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Chapter: Mechanical and Electrical - Thermal Engineering - Refrig Eration and Air Conditioning

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Refrigeration and Air Conditioning

Before 1830, few Am ericans used ice to refrigerate foods due to a lack of ice-storehouses and iceboxes. As these two things became more widely available, individuals used axes and saws to harvestt ice for their storehouses.

REFRIGERATION AND AIR CONDITIONING

 

 

PREREQUISITE DISCUSSION

 

Before 1830, few Am ericans used ice to refrigerate foods due to a lack of ice-storehouses and iceboxes. As these two things became more widely available, individuals used axes and saws to harvestt ice for their storehouses. This method proved to be difficult, dangerous, and certainly did not resemble anything that could be du plicated on a commercial scale.

 

Despite the difficulti es of harvesting ice, Frederic Tudor thought that he could capitalize on this new commo dity by harvesting ice in New England and shipping it to the Caribbean islands as well as the southern states. In the beginning, Tudor lost thousands of dollars, but eventually turned a profit as he constructed icehouses in Charle ston, Virginia and in the Cuban port town of Havana. These icehouses as well as better insulated ships helped reduce ice wastage from 66% to 8%. This efficiency gain influen ced Tudor to expand his ice market to othe r towns with icehouses such as New Orleans a nd Savannah.

This ice market further expan ded as harvesting ice became faster and cheap er after one of Tudor’s suppliers, Nathaniel Wyeth, invented a horse-drawn ice cutter in 1825. This invention as well as Tudor’s s uccess inspired others to get involved in the ice trade and the ice industry grew.

 

Ice became a mass-m arket commodity by the early 1830s with the price of ice dropping from six cents per pound to a half of a cent per pound. In New York City, ice consumption increased from 12,000 tons in 1843 to 100,000 tons in 1 856. Boston’s consumption leapt from 6,00 0 tons to 85,000 tons during that same period. Ice harvesting created a “cooling culture” as majority of people used ice and iceboxes to store their dairy products, fish, meat, and ev en fruits and vegetables. These early cold storage practices paved the way for many Ame ricans to accept the refrigeration technology that would soon take over the country.

 

CONCEPT

 

CONCEPT OF REFRIGERATION

 

Refrigeration is a pro cess in which work is done to move heat from one location to another. The work of heat trannsport is traditionally driven by mechanical work, but can also be driven by heat, magnetism , electricity, laser, or other means.

 

How does it work?


Thermal energy moves from left to right through five loops of heat trans fer:

 

1)    Indoor air loop

2)    Chilled water loop

3)    Refrigerant loop

4)    Condenser water loop

 

5)    Cooling water loop


 

SIGNIFICANCE

 

Refrigeration has had a large importance on industry, lifestyle, a griculture and settlement patterns. The idea of preserving food dates back to the ancien t Roman and Chinese empires. However, refrigeration technology has rapidly evolved in the last century, from ice harvesting to temperature-controlled rail cars. In order to avoid food spoilage, refrigeration plays an important role in day to day life, similarly, Air conditioning is also an important technological system to prevent the human from the hot atmosphere during summer seasons.

 

CLASSIFICATION OF REFRIGERATION SYSTEM

Types of Refrigeratio n

 

         Vapour Compression Refrigeration (VCR): uses mechanical energy

 

         Vapour Absorption Re frigeration (VAR): uses thermal energy

 

Vapour Compression Refrigeration

 

         Highly compressed flu ids tend to get colder when allowed to expand

 

         If pressure high enough

 

         Compressed air hotter than source of cooling

 

         Expanded gas cooler than desired cold temperature

 

•   Lot of heat can be rem oved (lot of thermal energy to change liquid to vapour)

 

•   Heat transfer rate re mains high (temperature of working fluid mu ch lower than

 

what is being cooled)

Vapour Compression Refrig eration Cycle


Evaporator

 

Low pressure liquid re frigerant in evaporator absorbs heat and changes to a gas

 

Compressor

 

The superheated vapo ur enters the compressor where its pressure is raised

 

Condenser

 

The high pressure sup erheated gas is cooled in several stages in the co ndenser

 

Expansion

 

Liquid passes through expansion device, which reduces its pressure an d controls the flow into the evaporator

 

Type of refrigerant

 

         Refrigerant determine d by the required cooling temperature

 

         Chlorinated fluorocarb ons (CFCs) or freons: R-11, R-12, R-21, R-22 and R-502

 

Choice of compressor, design of condenser, evaporator determined by

 

         Refrigerant

 

         Required cooling

 

         Load

 

         Ease of maintenance

 

         Physical space requirements

 

         Availability of utilities (water, power)

 

 

Vapour Absorption  Refrigeration

 


Evaporator



Absorber

 


High pressure generator


Condenser


Evaporative Cooling

 

         Air in contact with water to cool it close to ‘wet bulb temperature’

 

         Advantage: efficient cooling at low cost

 

         Disadvantage: air is ri ch in moisture

 

COMPARISON BETWEEN VAPOR COMPRESSION AND A BSORPTION SYSTEM


 

PERFORMANCE

 

Assessment of Refrigeration

 

         Cooling effect: Tons of Refrigeration

 

1 TR = 3024 kCal /hr heat rejected

 

         TR is assessed as:

 

TR = Q x×Cp x× ( Ti To) / 3024

 

Q =       mass flow rate of coolant in kg/hr

 

Cp =     is coolant specific heat in kCal /kg °C

 

Ti = inlet, temperature of coolant to evaporator (chiller) in 0°C To = outlet temperature of coolant from evaporator (chiller) in 0°C

 

Specific Power Consumption (kW/TR)

 

         Indicator of refrigeration system’s performance

 

         kW/TR of centralized chilled water system is sum of

 

         Compressor kW/TR

 

         Chilled water pump kW/TR

 

         Condenser water pump kW/TR

 

         Cooling tower fan kW/TR

 

Coefficient of Performance (COP)

 

         The  performance  of  refrigerators  and  heat  pumps  is  expressed  in  terms  of

coefficient  of performance (COP), defined as


Measure

 

         Airflow Q (m3/s) at Fan Coil Units (FCU) or Air Handling Units (AHU): anemometer

 

         Air density r(kg/m3)

 

         Dry bulb and wet bulb temperature: psychrometer

 

         Enthalpy (kCal/kg) of inlet air (hin) and outlet air (Hout): psychrometric charts

 

APPLICATIONS OF REFRIGERATRION

 

Ø Metal workers

Ø Oil refineries

Ø Chemical plants

 

Ø  Petrochemical plants

Ø  Transporting temperature-sensitive foodstuffs

Ø Dairy products

 

 

 

 

 

 

AIR CONDITIONERS

 

CONCEPT OF AIR CONDITIONING

 

Air conditioning (often referred to as aircon, AC or A/C) is the process of altering the properties of air (primarily temperatureand humidity) to more favourable conditions, typically with the aim of distributing the conditioned air to an occupied space to improve thermal comfort and indoor air quality.

 

TYPES OF AIR CONDITIONERS

 

         Room air conditioners

 

         Zoned Systems

 

         Unitary Systems

 

         Window Air-conditioning System

 

         Split Air-conditioning System

 

         Central air conditioning systems

 

1 ROOM AIR CONDITIONER

 

         Room air conditioners cool rooms rather than the entire home.

 

         Less expensive to operate than central units

 

         Their efficiency is generally lower than that of central air conditioners.

 

         Can be plugged into any 15- or 20-amp, 115-volt household circuit that is not shared with any other major appliances

 

2  ZONED SYSTEMS


 

3 CENTRAL AIR CONDITIONING

 

         Circulate cool air through a system of supply and return ducts. Supply ducts and registers (i.e., openings in the walls, floors, or ceilings covered by grills) carry cooled air from the air conditioner to the home.

 

         This cooled air becomes warmer as it circulates through the home; then it flows back to the central air conditioner through return ducts and registers

 


 

4.UNITARY SYSTEMS

 

A unitary air conditioning system comprises an outdoor unit including a compressor for compressing a refrigerant, an outdoor heat exchanger for heat exchange of the refrigerant and an expander connected to the outdoor heat exchanger, for expanding the refrigerant; a duct installed inside a zone of a building; a central blower unit having a heat exchanger connected to the outdoor unit through a first refrigerant pipe and a blower for supplying the air heat-exchanged by the heat exchanger to the duct; and an individual blower unit including a heat exchanger connected to the outdoor unit through a second refrigerant pipe and a fan for sending the air heat exchanged by the heat exchanger and disposed in a zone in the building, for individually cooling or heating the zone. Accordingly, cooling or heating operation is performed on each zone of the building, and simultaneously, additional individual heating or cooling operation can be performed on a specific space, so that a cost can be reduced and cooling or heating in the building can be efficiently performed.


 

5.WINDOW AIR-CONDITIONING SYSTEM

 

It is the most commonly used air conditioner for single rooms. In this air conditioner all the components, namely the compressor, condenser, expansion valve or coil, evaporator and cooling coil are enclosed in a single box. This unit is fitted in a slot made in the wall of the room, or often a window sill. Windows air conditioners are one of the most widely used types of air conditioners because they are the simplest form of the air conditioning systems. Window air conditioner comprises of the rigid base on which all the parts of the window air conditioner are assembled. The base is assembled inside the casing which is fitted into the wall or the window of the room in which the air conditioner is fitted. The whole assembly of the window air conditioner can be divided into two compartments: the room side, which is also the cooling side and the outdoor side from where the heat absorbed by the room air is liberated to the atmosphere. The room side and outdoor side are separated from each other by an insulated partition enclosed inside the window air conditioner assembly. In the front of the window air conditioner on the room side there is beautifully decorated front panel on which the supply and return air grills are fitted (the whole front panel itself is commonly called as front grill). The louvers fitted in the supply air grills are adjustable so as to supply the air in desired direction. There is also one opening in the grill that allows access to the Control panel or operating panel in front of the window air conditioner.


TYPES OF CENTRAL AC

 

         split-system

 

         An outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator

 

 

 

         Packaged

 

         The evaporator, condenser, and compressor are all located in one cabinet.

 

 

6 SPLIT AIR-CONDITIONING SYSTEM:

 

The split air conditioner comprises of two parts: the outdoor unit and the indoor unit. The outdoor unit, fitted outside the room, houses components like the compressor, condenser and expansion valve. The indoor unit comprises the evaporator or cooling coil and the cooling fan. For this unit you don't have to make any slot in the wall of the room. Further, the present day split units have aesthetic looks and add to the beauty of the room. The split air conditioner can be used to cool one or two rooms.


Energy Consumption

 

         Air conditioners are rated by the number of British Thermal Units (Btu) of heat they can remove per hour. Another common rating term for air conditioning size is the "ton," which is 12,000 Btu per hour.

 

         Room air conditioners range from 5,500 Btu per hour to 14,000 Btu per hour.

 

 

 

Energy Efficiency

 

         Today's best air conditioners use 30% to 50% less energy than 1970s

 

         Even if your air conditioner is only 10 years old, you may save 20% to 40% of your cooling energy costs by replacing it with a newer, more efficient model

 

 

 

SOLVED PROBLEMS

 

1.    A sling psychrometer gives reading of 250c dry bulb temperature 1 50c wet bulb temperature. The barome ter indicates 760 mm of hg assuming partial pressure of the vapour as 10 mm of Hg. Determine 1. Specific humidity 2. Saturation ratio.

 

Given Data:

 

Dry bulb  temper ature  td  =250c

Wet bulb tempe rature tw=150c Barometer pressure pb=760mm of Hg

 

Partial pressure pv= 10mm of Hg

 

To Find:

 

Specific humidit y

 

Saturation ratio.

 

Solution:

 

Specific humidity:

 

We know that Specific humidity


=                                                             0.41

Result:

 

1.                                                             Specific humidity     0.0083 kg/kg of dry air

2.                                                             Saturation ratio . =  0.41


2. A two stages, single acting air compressor compresses air to 20bar. The air enters the L.P cylinder at 1bar a nd 27oc and leaves it at 4.7bar. the air en ters the H.P. cylinder at 4.5bar and 27oc. the size of the L.P cylinder is 400mm diameter and 5 00mm stroke. The clearance volume In both cylinder is 4% of the respective stroke volume. The compressor runs at 200rpm, taking index of compression and expansi on in the two cylinders as 1.3, estimate 1. The indicated power required to run the co mpressor; and 2. The heat rejected in the intercooler per minute.

 

Given data:

Pressure (P4)= 20bar

Pressure (P1) = 1 bar = 1× 105 N/m2

 

Temperature (T1 ) = 27oC = 27+273 = 300K Pressure (P 2) = 4.7bar

 

Pressure (P3) = 4.5bar

 

Temperature (T3 ) = 27oC = 27+273 = 300K Diameter ( D1) = 400mm 0.4m Stroke (L1) = 500 mm = 0.5m

K = 0.04

N = 200rpm ; n = 1.3

 

To Find:

 

Indicated power required to run the compressor

 

Solution :


= 2 043.5× 103 J/min = 2034.5 KJ/min

 

Total indicated work done by the compressor,

 

W = WL + WH = 2123.3 + 2034.5

 

= 4157.8 KJ/min

 

Indicated power required to run the compressor = 4157.8 / 60

 

= 69.3KW

 

 

3. In an oil gas turbine installation , air is taken as 1 bar and 30oC . The air is compressed

to 4bar and then heated by b urning the oil to a temperature of 500oC . If the air flows at the rate of 90Kg/min . Find the power developed by the plant take γ for air as 1.4 Cp as 1KJ/KgK . If 2.4Kg of oil having calorific value of 40,000 KJ/Kg if b urned in the combustion chamber per min ute. Find the overall efficiency of the plant.

 

Given Data:

 

Pressure (P4 = P3) = 1bar Pressure (P1 = P2) = 4bar

Temperature (T2) = 500oC  = 500+273 = 773K

 

Mass flow rate of air(ma) = 90Kg/min = 1.5Kg/sec Mass flow rate of fuel (mf) = 2.4Kg/min = 0.04Kg/sec

 

Temperature (T4) = 30oC = 30+273 = 303K γ = 1.4 ; Cp = 1KJ/KgK ; Cv= 40,000 KJ/Kg

 

 

To Find:

Power developed   by   the   plant

 

Performance                            of  the  gas  turbine

 

Overall efficiency of the plant

 

Solution:

 


 

 

 

TECHNICAL TERMS

 

BTU - British thermal unit. A unit of heat energy - approximately the amount of energy needed to heat one pound of water by one degree Fahrenheit.

 

dBA a unit for measuring sound power or pressure, deciBel on the A scale.

 

Capacity the ability of a heating or cooling system to heat or cool a given amount of space.

 

For heating, this is usually expressed in BTU’s. For cooling, it is usually given in tons.

 

Compressor the pump that moves the refrigerant from the indoor evaporator to the outdoor condenser and back to the evaporator again. The compressor is often called “the heart of the system” because it circulates the refrigerant through the loop.

 

Condenser a device used to condense a refrigerant thereby rejecting the heat to another source, typically an air cooled or water cooled condenser.

 

Cassette a fan coil unit that fits mainly in the ceiling void with only a diffuser plate visible, diffuses conditioned air in one, two, three or four directions.

 

HVAC heating, ventilation and air conditioning.

 

Inverter system Constantly alters fan and motor speeds. This enables faster cooling of a room, and the inverter air conditioner doesn’t have to switch itself on and off to maintain a constant

temperature.

 

kw standard measurement of heat or power, 1kw = 1000 watts = 3412Btu/hr = 860kcal.

 

Load Calculation a mathematical design tool used to determine the heat gain and heat loss in a building so that properly sized air conditioning and heating equipment may be installed.

 

Refrigerant a substance that produces a refrigerating effect while expanding or vaporizing.

 

Reverse cycle the reverse cycle air conditioner internally reverses its operation to provide heating or cooling, as required.

 

Split System a central air conditioner consisting of two or more major components. The system usually consists of a compressor-containing unit and condenser, installed outside the building and a non-compressor containing air handling unit installed within the building. This is the most common type of system installed in a home.

 

Zoning the practice of providing independent heating and/or cooling to different areas in a structure. Zoning typically utilizes a system controller, zoning dampers controlled by a thermostat in each zone, and a bypass damper to regulate static pressure in the supply duct.

 

 

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