Hotel Engineering Notes- REFRIGERATION AND AIR-CONDITIONING

                       
                                     REFRIGERATION AND AIR-CONDITIONING

Temperature / Humidity Ranges for Comfort
Conditions	Relative Humidity	Acceptable Operating Temperatures
		°C	°F
Summer light clothing	If 30%, then If 60%, then	24.5 - 28 23 - 25.5	76 - 82 74 - 78
Winter warmclothing	If 30%, then If 60%, then	20.5 - 25.5 20 - 24	69 - 78 68 - 75

Ventilation: Ventilation is the process by which 

‘clean’ air (normally outdoor air) is intentionally provided to a space and stale air is removed. This may be accomplished by either natural or mechanical means. Ventilation is needed to provide oxygen for metabolism and to dilute metabolic pollutants (carbon dioxide and odour). It is also used to assist in maintaining good indoor air quality by diluting and removing other pollutants emitted within a space but should not be used as a substitute for proper source control of pollutants. Good ventilation is a major contributor to the health and comfort of building occupants.

Product	Recommended Relative Humidity – RH - (%)
Sugar Storage	20 - 35%
Breweries	35 - 45%
Coffee Powder	30 - 40%
Milk Powder Storage	20 - 35%
Seed Storage	35 - 45%

Air infiltration and exfiltration: In addition to intentional ventilation, air inevitably enters a building by the process of ‘air infiltration’. This is the uncontrolled flow of air into a space through adventitious or unintentional gaps and cracks

in the building envelope. The corresponding loss of air from

an enclosed space is termed ‘exfiltration’.

Air re-circulation: Air re-circulation is frequently used in commercial buildings to provide for thermal conditioning.

 Re-circulated air is usually filtered for dust removal but, since oxygen is not replenished and metabolic pollutants are not removed, re-circulation should not usually be considered as contributing towards ventilation needs.

Advantages of Air conditioning Better quality of work environment Controlled humidity Reduces corrosive atmosphere Better psychological impact Better comfort level Improves efficiency and activity Cleanliness Low noise level

Air conditioning is the removal of heat from indoor air for thermal comfort. In another sense, the term can refer to any form of cooling, heating, ventilation, or disinfection that modifies the condition of air. An air conditioner (often referred to as AC or air con.) is an appliance, system, or machine designed to stabilise the air temperature and humidity within an area (used for cooling as well as heating depending on the air properties at a given time), typically using a refrigeration cycle but sometimes using evaporation, commonly for comfort cooling in buildings and motor vehicles.

A room air conditioner most commonly fits into a window, is a Unitary system as opposed to a Central system, though there are models that can be installed into an exterior wall. Whether mounted in a window or wall, this type of air conditioner plugs into a standard electrical outlet and doesn't need special wiring.

 

A room air conditioner pulls hot air in from the outside and cools it with a fairly complicated process that involves a refrigerant gas, compression, heat absorption, condensation, coils and a fan that blows the cooled air into the room. It's essential to determine the size of the area you want to cool: If you buy too small a unit, it will keep running, increasing your electricity bills without making you feel much cooler. If the unit is too large for the space, it will cool but very inefficiently with humidity build-up, leaving you feeling cold and clammy.

A central air conditioner cools your entire house at once using a condenser (usually located outside) and a fan-and-coil system and ductwork that brings the cooled air to each room and returns the air for cooling again. It usually works in tandem with a forced-air furnace and its related ducting; for lack of that type of furnace, the cooling coils and fan will be in the attic, with ductwork coming from it to deliver the cooled air.

Dry Bulb Temperature	It is the temperature recorded by a thermometer which is not affected by moisture.
Dew Point Temperature	It is the temperature of air at which water vapour in air starts condensing.
Specific Humidity or Humidity Ratio	It is the mass in kg. of water vapour contained in the air-water mixture per kg. of dry air. It is the ratio of mass of water vapour to the mass of dry air in a certain volume of mixture.
Wet Bulb Temperature	The bulb is covered with muslin wick wetted with water is moved past unsaturated air at velocity of 300 m/min. The temperature reading obtained is wet bulb temperature.
Actual Humidity	Actual quantity of water in a given amount of air.
Load on air-conditioner	Amount of heat that must be removed from air of a given space.

Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. A device that performs this function may also be called a heat pump. This is the removel of heat from a body to make it colder than its surroundings.

Laws of refrigeration: (i) Fluids absorb heat while changing from liquid to a vapour state and vice-versa (called evaporation and condensation respectively). (ii) The temperature at which the change of state occurs is constant if pressure remains constant.

Unit of Refrigeration: This is generally given in tonnes of refrigeration (TR). One tonne of refrigeration means one tonne of water at 0^oC converted to one tonne of ice at 0^oC. (1 TR = 3024 kcal/hr = 50.4 kcal/min = 12600kj/hr = 3.517kW.)

(1cal = 4.1868 J). A ton of refrigeration is approximately equal to the cooling power of one short ton (2000 pounds or 907 kilograms) of ice melting in a 24-hour period. The value is defined as 12,000 BTU per hour, or 3517 watts. Residential central air systems are usually from 1 to 5 tons (3 to 20 kilowatts (kW)) in capacity.

Coefficient of Performance: Cop = (Heat removed in kcal per unit time) / (Work supplied in kcal per unit time)

Refrigeration in catering Preservation of food/ice-cream, Cooling of food to a temperature suitable for serving, Cooling of drink, Ice-water, Cooling of food and drink for sale, Ice-making, Bakery, Fish Storage, Vegetables cold storage.

Methods of refrigeration: (a) Ice Refrigeration: Ice is put around the object which is to be cooled. In this the heat is taken from the object by the ice and it gets converted to water. (b) Evaporative System: Volatile liquids absorb its latent heat of vaporisation from the object that is to be cooled and gives this heat to the coolant in the condenser and again becomes liquid.

(iii) Gas / Air expansion syatem: First, compress a gas adiabatically (i.e. a process in which no heat is transferred from the system ) and then cool this high pressure gas keeping pressure constant. Then cool this high pressure  low initial temperature gas to atmospheric pressure. It is found that the temperature of the gas is less than 0^oC.

Vapour-compression refrigeration system

Components of Vapour Compression System Compressor: Rotary, Centrifugal or Condenser: Shell or Tube type Air Cooler: Natural, Forced or Water Spray Type Expansion Valve: Automatic, Thermostatic Evaporator: Shell and Tube, Double pipe type

The vapour-compression uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. All such systems have four components: a compressor, a condenser, a Thermal expansion valve (also called a throttle valve), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be condensed with typically available cooling water or cooling air. That hot vapor is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air (whichever may be the case).

The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the

temperature of the enclosed space to be refrigerated. The cold

mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser.

To complete the refrigeration cycle, the refrigerant vapour from the evaporator is again a saturated vapor and is routed back into the compressor.

Vapour Absorption Refrigeration Systems: (VARS) belong to the class of vapour cycles similar to vapour compression refrigeration systems. However, unlike vapour 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 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, 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 vapour generator producing ammonia vapour at high pressure. The high pressure ammonia vapour 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 a low pressure. During throttling, ammonia is partially vapourized and its temperature decreases.

This low temperature ammonia is fed to an evaporator where it is vapourized removing energy from the evaporator. Then this low-pressure ammonia vapour is absorbed in the weak solution of ammonia. The resulting strong ammonia solution is pumped back to the vapour 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

Comparison between Vapor Compression and Absorption system:

Absorption system	Compression System
Uses low grade energy like heat. Therefore, may be worked on exhaust systems from I.C engines,etc.	Using high-grade energy like mechanical work.
Moving parts are only in the pump, which is a small element of the system. Hence operation is smooth.	Moving parts are in the compressor. Therefore, more wear, tear and noise.
The system can work on lower evaporator pressures also without affecting the COP.	The COP decreases considerably with decrease in evaporator pressure.
No effect of reducing the load on performance.	Performance is adversely affected at partial loads.
Liquid traces of refrigerant present in piping at the exit of evaporator constitute no danger.	Liquid traces in suction line may damage the compressor.
Automatic operation for controlling the capacity is easy.	It is difficult.

Properties of Refrigerants: The refrigerants should be,

Non-poisonous, Non-toxic, Non-corrosive, Non-explosive, Non-inflammable, Low boiling point, Condensing pressure low, High latent heat of vaporization, Low specific heat, Inert to oil, Easy availability.

Temperatures recommended for storage of perishables

Type of Food	Optimum Temperature
Fruits and vegetables (except bananas)	1.1 oC to 7.2 oC
Dairy products	3.3 oC to 7.8 oC
Meat and poultry	0.6 oC to 3.3 oC
Fish and shell fish	5.0 oC to 1.1 oC
Frozen foods	1.8 oC to 6.7 oC

Precautions in refrigeration systems: Keep the refrigerator well away from boilers and cooking appliances. b) Keep the air condenser cooled by keeping the system away from walls for better circulation of air. c) The goods while keeping inside the refrigerator should not be hot, they should be at room temperature (i.e. about 17 ^oC to 23 ^oC) c) Keep the fins of the condenser clean and free of lint  & dust accumulation d) Keep the door gaskets clean and dent free.

Air Conditioning: Air-conditioning is achieved by a cycle of expansion and compression of a refrigerant, where the compression converts cold gas to high pressure hot gas and the expansion converts liquid refrigerant to cold gas which in turn cools the desired area. 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. The term Freon is generically "used for any of various nonflammable fluorocarbons used as refrigerants and as propellants for aerosols."

This is how the evaporation cycle in an air conditioner works:

The compressor compresses cool Freon gas, causing it to become hot, high-pressure Freon gas (shown in the diagram).

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, low-pressure Freon gas (shown in the diagram).

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.

 

Central chilled water air conditioning systems - All Air Systems

 An all-air system provides complete sensible and latent cooling capacity in the cold air supplied by the system.  Heating can be accomplished by the same air stream, either in the central system or at a particular zone.  All-air systems can be classified into 2 categories:-

-Single duct systems

-Dual duct systems

 System Advantages

 1. The central plant is located in unoccupied areas, hence facilitating operating and maintenance, noise control and choice of suitable equipment.

 2. No piping, electrical wiring and filters are located inside the conditioned space.

 3. Allows the use of the greatest numbers of potential cooling seasons house with outside air in place of mechanical refrigeration.

 4. Seasonal changeover is simple and readily adaptable to climatic control.

 5. Gives a wide choice of zonability, flexibility, and humidity control under all operating conditions.

 6. Heat recovery system may be readily incorporated.

 7. Allows good design flexibility for optimum air distribution, draft control, and local requirements.

 8. Well suited to applications requiring unusual exhaust makeup.

 9. Infringes least on perimeter floor space.

 10. Adapts to winter humidification.

 System Disadvantages

 1. Requires additional duct clearance which can reduce the usable floor space.

 2. Air-balancing is difficult and requires great care.

 3. Accessibility to terminals demands close cooperation between architectural, mechanical and structural engineers.

Central chilled water air conditioning systems - All-water Systems

 All-water systems are those with fan-coil, unit ventilator, or valance type room terminals with unconditioned ventilation air supplied by an opening through the wall or by infiltration.  Cooling and dehumidification is provided by circulating chilled water through a finned coil in the unit.  Heating is provided by supplying hot water through the same or a separate coil.

 System Advantages:

 1. Flexible and readily adaptable to many building module requirements.

 2. Provides individual room control.

 System Disadvantages

 1. No positive ventilation is provided unless wall openings are used.

 2. No humidification is provided.

 3. Seasonal change over is required.

 4. Maintenance and service work has to be done in the occupied areas.

In 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), 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, and at least one company has a 2,700 ton chiller for special uses. Chilled water temperatures can range from 35 to 45 degrees Fahrenheit (1.5 to 7 degrees Celsius), depending upon application requirements.

The Potential for Raising Chilled Water Temperature: Chilled water systems are commonly designed to provide full cooling load with a chilled water temperature of about 42°F (i.e. 5.5°C). Plant operators typically leave the chilled water temperature fixed at this value or some other. This is inefficient for most applications, such as air conditioning, where the load is well below its maximum most of the time. Typically, you can raise the chilled water temperature by 5°F to 10°F for much of the time. Even at full load, the typical oversizing of airside components (air handling units, fan-coil units, etc.) usually allows some increase in chilled water temperature.