open to atmosphere so that the air is discharged in the open.
Unit Air Conditioners – Split Type AC
In this system, the cold side of the unit (such as expansion valve, evaporator coil) is physically separated from the hot side (compressor, condenser).
The refrigerant flows through a long pipeline connecting the hot side and the cold side.
The cold part inside the room and the hot part outside the building.
The main advantages of split type air conditioners are:
Low cost for relatively larger size units
Absence of noise as the compressor is placed outside the building
Building Transport System
Most multi-storied hotels have many floors above the ground floor and also one/two floors below the ground floor. Hence appropriate and efficient building transport systems are not only important but necessary also for operation. The systems should be a mix of manual and automatic operations.
There are two types of building transport system namely:
Vertical Transport System and Horizontal Transport System
Vertical Transport System
Stairs
Elevators / Lifts
Escalators (Moving Stairs)
Gravity Chutes (for goods)
Stairs
This is the most common and essential for vertical movement of people and goods.It is also mandatory as it is an escape route in case of fire in buildings.In case of power failure, lifts do not operate and hence staircase is the only alternative for vertical movement.It must be fireproof so that guests can safely exit to upper and lower building floors.Provision of handrails and non-slip steps are recommended for safety purpose.
Elevators
An elevator is a transportation device used to transport people and goods vertically. Elevators are generally automatic safety units for up and down transport purpose. It consists of a platform travelling in vertical guides in a shaft with hoisting and lowering mechanism and a source of power. The enclosure moving up and down in the guides is known as car.
There are two types of lift systems:
Cable elevators – They are more common because they are fast, smooth and quite.
Hydraulic elevators – are elevators which are powered by a piston that travels inside a cylinder.
An electric motor pumps hydraulic oil into the cylinder to move the piston. The piston smoothly lifts the elevator cab/car. They are used when the number of floors to be serviced is less.
The elevators require regular inspection and maintenance and generally have a life of 20 years.
Escalators
The term escalator is a combination of elevator and “scala” the latin word for steps.
Escalators are the moving step type lifts operating at constant speed between two levels in an inclined course for moving large number of people in a short time. It consists of a staircase whose steps move up and down on tracks which keep them horizontal. Most escalators also have a moving handrail which approximately keeps pace with the movement of the steps. The direction of movement (up and down) can be permanently the same or can be controlled by operators according to the requirement.
Generally two units are required side-by-side at each level, one moving upward and the other downward.
Gravity Chutes
Gravity chutes are used by hotels for transporting soiled linen, garbage and waste from different floors of the hotel to an outlet point (generally kept on the ground floor). It is an efficient system as minimum power is involved in operating it. Gravity chutes save workers motion, time and increase their productivity.
Horizontal Transport System
Conveyor belts (moving sidewalk, moving walkway or travellator)
Electric cars
Manual trolley for transportation of luggage.
Travellators (Moving pavements)
Travellators also is known as horizontal moving sidewalks, moving walkways is a slow speed conveyor belt to transport people, they can walk along or stand on it.They are often installed in pairs, for movement in the opposite direction. Travellators may be used when there is a substantial distance between the hotel entry point and the reception point.
Electric Cars
An electric car is an electrically operated open car with 6 to 8 seats. This is an alternative to travellators and particularly very useful for disabled persons.
Manual Trolley
A manual trolley is used for transportation of luggage of guest.
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