# Mechanical Engineering Dictionary-Thermodynamics

Important Terms and their Meaning of Mechanical Engineering-Thermodynamics:

ABSOLUTE TEMPERATURE – The temperature of a substance measured above absolute zero.

ABSOLUTE ZERO TEMPERATURE – Temperature at which all molecular motion ceases, according to the kinetic theory of gases. A point which has been determined on the thermodynamic scale (by theoretical considerations) beyond which a further decrease in temperature is inconceivable. This is equal to – 459.6° on the Fahrenheit scale and – 273.1° on the centigrade scale.

ABSOLUTE TEMPERATURE SCALES – Used for calculating changes in refrigerant vapour pressures.

ADIABATIC EXPANSION or COMPRESSION – Expansion or compression where the temperature rises during compression and falls during expansion without any loss of heat to the cylinder walls or absorption of heat from the walls.

ADIABATIC HEAT DROP – The heat energy released and theoretically capable of transformation into mechanical work during the adiabatic expansion of unit weight of steam or other vapour or gas.

ADIABATIC SYSTEM – is the system which is insulated from its surroundings. In this system no heat transfer takes place i.e., either into the system or out of the system. It can however exchange work with the surroundings.

AIR – A gas consisting principally of a mechanical mixture of 23.2 percent (by weight) of oxygen, 65.5 percent of nitrogen and 1.3 percent of argon.

AIR STANDARD EFFICIENCY – The thermal efficiency of an internal combustion engine working on the appropriate air standard cycle.

AMBIENT TEMPERATURE – Temperature (usually of the air) surrounding the operating equipment.

APPLIED THERMODYNAMICS – Also called engineering thermodynamics deals with special applications such as energy transfer as power generation, refrigeration and, compression and expansion of gases and fluids.

ATKINSON CYCLE – A working cycle for internal combustion engines, in which the expansion ratio exceeds the compression ratio, more efficient than the Otto Cycle, but mechanically impracticable.

ATMOSPHERIC LINE – A datum line drawn on an indicator diagram by allowing atmospheric pressure to act on the indicator piston or diaphragm.

ATMOSPHERIC PRESSURE – Force exerted by the weight of the atmosphere on every point with which it is in contact. It is generally taken as 1.03 kscm at sea level.

AVAGADRO’S LAW – Equal volumes of different gases at the same temperature and pressure contain the same number of molecules.

AVAILABLE ENERGY – That portion of a quantity of heat which could be transformed into work by means of a reversible engine.

BASIC UNITS – are length, mass, time, temperature and angle.

BLACK BODY – A body which absorbs all the radiation falling on it i.e., has a non-reflecting surface. A black body emits the maximum amount of radiation possible at a given temperature, and the amount is proportional to the fourth power of the absolute temperature.

BOILING POINT – The temperature at which a liquid boils for any given surrounding atmospheric pressure. Now the saturation pressure of the vapour equals that of the atmosphere.

BOUNDARY – is a real physical surface or an imaginary surface enclosing some matter. The boundary may be a fixed one or a varying one.

BOYLE’S LAW – The absolute pressure of a gas will vary inversely as the volume, if the temperature remains constant. Or conversely, the volume will vary inversely as the absolute pressure, if the temperature remains constant.

BRAKE HORSE POWER – Useful power available from an engine. Also called SHAFT HORSE POWER.

CALORIE – The heat per unit weight, one gram, required to raise the temperature of water through one degree centigrade.

CELSIUS – The scale of changes of temperature which uses 0 degree as the freezing point and 100 degree as the boiling point for water at standard pressure.

CHARLE’S LAW – At constant pressure, the volume of a gas is proportional to its absolute temperature. At constant volume, the pressure is proportional to its absolute temperature.

CLOSED SYSTEM – The system which will have boundaries across which both heat and work can penetrate, but no mass will be permitted to cross them.

Cp – Specific heat at constant pressure-Heat to be supplied to raise the temperature of 1 kg of gas through 1°C, the pressure being kept constant (in other words external work is done).

Cv – Specific heat at constant volume-Heat to be supplied to raise the temperature of 1 kg of gas through 1°C, the volume being kept constant (in other words no external work is done).

COEFFICIENT OF EXPANSION – The fractional increase in length or volume per degree rise in temperature.

COEFFICIENT OF LINEAR EXPANSION – Amount of expansion per unit length, per degree rise in temperature.

COEFFICIENT OF PERFORMANCE OF A HEAT PUMP – COPHP = Heat added to the hot body/work supplied.

COEFFICIENT OF PERFORMANCE OF A REFRIGERATOR – COPRef = Heat removed from cold body/work supplied.

COEFFICIENT OF THERMAL CONDUCTIVITY – The quantity of heat that will flow across a unit area in unit time if the temperature gradient across this area is unity.

COLD – A comparative lack of heat, indicating chillness.

COMPRESSED SOLID – is a solid at a temperature below its saturation temperature.

COMPRESSIBILITY FACTOR, Z – is the factor introduced to modify the ideal gas equation Pv = RT, and to describe the behaviour of a real gas. Z = Pv/RT.

CONDENSATE – The liquid formed by the condensation of a gaseous substance.

CONDENSATION – The change of state of a substance from the gaseous to the liquid form.

CONDENSING – The process of giving up latent heat of vaporization in order to liquefy a vapour.

CONDUCTION – Transfer of heat from one part of a material to another or to a material with which it is in contact.

CONDUCTIVITY – The relative value of a material, as compared with a standard, in affording a passage through itself or over its surface for heat.

CONSTANT VOLUME PROCESS – is one wherein a gas is heated (or cooled) in a fixed enclosed space (no change in volume occupied by the gas). There will be no workdone by the gas. The whole heat supplied will be stored in the form of internal energy.

CONSTANT PRESSURE PROCESS – Also called isobaric process. Heat supplied to a system exhibits as the change in enthalpy.

CONSTANT TEMPERATURE PROCESS – Also called isothermal process. There is no change in temperature and hence internal energy and enthalpy remain constant. Heat supplied = work done.

CONVECTION – Passage of heat from one point to another by means of a gravity fluid circulation due to changes in density resulting from picking up and giving up heat. Also transfer of heat to or from a fluid (liquid or gas) flowing over the surface of a body.

COSMIC RADIATION – Radiation of many sorts, but mostly atomic nuclei (protons) with very high energies, originating outside the earth’s atmosphere.

COUNTER FLOW HEAT EXCHANGER – A heat exchanger in which the warm substance flows in the opposite direction to the flow of the cool substance.

CRITICAL STATE OF A SUBSTANCE – is that state at which liquid and vapour coexist in equilibrium. At critical state, latent heat of evaporation becomes zero.

CRITICAL PRESSURE – The critical pressure of a vapour is the pressure required to liquefy it at the critical temperature and is the highest pressure on the temperature -pressure graph for saturated vapour.

CRITICAL TEMPERATURE – Temperature of the vapour above which no pressure, however high, will produce liquefaction.

CRITICAL VELOCITY – The velocity above which fluid flow is turbulent.

CYCLIC PROCESS – is a process (or a series of processes) which returns the system to the state it was before the process began.

DALTON’S LAW OF PARTIAL PRESSURES – At a common temperature, a mixture of gases will exert on the sides of the vessel a total pressure equal to the sum of the pressures which each constituent would exert separately if it alone occupied the vessel.

DERIVED UNITS – are those derived from basic units, e.g., force, work, density etc.

DIMENSIONS – are the properties by which the physical quantity of an object may be described.

DRY BULB TEMPERATURE – The actual temperature of air, as opposed to wet bulb temperature.

DRY STEAM – Steam containing no moisture. It may be either saturated or superheated steam.

DRY SATURATED STEAM – Saturated steam, as generated from water, that contains no moisture in suspension.

EFFICIENCY – Ratio of the useful work performed by a prime mover to the energy expended. In other words the output divided by the input.

ENERGY – Stored work, that is the ability to do work.

ENTHALPY – Name given to the total heat in the fluid at any temperature.

ENTROPY TEMPERATURE DIAGRAM – In thermodynamics, the base of a heat diagram, the area of which is heat units and the altitude of which is absolute temperature.

EQUIVALENT EVAPORATION – Amount of water in kg. that would be evaporated from water at 100°C into steam at 100°C and 1.03 kscm, by the heat put into steam actually evaporated in one hour by 1 kg of fuel.

EVAPORATION – Process by which a liquid changes into a vapour as a result of absorbing heat.

EVAPORATIVE CONDENSER – Combines the principles of forced circulation convection currents with the ability of a vaporizing liquid to absorb heat.

EXTERNAL LATENT HEAT – When vaporization takes place, the amount of heat required because of the work in pushing back the atmosphere to make room for the steam.

EXTRINSIC PROPERTIES – Also called extensive properties, are those properties which are dependent on the mass of the system. Examples are volume, weight and total energy.

FACTOR OF EVAPORATION – A quantity which when multiplied by the amount of steam generated at a given pressure from water at a given temperature, gives the equivalent evaporation from and at 100°C.

FLOW WORK – The product PV (pressure and specific volume) represents flow work in a steady flow system.

FORCED CONVECTION – Circulation of the fluid is made positive by some mechanical means such as a pump for water or a fan for hot gases.

FORCED DRAFT COOLING TOWER – Cools water by mechanically forcing air through the water spray in the tower.

FREE EXPANSION – is a process wherein a fluid from a pressure chamber expands into a vacuum chamber through an orifice of large dimensions.

FREEZING POINT – The temperature at which water turns into ice (0°C at sea level).

FROM AND AT 100°C – In boiler operation, it is an evaporation that would be the equivalent of the actual evaporation when the feed water enters the boiler at 100°C and steam is formed at 100°C, at standard atmospheric pressure.

FROSTING EVAPORATORS – Those evaporators which always operate at temperature below 0°C.

FUNDAMENTAL DIMENSIONS – Internationally accepted units are time, length, mass, force and temperature.

GAUGE PRESSURE – Pressure above or below atmospheric pressure.

GAY LUSSAC LAW – The volume of a gas will vary directly as the absolute temperature, if the pressure remains constant.

GASES – are substances in which the molecules move freely and exist in an unbound state. Gases can be compressed easily because of the large empty space available between the molecules.

HEAT – A form of energy and is due to the motion of the molecules of which all substances are composed. Unit of heat is kilocalorie.

HEAT OF COMPRESSION – Increase of temperature brought about by compression of air or air fuel mixture.

HEAT OF CONDENSATION – The heat that is removed per kilogram of vapour to cause it to condense. It has the same numerical value as the heat of vaporization.

HEAT ENGINE – It is an energy transformer. It transforms heat energy into mechanical energy and render the latter available for doing useful work.

HEAT PUMP – is a device which extracts heat from low temperature surroundings and sends it to a high temperature body, while operating in a cycle. Heat pump maintains a body or system at a temperature higher than the temperature of the surroundings, with the work supplied to it.

HEAT SINK – A means for disposing of unwanted heat, usually by using it to increase the temperature of water, which is then run to waste.

HEAT SOURCE – Supplier of heat to the working agent of a heat engine-a fraction of the heat supplied being changed into work.

HEAT TRANSFER – Movement of heat energy from one place to another (warmer to cooler portion).

HETEROGENEOUS SYSTEM – is a system which is made of more than one phase.

HOMOGENEOUS SYSTEM – is a system consisting of a single phase.

HYPERBOLIC PROCESS – is one in which a gas is heated in such a way that at any instant its pressure multiplied by its volume remains constant.

IDEAL GAS – is one which will obey all the gas laws, under all conditions of temperature and pressure. For an ideal gas, the internal energy and enthalpy are functions of temperature alone.

INDICATED HORSE POWER – The actual power developed or used within a cylinder as calculated from the indicator diagram.

INERT GAS – A gas which under normal conditions does not react with or combine with other substances for example, argon, helium, neon, krypton, xenon.

INTERNAL ENERGY – is the energy arising from the motion and from configuration of the internal particles (atoms and molecules). It is the energy stored in the gas and is used for raising its temperature. v = CvT., Δu = Cv (T2 – T1).

INTRINSIC PROPERTIES – also called intensive properties, are those properties which are independent of the mass of the system. Examples are pressure, temperature and density.

ISENTROPIC PROCESS – which is called reversible adiabatic process is one in which no heat is supplied and work done is at the expense of internal energy. Law followed is Pvγ = constant.

ISOLATED SYSTEM – is a special case of the closed system which will not be influenced by the surrounding. In this system, no mass transfer, no energy transfer occurs. Work and heat cannot cross the boundary of the system.

ISOTHERM – The line drawn through points or areas of similar temperature.

ISOTHERMAL EXPANSION or COMPRESSION – The expansion or compression of a gas at constant temperature, that is with the gas temperature remaining the same during the process.

JOULE’S LAW OF INTERNAL ENERGY – The internal energy of a given quantity of a gas depends only on the temperature of the gas. As temperature changes, the internal energy also changes.

KELVIN PLANK STATEMENT – It is impossible to construct an engine undergoing a cyclic process, which will convert all the heat supplied to it into an equivalent amount of work.

KILO PASCALS (kPa) – Measurement of pressure in the metric system 1 kilo Pascal is approximately equal to 6.895 pounds per square inch.

KINETIC ENERGY – Energy due to momentum, that is the energy of a moving body, which is equivalent to saying, dynamic inertia. The kinetic energy of a moving body is the work which the body is capable of performing against a resistance before it is brought to rest, that is, it equals the work which has brought it from its state of rest to its actual velocity.

LATENT HEAT – The quantity of heat required to change the state or condition under which a substance exists without changing its temperature.

LATENT HEAT OF FUSION – The amount of heat which must be added to one kilogram of material to change its state from a solid to a liquid or which must be subtracted from one kilogram of a liquid to change it to a solid.

LATENT HEAT (internal) – The amount of heat that the water will absorb (latent heat minus heat equivalent of work of evaporation), at the boiling point without a change in temperature.

LATENT HEAT OF VAPOURIZATION – Amount of heat to be added to (or subtracted from) one kilogram of the refrigerant to cause it to vaporize (or condense). Also, the amount of heat energy in a gas which is in addition to that found in the liquid at the same temperature.

LAW OF CONSERVATION OF ENERGY – States that energy can neither be created nor destroyed. However, it can be stored in various forms and can be transferred from one form to another. In other words, the total energy of a body or substance always remains constant.

LIQUIDS – are substances in which the molecules are loosely bounded. A definite volume is maintained by the bond forces, but can change the shape according to the shape of the container.

MATTER – General name for all material substances, gaseous, liquid or solid forming the earth and its surrounding atmosphere.

MEAN EFFECTIVE PRESSURE – The difference between the mean forward pressure and the mean back pressure acting on a moving piston during a cycle of operation.

MECHANICAL CYCLE – is one wherein during the various processes, the properties of the working fluid change.

MECHANICAL EFFICIENCY – Ratio of mechanical energy available at the brake or flywheel to the mechanical energy put into the moving mechanism at the piston of the engine.

MECHANICAL EQUIVALENT OF HEAT – 1 kcal is equal to 1427 Mt.kg of mechanical energy.

MOLE OF GAS – Quantity of gas, the weight of which is equal to the molecular weight in pounds.

MOLAR HEAT – is the quantity of heat required to raise the temperature of one mole of a substance through one degree.

MOLLIER DIAGRAM – A graphical representation of the properties of a fluid in which enthalpy is plotted as abscissa and pressure as ordinate.

NATURAL CONVECTION – Circulation or the transfer of fluid due to a difference in density resulting from temperature changes.

NATURAL DRAFT COOLING TOWER – Cooling tower which cools water by moving air at low velocities through the tower, without the aid of any mechanical means.

NON FLOW PROCESS – It is the one in which there is no mass interaction across the system boundaries during the process. This occurs in a closed system.

NTP – The normal temperature and pressure refer to the conditions of temperature 0°C (273 K) and pressure of 760 mm of mercury.

OPEN SYSTEM – It has no closed boundary, but has one or more openings. Fluid may enter or leave the system, it undergoes thermodynamic process, while moving through the system.

PARALLEL FLOW HEAT EXCHANGER – A heat exchanger in which the warm and cool fluids flow in the same direction but separated from one another.

PERFECT GAS – Gas which behaves in accordance with the gas law PV = MRT, where P-absolute pressure, V-volume, M-mass, T-absolute temperature and R-constant for the particular gas depending on its molecular weight.

PERPETUAL MOTION MACHINE OF FIRST KIND – is a machine which will give continuous work without taking any energy from other system or surrounding.

PERPETUAL MOTION MACHINE OF SECOND KIND – is a machine which abstracts heat continuously from a reservoir and converts it completely into work or 100% efficient engine.

PERPETUAL MOTION MACHINE OF THIRD KIND – is applied to devices that, once set in motion, continue in motion for an indefinitely long time without slowing down.

PHASE OF A SUBSTANCE – It refers to a quantity of matter which is homogeneous or uniform throughout its physical structure and chemical composition.

PHASE DIAGRAM – is a pressure temperature diagram showing more than one of the saturation lines (liquid-vapour, liquid-solid, solid-vapour and other of a pure substance).

PATH FUNCTIONS – depend on which path is followed between the states 1 and 2. Examples are heat, work, internal energy and entropy.

POINT FUNCTIONS – are the properties of a system at a state.

POLYTROPIC PROCESS – follows the law Pvn = constant. In this process, heat is supplied in such a way there is some work done by the gas (i.e., during expansion) and there is also a change in internal energy.

PROCESS – It is the transformation of a system from one state to another.

PROPERTY OF A FLUID – is its characteristic such as pressure, volume, temperature, internal energy, enthalpy and entropy which are measurable directly or indirectly.

POTENTIAL ENERGY – Energy possessed by a substance by virtue of its position with respect to a datum.

POWER – The rate at which work is done, i.e., 1 HP = 4500 Mt.kg/ minute.

PRESSURE – As defined by Rankine-A force of the nature or a thrust, distributed over a surface measured as kg/sqcm.

PURE SUBSTANCE – is any material of single chemical structure or of homogeneous and invariant chemical structure.

QUALITY OF STEAM – Refers indirectly the amount of water or unevaporated moisture in steam.

QUASI-STATIC PROCESS – is a process which is carried out in such a manner that at every instant the system departs only infinitesimally from the thermodynamic equilibrium state.

R – An experimentally determined constant which is equal to the mechanical work done by the expansion of unit weight of a perfect gas at a constant pressure while heat is added to increase its temperature through one degree.

RADIATION – Continuous form of interchange of heat energy from one body to another by means of electromagnetic waves without causing a change in the temperature of the medium between the two bodies involved. These energy waves may be reflected, penetrate the material or be absorbed.

REAL GAGES – are those that deviate from the characteristic gas relation, Pv = RT.

REFLECTIVITY – Ability of a material to reflect radiant heat.

REFRIGERATOR – Maintains a body at a temperature lower than that of the surroundings while operating in a cycle. External work is supplied for realizing it.

RENAULT’S LAW – The two specific heats of a gas Cv and Cp do not change with the temperature and pressure.

REVERSIBLE PROCESS – Any process performed so that the system and all its surroundings can be restored to their initial states by performing the process in reverse.

RUDOLF CLAUSIUS STATEMENT – Heat can flow from a hot body to a cold body unaided, but it cannot flow from a cold body to a hotter body without the expenditure or supply of mechanical work.

SATURATION TEMPERATURE – Temperature of the liquid surface corresponding to the pressure of the vapour in contact with it.

SATURATED PHASE – is any phase of a substance existing under saturated conditions, wherein two or more phases of a pure substance can exist together in equilibrium.

SATURATED STEAM – Steam, as it is generated from water, just barely on the steam side of the fence.

SATURATED VAPOUR – Vapour whose temperature and pressure are in accordance with the temperature vapour pressure relationship for the particular substance. Vapour in contact with its liquid is saturated.

SECONDARY DIMENSIONS – are those quantities which are described using primary dimensions.

SEMI-PERFECT GAS – is one which follows the ideal gas relations with specific heats being functions of temperature.

SENSIBLE HEAT – Heat energy that causes a change of temperature of a solid, liquid or gas, changes the speed with which molecules move. When unit weight of a substance is heated by one degree to gain, the sensible heat is equal to the specific heat.

SENSIBLE HEAT OF WATER – Heat added to one kg of water from 0°C to the boiling point at the given pressure. Also called ENTHALPY OF WATER.

SOLIDS – are substances in which the molecules are closely packed and may not move freely. Molecules can only vibrate about a mean position. Very large force is required to change the shape of a solid substance.

SPECIFIC HEAT – The ratio of the amount of heat which will raise the temperature of a given weight of a substance by one degree to the amount of heat which will produce the same raise in temperature in the same weight of water.

SPECIFIC HEAT OF SUPERHEATED STEAM – Amount of heat required to raise the temperature of superheated steam, at constant pressure by 1°C.

SPECIFIC PROPERTY – is the value of any extensive property divided by the mass of the system. This is an intensive property, e.g., specific volume, specific weight.

SPHEROIDAL STATE – The condition of a liquid, as water, when being thrown on a highly heated metal surface, it rolls about in spheroidal drops or masses, at a temperature several degrees below ebullition and without actual contact with the heated surface. This phenomenon is due to the repulsive force of heat and the intervention of a cushion of vapour.

STATE OF A SYSTEM – refers to the unique condition of the system at any particular moment. The state is indicated by the properties such as temperature, volume, pressure, internal energy, enthalpy and entropy. These are point functions.

STEADY FLOW SYSTEM – is an open system in which the rate of mass transfer and energy transfer remain constant with respect to time.

STEAM – The vapour of water. It is a colourless expansive invisible gas.

STEAM TABLES – Tables containing values of various properties of saturated steam such as boiling point, specific volume, sensible heat, latent heat and total heat calculated for a wide range of pressures.

STEADY FLOW PROCESS – is a flow process wherein the conditions within the control volume do not vary with time.

STORED ENERGY – is the energy which is contained by the medium within the system boundaries. Examples are potential energy and internal energy.

STP – The standard temperature and pressure refer to the conditions of temperature 15°C (288 K) and pressure 760 mm of mercury.

SUBCOOLED LIQUID – is a liquid existing at a temperature lower than its saturation temperature (or, in other words, at a pressure higher than its saturation pressure).

SUPERHEAT – Addition of heat to a fluid after it has completely vaporized. In this situation, temperature increases but pressure does not.

SUPERHEATED VAPOUR – Vapour removed from contact with its liquid and at a temperature higher than that which corresponds to its pressure as indicated by the temperature – pressure vapour relationship for that particular substance.

SUPERHEATED STEAM – Steam at a temperature higher than the saturation temperature for the given pressure.

SURROUNDING – is everything outside the boundary which may influence the behaviour of the system. Transfer of mass and energy may take place between the system and boundary.

SYSTEM IN EQUILIBRIUM – means it does not undergo any change on its own accord.

SYSTEM IN CHEMICAL EQUILIBRIUM – is an isolated system which may contain substances that can react with one another chemically, but all these chemical reactions cease after a certain time.

SYSTEM IN MECHANICAL EQUILIBRIUM – is an isolated system, when left to itself, may change with time all its properties such as pressure, elastic stress etc., but these changes may cease after a certain time.

SYSTEM IN THERMAL EQUILIBRIUM – is an isolated system, wherein there may be variations of temperature from point to point, but these variations vanish after a certain time.

SYSTEM IN THERMODYNAMIC EQUILIBRIUM – is one which is in mechanical equilibrium, thermal equilibrium and chemical equilibrium.

TEMPERATURE – A measure of the intensity of heat in a substance with reference to a fixed reference point and of its ability to pass its heat into anything at a lower temperature than itself.

THERMAL CONDUCTIVITY – Ability to a material to allow the passage of heat. Stated as the number of kilocalories that can flow in one hour through a block of material, one square meter in area and one meter thick with one degree centigrade difference in temperature between the opposite surfaces.

THERMAL GRADIENT – The slope of the temperature curve produced by measuring the temperature at various points across a body or heated section.

THERMODYNAMICS – Study of energy, heat and work, properties of media employed and the processes involved.

THERMODYNAMIC SYSTEM – is any space or any matter or group of matter within a prescribed boundary, to which attention is directed for the purpose of analysis.

THERMODYNAMIC CYCLE – is said to occur when a working fluid of a system undergoes a number of operations and processes which take place in a certain order and finally return the fluid to the initial state itself.

THERMODYNAMICS – ZEROTH LAW – That two systems having equal temperatures with a third system also have equal temperatures with each other.

THERMODYNAMICS – FIRST LAW – In ordinary processes, matter can neither be created nor destroyed. Energy can take different forms but cannot be created or destroyed.

THERMODYNAMICS – SECOND LAW – There is a definite limit to the amount of mechanical energy that can be obtained from heat energy.

THERMOELECTRIC INDICATORS – Temperature measuring instruments which operate on the principle that minute quantities of electric current may be produced by heating two dissimilar metals which are joined at one end.

THERMOSTAT – Operating control which reacts to temperature.

THROTTLING PROCESS – is one that occurs when a gas or vapour is expanded through an aperture of minute dimensions, such as a slightly opened valve or a narrow throat. In this process, no heat transfer takes place, no work is done.

TOTAL HEAT – Sum of the sensible heat and latent heat. Also called enthalpy of steam or vapour.

TRANSIT ENERGY – is that energy which crosses the system boundaries. Examples are heat, work, velocity energy and electrical energy.

TRIPPLE POINT – The temperature and pressure at which a substance can exist simultaneously in solid, liquid and vapour forms.

TUBE AND FIN RADIATOR CORE – One type of radiator core, consisting of tubes to which cooling fins are attached, water flows through the tubes.

ULTRAVIOLET RADIATION – Radiation of short wave length or high frequency, just below the X-ray range, which come from the SUN and the electric arc. These may cause eye damage and skin burns unless the correct precautions are followed.

UNIVERSE – is the system and the surrounding put together.

UNSTEADY FLOW SYSTEM – is an open system in which the rate of mass transfer and energy transfer vary with respect to time.

VACUUM – A space devoid of matter, that is, a space in which the pressure is zero absolute.

VAPOUR – A gaseous condition of a substance near its point of liquefaction.

VAPOURIZATION – Change of state from liquid to vapour or gas.

VAPOUR PRESSURE – Pressure exerted by the vapour of any particular liquid on the containing vessel, as a result of the molecules near its surface freeing themselves from the attraction of their neighbours and flying off into space. Its magnitude depends solely on the temperature of the liquid surface.

VOLUMETRIC EFFICIENCY – The ratio of the volume of gas actually pumped by a compressor or pump to the theoretical displacement of the piston of the compressor or pump or reciprocating IC engine.

WEIGHT – It refers to the force exerted by gravity on the given mass. Weight is proportional to mass, but the proportionality factor is different at different locations.

WET STEAM – Steam containing intermingled moisture, mist or spray.

WORK – The overcoming of resistance through a certain distance by the expenditure of energy.

WORKING AGENT – Substance in a heat engine which alternately takes in and rejects heat, expanding and contracting when doing so, and overcoming the resistance opposing these changes.

ZEROTH LAW OF THERMODYNAMICS – States that if two systems are separately in equilibrium with a third system, then the two systems will be in thermal equilibrium with each other.