Thermodynamics, Heat Transfer and Refrigeration

System

quantity of matter or a region in space chosen for study

Surroundings

mass or region outside the system

Boundary

real or imaginary surface that separates the system from its surroundings

Closed system

(control mass or just system when the context makes it clear) consists of a fixed amount of mass, and no mass can cross its boundary

Isolated system

energy is not allowed to cross the boundary

Open system

a control volume, as it is often called, is a properly selected region in space

Zeroth law

when two bodies have equality of temperature with a third body, they in turn have equality of temperature with each other

First Law

also known as the Conservation of Energy principle, states that energy can neither be created not destroyed, it can only change forms

Second Law

deals with the quality of energy (energy degradation)

Kevin-Planck statement

It is impossible to construct a device that will operate in a cycle and produce no effect other than the raising of a weight and the exchange of heat with a single reservoir

Clausius statement

It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a cooler-body to a hotter body

Third Law

states that the entropy of a perfect crystal is zero at the absolute zero of temperature

Extensive Properties

those properties that vary directly with mass

Intensive properties

those properties that are independent of the mass

Phase

quantity of matter that is homogeneous throughout (solid, liquid, gas)

Properties

condition of a substance as described by certain observable macroscopic parameter

Isothermal

constant temperature

Isobaric

constant pressure

Isometric

constant volume

Reversible Adiabatic

constant entropy

Isenthalpic

constant enthalpy

Polytropic

PVn = C

Cycle

a series of processes one after the other such that the initial and final states are the same

Internal Energy

energy stored within a body or substance by virtue of the activity and configuration of its molecules and of the vibration of the atoms within the molecules

Work

product of the displacement of the body and the component of the force in the direction of the displacement. It is the energy in transition

Heat

energy in transit (on the move) from one body or system to another, solely because of a temperature difference between the bodies or systems

Boyle's Law

If the temperature of a given quantity of gas is held constant, the volume of the gas varies inversely with the absolute pressure during a change of state

Charles's Law

If the pressure on a particular quantity of gas is held constant, then, with any change of state, the volume will vary directly as the absolute temperature

Gay-Lussac's Law

If the volume of a particular quantity of gas is held constant, then, with any change of state, the pressure will vary directly as the absolute temperature

Fourier's law

time rate of energy transfer by conduction is quantified macroscopically by this law

Newton's Law of Cooling

time rate of energy transfer from the surface to the air can be quantified by this law

Stefan-Boltzmann Law

rate at which energy is emitted, Qe, from a surface of are A

Radiation

occurs in the form of a wave motion similar to light waves wherein the energy is transmitted from one body to another without the need for intervening matter

Convection

occurs when heat moves from one place to another by means of currents that are set up within some fluid medium

Conduction

process of heat transfer by molecular motion, supplemented in some cases by the flow of free electrons, through a body (solid, liquid, or gaseous) from a region of high temperature to a region of low temperature

Absorptivity

ratio of rate at which a given surface absorbs radiation to the rate of radiation of a black body at the same temperature

Black Body

hypothetical body capable of absorbing all radiation at the maximum rate for a given temperature.

Emissivity

ratio at which the given surface emits radiation to the rate of radiation of a black body at the same temperature

Enthalpy

is a property of state; is the sum of internal energy of a substance and its pressure - specific volume of product at the given state. It is a measure of the energy of the air expressed in kJ/kg dry air.

Heat

form of energy that provides differences in temperature of molecular materials

Heat Balance

difference between the heat gained (or generated) within the building and the entire heat test from the building

Latent Heat

It is the heat gained or lost without a change in temperature when a substance passes from one state to another as a solid, liquid, or gas

Latent Heat of Evaporation

is the change of heat (energy) required to change unit mass of a substance from a liquid to a gas.

Specific Heat

heat energy required to raise the temperature of unit mass of a substance by 1°C

Specific Volume

volume of space occupied by a pound of dry air at standard atmospheric pressure. It is expressed in m³/kg dry air.

Density

it is the mass per unit volume

Specific Weight

force of gravity (weight) per unit volume

Specific Gravity/ Relative Density

ratio of weight to the weight of water of equal volume

Saturation temperature

temperature at which liquids start to boil or the temperature at which vapor begin to condense

Sub-cooled liquid

one, which has a temperature lower than the saturation temperature corresponding to the existing pressure

Compressed liquid

one, which has a pressure higher than the saturation pressure corresponding to the existing temperature.

Saturated liquid

liquid at the saturations (saturation temperature or saturation pressure), which has temperature equal to the boiling point corresponding to the existing pressure. It is a pure liquid which has no vapor content.

Vapor

name given to any gaseous phase that is in contact withy the liquid phase, or that is in the vicinity of a state where some of it might be condensed

Saturated Vapor

a vapor at the saturation conditions (saturation temperature and saturation pressure). It is 100% vapor which has no liquid or moisture content.

Superheated Vapor

vapor having a temperature higher than the saturation temperature corresponding to the existing pressure

Wet Vapor

combination of saturated vapor and saturated liquid.

Quality

percent by weight that is saturated vapor

Critical Point

represents the highest pressure and highest temperature at which liquid and vapor can coexist in equilibrium.

Refrigeration

process of cooling

Air conditioning

process of treating air to control simultaneously its temperature, humidity, cleanliness, and distribution to meet the comfort requirements of the occupants of the conditioned space

Evaporator

provides a heat transfer surface through which heat can pass from the refrigerated space or product into the vaporizing refrigerant

Suction line

conveys the low-pressure vapor from the evaporator to the suction inlet of the compressor

Vapor compressor

removes the vapor from the evaporator and raises the temperature and pressure of the vapor to a point such that the vapor can be condensed with normally available condensing media

Discharge line

delivers the high pressure, high-temperature vapor from the discharge of the compressor to the condenser

Condenser

provides a heat transfer surface through which heat passes from the hot refrigerant vapor to the condensing medium

Receiver tank

provides storage for the condensed liquid so that a constant supply of liquid is available to the evaporator as needed

Liquid Line

carries the liquid refrigerant from the receiver tank to the refrigerant flow control

Refrigerant flow control

meters the proper amount of refrigerant to the evaporator and reduces the pressure of the liquid entering the evaporator so that the liquid will vaporize in the evaporator at the desired low temperature

Refrigerant

medium of heat transfer which absorbs heat by evaporating at a low temperature and gives up heat by condensing at a high temperature and pressure compensates for this drawback in air-borne service

Air

"principal use as a refrigerant is in the air-cycle refrigeration unit for aircraft. Operating with this requires more work, but the light weight more than compensates for this drawback in air-borne service

Ammonia

its use is in large industrial and low-temperature installations. Although its use in such low-temperature applications as frozen food and dairy plants has been challenged by Refrigerants 22 and 12, many new systems of this come into operations each year

Carbon dioxide

its high horsepower requirements and high condensing pressures have now limited its use to the low -temperature cycle in a cascade system. In the cascade system, this condenses by giving its heat to the evaporator of a highertemperature unit using a different refrigerant.

Refrigerant 11

it has a high value of volume flow per ton which makes it suitable for service in centrifugal compressors. These are the most-used refrigerants in centrifugal compressor system

Refrigerant 12

most widely used refrigerant.It is used primarily with reciprocating compressors for service in household refrigeration appliances, commercial and industrial air conditioning, and in a multitude of other types of refrigeration systems. It possesses desirable properties, having convenient operating pressures, low power requirement per ton, and is nontoxic and noncorrosive

Refrigerant 22

it is nontoxic and has a low power requirement per ton. It is competitive with Refrigerant 12 in small air-conditioning units. It is competitive with ammonia in industrial low-temperature systems in cases where toxicity of the refrigerant is of concern.

Product load

made up of the heat that must be removed from the refrigerated product in order to reduce the temperature of the product to the desired level.

Infiltration

uncontrolled entry of unconditioned outside air directly into the building

Internal load

Heat gain due to the release of energy within a space (lights, people, equipment, etc.). The amount of heat gain in the space due to lighting depends on the wattage of the lamps and the type of fixture.

Ventilation

air intentionally brought into the building by mechanical means

Dew Point Temperature

temperature at which the water vapor in the air is saturated

Humidity

is the water vapor in the air

Absolute humidity/ Vapor density

is the mass of water vapor per unit volume of air

Relative humidity

ratio of the actual partial pressure exerted by the water vapor in any volume of air to the partial pressure that would be exerted by the water vapor if the water vapor in the air is saturated at the temperature of the air.

Humidity ratio

specific humidity, is an expression of the mass of water vapor per unit mass of dry air

Saturation ratio

ratio of the mass of water vapor in the air per unit mass of dry air to the mass of water vapor required for saturation of the same air sample

Dry bulb temperature

temperature as measured by an ordinary dry bulb thermometer

Wet bulb temperature

temperature as measured by a wet bulb thermometer. A wet bulb thermometer is an ordinary thermometer whose bulb is enclosed in a wetted cloth sac or wick

Degrees of superheat

difference between the dew point and dry bulb temperature of air

Saturation line

condition of the mixture lies on the saturation line, the air is said to be saturated, meaning that any decrease in temperature will result in condensation of the water vapor into liquid

Sensible heating/ cooling

psychrometric process; refers to a rate of heat transfer attributable only to a change in dry-bulb temperature of the air. It involves the increase or decrease in the temperature of air without changing its humidity ratio

Humidification

psychrometric process; process of adding water to the air which may be adiabatic (constant wet bulb temperature process since the latent heat added resulted to a decrease in dry bulb temperature) or non adiabatic humidification (with addition of heat)

Cooling and dehumidification

psychrometric process; results in a reduction of both the dry-bulb temperature and the humidity ratio. The total heat removed (refrigeration capacity) is the total heat removed in the cooling coil

Chemical dehumidification

psychrometric process; removal of moisture using chemical or gel to absorb the moisture in the air. This process is a constant enthalpy process

Mixing process

psychrometric process; process of mixing two different streams of air at the desired temperature and humidity ratio.

Shutoff valve

installed in the line to block flow completely when it is closed. This is installed wherever any component of the system should at certain times be isolated from the remainder of the system

Back-pressure valve/ Evaporator-pressure regulator

installed in the suction line to maintain a sufficiently high pressure in an evaporator to prevent freezing or excessive dehumidification of air in an air cooling coil

Solenoid valves

electrically powered magnetic valves which serve as automatic shutoff valves. These are often installed in the liquid line and close when the compressor is not operating

Fittings

In small copper tubing, flare fittings are popular, while the larger tubing lines are soldered. Steel pipe is joined by screw fittings, special refrigerant flanges, or the joints are welded.

Oil separator

used to recover as much of this oil as possible before it goes through the condenser and collects in the evaporator

Vibration eliminator

installed in the suction and discharge lines to reduce the transmission of vibration from the compressor. Vibrations cause irritating noise and weaken the solder connections in the piping