Introduction to HVAC Building Systems

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Part 1

Introduction to HVAC Building Systems

IntroductionEngineers, designers, and draftspersons are constantly working on new plans for heating,

ventilating, and air-conditioning systems in new buildings as well as plans for therenovation of existing buildings. Much of the design of such systems is accomplished by

the use of mathematics, good judgment, and drafting. Mathematical formulas are used in

calculating the heat loss and heat gain of various structures, the required volume of air,the sizes of ducts needed to carry this air, and similar details. Good judgment is gained

from experience and is used to determine the type of fuel used for a particular building

and to select the best HV AC (Heating, Ventilating, and Air Conditioning) system and

equipment. Drafting is the means of using lines, symbols, dimensions, and notations toconvey the engineer's calculations and design to the workmen on the job.

Drafting is really a language-an instrument of communication. Therefore, a potential

draftsperson must master this language so that his/her drawings will show a complete

description of the finished product and so that the trades’ workers can understand what isexpected of them.

Fig1-1 Isometric design sketch of air handling system




Building ConstructionMuch of the material in this section deals with the relationship of the HV AC

draftsperson to the building construction industry. Familiarity with this relationship isconsidered necessary to give the draftsperson a proper background for approaching

his/her work more intelligently.

Relation of the Architect to the Job

Whenever the construction of a new building is contemplated, an architectural firm is

usually commissioned to prepare complete working drawings and specifications for the

building. These construction documents usually include the architectural drawings which

show the design and building construction details-the floor plan layouts, verticalelevations of all building exteriors, various cross sections of the building, and other

details of construction. While the number of drawings will vary depending on the size

and complexity of the job, the drawings will almost always fall into the following fivegeneral groups:

• Site Work - The site or plot plan usually will include the location of the

building on the property as well as the outside utilities which will serve the building. Topography lines are sometimes included on the site plan also,

especially when the building site is on a slope.

• Architectural - The architectural drawings usually will include: elevations of

all exterior faces of the building; floor plans showing walls and partitions for eachfloor; and sufficient cross sections to indicate clearly the various floor levels and

details of the foundation, walls, floors, ceilings, and roof construction. Large-

scale detail drawings may also be included.

• Structural - Structural drawings are usually included for long-span wood-trussconstruction, and all drawings of reinforced-concrete and structural steel

construction are prepared by structural consulting engineers.

• Electrical - The electrical drawings will cover the complete design and layoutof electrical wiring systems for light, power, and communication.

• Mechanical - The mechanical drawings will cover the complete design and

layout of the plumbing, piping, heating, ventilating, and air-conditioning systems

and related mechanical construction. Electrical-control wiring diagrams for theheating and cooling systems are very often included on the mechanical drawings

also.

In general, the HVAC drawings will indicate the location of all HV AC equipment; the

supply, makeup, and return air ducts; the fans for ventilation; the size and location of

grilles, registers, and diffusers; and any other items falling under the HV AC

specifications. The drawings will also include equipment schedules and large-scale




construction details, when they are required. On very small jobs the architect may merely

show a one-line diagram of the ductwork on the architectural floor plans with therequired volume of air and British thermal units (Btu) to each area. The ductwork is

usually sized, in this case, by the mechanical contractor installing the job.

Fig. 1-2 shows an illustration of a good HVAC drawing, while Fig. 1-3 shows an HVACdrawing which is very sketchy, leaving much doubt as to what exactly is wanted.

Fig. 1-2. Illustration of a good HVAC drawing, clearly showing all details ofconstruction.

Fig. 1·3. Illustration of another HVAC drawing of only fair quality, leaving some doubtabout what is required for construction.




Relation of the Consulting Engineer to the Job

When electrical and mechanical systems in buildings became more extensive andcomplex, architects began hiring consulting engineers to design and layout these systems.

The consulting engineer usually will act as liaison between the architect and the electrical

and mechanical contractors; the consulting engineer will handle the' details of the

electrical and mechanical construction from the time the design and layout of the work isstarted, through the bidding and construction sequences; to the final approval and

acceptance of the finished job.

Although consulting firms vary in both size and types of services offered, most have an

electrical, a plumbing, and a heating, ventilating, and air-conditioning design department.

It is in the latter that most HVAC draftspersons are employed.

The HVAC Draftsperson

The HVAC designer will use the architect's drawings as a reference when designing a

suitable HVAC system for the building in question. This involves calculating the heat

gain for air conditioning, the heat loss for heating, the required air changes per hour forventilation, etc. Usually the designer's layout is then roughly sketched as shown in Fig. 1-

4 and, in turn, is given to the HVAC draftsman to complete.

The HVAC draftsperson's responsibility is to provide drawings of the design which areneat, detailed, and accurate and which will indicate, beyond any question of a doubt,

exactly what is required to install a correct HVAC system in the building. Fig. 1- 5 shows

the engineer's sketch from Fig. 1-4, after an HV AC draftsperson completed his/her work.

The units to follow are designed to provide the student with the knowledge necessary to

prepare HV AC drawings of high quality. More examples of such drawings can be found

in Appendix of this book.

Fig. 1-4. Illustration of an engineer's rough sketch before drafting.




Fig. 1·5. The same engineer's sketch after it has been neatly drawn by an HVACdraftsman.

HVAC Equipment and TerminologyThis section explains the equipment that produces or moves heat in buildings. Operating

cycles are described so that readers can understand the factors that affect heating andcooling efficiency. Strive to accomplish the following four objectives as you read this

section

• Build your knowledge and vocabulary by learning the differences between stripheat and furnaces; absorptive and compressive cooling cycles; and an "air

conditioner" and a heat pump.

• Explain the advantages of hydronic heating systems compared to forced-air, andargue in favor of water-air heat distribution in large buildings.

• Learn that refrigeration and cooling cycles do not create "cool"; instead they

remove heat from a cool place and discard heat in a warm place.

• Be able to discuss the significant factors that affect heating and cooling equipmentefficiency, and to calculate efficiency consequences of varying SEER and COP

ratings.




Heating EquipmentHeating equipment may use combustion or electrical energy. Selection is usually an

economic decision based on initial and operating cost estimates. The quantity of heat produced by heating equipment is measured in BTU (British thermal units); 1 BTU is

defined as the quantity of heat required to increase the temperature of 1 pound of water

1°F. Boilers, furnaces, and heat pumps are rated in BTUH. They are selected to meet a building's calculated peak heat loss.

Combustion Heating

Natural gas, oil, and coal are all potential fuels for building heating. See Figure1-6. When

available, natural gas-fired equipment usually offers the lowest initial cost because fuel

storage is not required. Oil fired systems are more expensive to install because theyinclude a storage tank and a more complicated burner. Coal is the most expensive

installation because of storage, handling, and ash disposal requirements. Good operating

cost projections should be developed to ensure selection of a heat source and equipmentthat will provide the lowest life cycle cost.

Fig 1-6 BTU rating for different fuels

Furnace or Boi lerFurnaces heat air, while boilers heat water. Warm-air furnaces can provide effective

heating in smaller buildings where the longest duct run extends less than 100 feet from

the furnace. Boilers are preferred for larger projects because hot water carries heat overlong distances more economically than warm air. In particular, water piping requires less

building space than duct work, and pumps are more efficient than fans in moving large

quantities of heat (see Figures 1.7).

Fig 1-7




Electric Heat

Two different types of electric heating are used in buildings. Resistance heaters deliver3,400 BTU for each kilowatt (k W) of electrical input. Heat pumps deliver more BTUs

per kW because they take extra heat from the winter environment.

Resistance (Electr ic Furnace)

Resistance heating uses electric current to heat Nichrome wire (see Figure 1.8). Familiar

small examples include hair dryers and coffee pots. In buildings, resistance heaters arecalled strip heat if they heat air, and electric boiler if they heat water. Resistance heaters

also come in the form of baseboards that are equipped with built-in thermostatic controls.

They are available from 500 watts up, in increments of about 250 watts. Standard-watt-

density baseboards are normally 250 watts (an output of about 850 BTUH) per linearfoot, while low-watt density baseboards are 140 to 235 watts (an output of about 480 to

800 BTUH) per linear foot unit. Resistance heating is usually the least expensive

equipment to install, and the most expensive to operate. Do not consider resistance

heating unless building heating requirements are minimal or electricity costs areespecially low.

Heat Pump

A heat pump is a reversible refrigeration machine that takes heat from a cool source and

delivers it to a warmer location (see Figure 1.8). Initial cost for a heat pump is higherthan strip heat and most combustion heating equipment, but a heat pump can cool a

building during summer months.

Electric heat pumps can operate as economically as combustion equipment in mild

climates because of the "free" heat they take from the outdoor environment. Likecombustion furnaces or boilers, heat pumps can be designed to warm air or heat water

depending on the size of the building served.

Fig 1-8 Electric Furnace and Heat Pump




Hydronic Heating

Four components shown in Figure 1.9 comprise a "hydronic" heating installation.Hydronic systems use water as a heat transfer fluid because water can carry BTUs more

economically than air. Pumps use less energy than fans, and piping requires less building

space than ducts.

Heating system designers choose one of three hydronic heat exchanger types:

• Convectors or radiators

• Radiant surface

• Forced-air

Convectors or radiators Forced-Air

Radiant FloorFig 1-9




Refrigeration CycleCooling equipment moves heat from cool indoor spaces to warmer outdoor locations (see

Figure 1-10). It moves heat by causing a refrigerant to evaporate and condense.Refrigerants capture a lot of heat when they evaporate, and the captured heat is released

when refrigerant vapor condenses.

The evaporating or condensing temperature of a refrigerant fluid is dependent on the

pressure acting on the fluid. Water evaporates (boils) at sea level pressure at 212°F.

However, water on a mountaintop will boil at a lower temperature, and water in a pressure cooker must be raised to a higher temperature before boiling occurs. Water can

be used as a refrigerant, but a deep vacuum is required to sufficiently lower itsevaporating temperature. Most building cooling equipment uses halocarbon refrigerant

compounds instead of water.

Fig 1-10

Air Conditioner

The section view in Figure 1.11 shows awindow mounted air conditioner. As the

indoor and outdoor temperatures indicate, it

moves heat uphill from a cool location to a

warm one. An air conditioner's efficiency isa function of indoor/outdoor temperature

difference. Greater temperature difference

means more input energy and lessefficiency, just as driving a car uphill

requires more gasoline input and yields

fewer miles per gallon.

Fig 1-11Adding resistance heat strips or using a reversible refrigeration cycle will allow the air

conditioner to heat or cool. Although just a small window unit is illustrated, therefrigeration cycle components and temperatures are similar in large equipment.




Moving Heat

The schematic drawing in Figure 1-12 showstemperatures and pressures within the refrigerant

circulating loop. Low pressure on the suction side

of the compressor permits the refrigerant to

evaporate and capture heat from 75°F indoor air.High pressure on the discharge side of the

compressor permits heat release (by refrigerant

condensation) to 95°F outdoor air. The schematicshows a window unit, but the cycle is similar for

large cooling equipment.

Fig 1-12

Ai r-Condit ioning Eff ic iency

Miles per gallon (mpg) is an index of automobile efficiency. Air-conditioner efficiency is

measured by a similar index called SEER (seasonal energy efficiency ratio). The SEERrating is an index of an air conditioner's miles per gallon. It is the number of BTUs

removed by 1 watt of electrical energy input. Air conditioners are available with SEER

ratings that range from 8 to more than 14.

An automobile's mpg changes with load; a car will get less mileage going uphill and

more mileage going downhill. An air conditioner's efficiency also depends on load.

Cooling load is the difference between heat source and heat sink temperature. Indoor airis the usual heat source, and outdoor air or cooling tower water is the usual heat sink. An

air conditioner's efficiency can be increased by increasing indoor temperature or by

lowering outdoor temperature. Either temperature change means less work for the

equipment.

Cooling Towers

Water-cooled refrigeration equipment

can achieve higher SEER ratings than

air-cooled equipment because it is possible to cool water to near the wet-

bulb temperature of outdoor air. SEER

ratings of chillers range from 15 to morethan 24. A cooling tower is an outdoor

shower that cools water by evaporation

(see Figure 1-13). Cooling towers cancool water 10° to 15°F below ambient

air temperature, and this cool water is

used to lower condensing temperature and increase SEER. Fig 1-13

Each ton (> of cooling load requires about 3 gallons per minute of cooling tower water,and about 5% of this water is lost to evaporation and drift. River, lake, or well water may

offer lower temperatures and higher SEER potential.




Heat Pumps

A heat pump is a reversiblerefrigeration cycle machine that

can cool or heat. As a cooling unit,

it takes heat from a building and

rejects it to the warm summerenvironment. As a heater, it takes

heat from a cool winter

environment and delivers it to a building. A four-way reversing

valve is used to reverse refrigerant

flow, permitting the heat pump toheat or cool (see Figure 1-14).

Fig 1-14

Like an air conditioner, heat pump efficiency depends on the temperature difference between the environment and indoor air. As the environment gets cooler, a heat pump

will deliver less heat.

Fig 1-15




Terminology

ABSOLUTE HUMIDITY - The weight of water vapor in a given amount of air. Grains per cubicfoot

ABSOLUTE PRESSURE - Pressure measured with a base of zero.

ABSOLUTE TEMPERATURE - A temperature scale expressed in degrees F0 or C0 usingabsolute zero as a base. Referred to as the Rankin or Kelvin scale.

ABSOLUTE ZERO - The temperature at which molecular activity theoretically ceases. -456.69 F0or -273.16

AIR CONDITIONING - The process of controlling the temperature, humidity, cleanliness anddistribution of the air.

AIR, STANDARD CONDITIONS - Conditions at which BTU ratings for summer air conditioning

equipment is rated. 95 F0 dry bulb, 75 F0wet bulb at the condenser inlet and 80 F0 dry bulb, 67F0 wet bulb at the evaporator inlet.

AMBIENT - Refers to the temperature surrounding a body or unit under test.

ATMOSPHERIC PRESSURE - The weight of a 1 square inch column of the earth's atmosphere. At sea level this pressure is 14.696 pounds per square inch.

BIMETAL - Two metals with different rates of expansion fastened together. When heated orcooled they will warp and can be made to open or close a switch or valve.

BOILING POINT - The temperature at which the addition of any heat will begin a change of state

from a liquid to a vapor.

BRITISH THERMAL UNIT (BTU) - The amount of heat necessary to change the temperature of 1pound of pure water 1 degree Fahrenheit (oF).

CAPILLARY TUBE - A refrigerant control consisting of a small diameter tube which controls flowby restriction. They are carefully sized by inside diameter and length for each particularapplication.

CENTIGRADE - A temperature scale with the freezing point of water 00 and the boiling point1000 at sea level.

CHECK VALVE - A valve designed to permit flow in one direction only.

COMPRESSION - The reduction of volume of a vapor or gas by mechanical means.

COMPRESSION RATIO - The ratio determined by dividing the discharge pressure, in PSI, by thesuction pressure in PSI.

COMPRESSOR - A mechanical device used to compress gases. Three main types are:reciprocating, centrifugal and rotary.




CONDENSATION POINT - The temperature at which the removal of any heat will begin a changeof state from a vapor to a liquid.

CONDENSING MEDIUM - The substance, usually air or water, to which the heat in a condenseris transferred.

CONDENSING UNIT - The portion of a refrigeration system where the compression andcondensation of refrigerant is accomplished. Sometimes referred to as the 'high side'.

CONDUCTION - The transfer of heat from molecule to molecule within a substance.

CONTACTOR - An electromagnetic actuated relay. Usually used to refer to the relay whichcloses the circuit to a compressor.

CONVECTION - The transfer of heat by a moving fluid.

COOLING ANTICIPATOR - A resistance heater (usually not adjustable) in parallel with thecooling circuit. It is 'on' when the current is 'off", adding heat to shorten the off cycle.

COP - Ratio of work performed or accomplished as compared to the energy used.

CRANKCASE HEATER - Electric heating element that is used to heat the compressor crankcaseto prevent migrating refrigerant from condensing and diluting the crankcase oil during the off-cycle.

CUBIC FEET PER MINUTE - A common means of assigning quantitative values to volumes of airin transit, usually abbreviated CFM.

CYCLE - The complete course of operation of a refrigerant back to a selected starting point in asystem. Also used to describe alternating current through 360 space degrees.

DENSITY - Mass or weight per unit of volume. For example, standard air = .075 pounds per cubicfoot.

DISCHARGE LINE - A tube used to convey the compressed refrigerant vapor from thecompressor to the condenser inlet.

DISCHARGE PRESSURE - The pressure read at the compressor outlet. Also called headpressure or high side pressure.

DRY AIR - Air which contains no moisture vapor.

DRY BULB TEMPERATURE - Temperature read with an ordinary thermometer.

EFFECTIVE TEMPERATURE - An arbitrary concept which combines into a single value theeffect of temperature, humidity, and air movement as sensed by the human body.

ENTHALPY - Total amount of heat in one pound of a substance calculated from acceptedtemperature base, expressed in BTU's per pound mass.

EQUIVALENT LENGTH - That length of straight tubing which has the same pressure drop as thefitting, valve or accessory (of the same nominal size) being considered.




EVAPORATIVE COOLING - The cooling effect of vaporization of a liquid in a moving air stream.

EVAPORATOR - A device in which a liquid refrigerant is vaporized. Some superheating usuallytakes place.

EVAPORATOR SUPERHEAT - The actual temperature of the refrigerant vapor at the evaporator

exit as compared to the saturated vapor temperature indicated by the suction pressure.

EXTERNAL STATIC PRESSURE - The sum of the static and velocity pressures of a moving airsystem at the point of measurement.

FAHRENHEIT - A temperature scale with the freezing point of water 320 F and the boiling point2120 F at sea level.

FEET PER MINUTE - A term assigned to a velocity of a moving air stream, usually express FPM.

FILTER-DRIER - A device that removes moisture, acid and foreign matter from the refrigerant.

FLASH GAS - Instantaneous evaporation of some liquid refrigerant at the metering device due topressure drop which cools the remaining liquid refrigerant to desired evaporation temperature.

FREEZING POINT - The temperature at which the removal of any heat will begin a change ofstate from a liquid to a solid.

GAUGE PRESSURE - Pressure measured with atmospheric pressure as a base.

HEAT - A form of energy causing the agitation of molecules within a substance.

HEAT EXCHANGER - A device for the transfer of heat energy from the source to the conveyingmedium.

HEAT FLOW - Heat flows from a warmer to a cooler substance. The rate depends upon thetemperature difference, the area exposed and the type of material.

HEAT OF COMPRESSION - The heat added to a vapor by the work done on it duringcompression.

HEAT OF THE LIQUID - The increase in total heat (Enthalpy) per pound of a saturated liquid asits temperature is increased above a chosen base temperature. (Usually - 400F for refrigerants).It is expressed in BTU's.

HEAT TRANSFER - The three methods of heat transfer are conduction, convection and radiation.

INCHES OF MERCURY - Atmospheric pressure is equal to 29.92 inches of mercury.

LATENT HEAT - Heat that produces a change of state without a change in temperature; i.e., iceto water at 32 F0 or water to steam at 212 F0 .

LATENT HEAT OF CONDENSATION - The amount of heat energy in BTU's that must beremoved to change the state of one pound of a vapor to one pound of liquid at the sametemperature.




LATENT HEAT OF FUSION - The amount of heat energy, in BTU's required to change the stateof one pound of a liquid to one pound of solid at the same temperature.

LATENT HEAT OF MELTING - The amount of heat energy, in BTU'S, that must be removed tochange the state of one pound of solid to one pound of vapor at the same temperature.

LATENT HEAT OF VAPORIZATION - The amount of heat energy in BTU's required to changethe state of one pound of a liquid to one pound of vapor at the same temperature.

LIFT - To elevate a fluid from one level to a higher level.

LIQUID LINE - A tube used to convey the liquid refrigerant from the condenser outlet to therefrigerant control device of the evaporator.

MANOMETER - A tube filled with a liquid used to measure pressures.

MBH - One MBH is equivalent to 1,000 BTU's per hour.

MEAN TEMPERATURE DIFFERENCES - The mean of difference between the temperature of afluid receiving and a fluid yielding heat.

MELTING POINT - The temperature at which the addition of any heat will begin a change of statefrom a solid to a liquid.

MERCURY MANOMETER - Used to measure vacuum in inches of mercury.

MICRON - A unit used to measure high vacuums. One micron equals 1/25,400 of one inchmercury.

MUFFLER - Device installed in hot gas line to silence discharge surges.

OIL SEPARATOR - A device for separating out oil entrained in the discharge gas from thecompressor and returning it to the crankcase.

PARTIAL PRESSURE - The pressure exerted by any individual gas in a mixture.

PITCH - The slope of a pipe line for the purpose of improving drainage.

PITOT TUBE - A device comprising a small diameter orifice projecting directly into an air streammeasuring total pressure and surrounded by an annular section with small diameter entrancesnormal to the flow, measuring static pressure; both sections are usually connected to amanometer to indicate velocity pressure.

PRECHARGED LINES - Refrigerant line's which are filled with refrigerant and are sealed at bothends. The seals are broken when the lines are installed and the line charge becomes part of thetotal system charge.

PRESSURE DROP - The decrease in pressure due to friction of a fluid or vapor as it passesthrough a tube or duct or/and lift.




PRESSURE - TEMPERATURE RELATIONSHIP - The change effected in temperature whenpressure is changed or vice versa. Only used at saturated conditions. An increase in pressureresults in a temperature increase. A decrease in temperature results in a pressure decrease.

PUMPDOWN - Process of pumping refrigerant out of the evaporator and suction line at the end ofthe on- cycle by closing a solenoid valve in the liquid line and letting the compressor shut-off by

the low pressure control.

PSYCHROMETER - A devices having both a dry and wet bulb thermometer. It is used todetermine the relative humidity in a conditioned space. Most have an indexed scale to allow directconversion from the temperature readings to the percentage of relative humidity.

PSYCHROMETRIC CHART - A chart on which can be found the properties of air under varyingconditions of temperature, water vapor content, volume, etc.

QUICK CONNECT - Name given to the end connections on precharged lines which screw on tomated fittings of the outdoor and indoor sections. Tightening the quick connections ruptures theseals on the fittings and the line charge becomes part of the total system charge.

RADIATION - The transfer of heat without an intervening medium. It is absorbed on contact witha solid surface.

RECEIVER - A vessel for holding refrigerant liquefied by the condenser.

REFRIGERANT - A substance which produces a refrigerating effect while expanding orvaporizing.

REFRIGERANT CONTROL - A device used to meter the amount of refrigerant to an evaporator.It also serves as a dividing point between the high and low pressure sides of the system.

REFRIGERANT DISTRIBUTOR - A device which meters equal quantities of refrigerant to

independent circuits in the evaporator coil.

REFRIGERANT MIGRATION - The movement of refrigerant through the system to thecompressor crankcase during the off-cycle, caused by its attraction to oil.

REFRIGERANT OPERATING CHARGE - The total amount of refrigerant required by a a systemfor correct operation.

REFRIGERANT VELOCITY - The rate at which refrigerant is moving at a given point in a system,usually given in feet per minute (FPM).

REFRIGERATION - The transfer of heat from a place where it is not wanted to a place where itspresence is not desirable.

REFRIGERATION EFFECT - The amount of heat a given quantity of refrigerant will absorb inchanging from a liquid to a vapor at a given evaporating pressure.

RELATIVE HUMIDITY - The percentage of water vapor present in a given quantity air comparedto the amount it can hold at its temperature.




RELAY - A device used to open and close an electrical circuit. The relay may may be actuated bya bimetal electrically heated strip, a rod wrapped with a fine resistance wire causing expansionwhen energized, a bellows actuated by expansion of a fluid or gas or an electromagnetic coil.

REVERSING VALVE - A device in a heat pump that is electrically controlled to reverse the flow ofrefrigerant as the system is switched from cooling to heating; also called a four-way valve.

RISER - A vertical tube or pipe which carries refrigerant in any form from a lower to a higher level.

SATURATED VAPOR - Vapor in contact with a liquid.

SATURATION - A condition of stable equilibrium of a vapor and a liquid. SENSIBLE HEATHeat that can be measured or felt. Sensible heat always causes a temperature rise.

SIGHT GLASS - A glass installed in the liquid line permitting visual inspection of the liquidrefrigerant for the purpose of detecting vapor in the liquid. They also generally have a deviceincluded to monitor moisture content of the refrigerant.

SLUGGING - A condition in which a quantity of l iquid enters the compressor causing hammeringand possible compressor damage.

SPECIFIC HEAT - The amount of heat necessary to change the temperature of one pound of asubstance 10 F.

SPECIFIC VOLUME - The volume of a substance per unit of mass; i.e., standard air 13.33 cubicfeet per pound. The reciprocal of density.

STANDARD AIR DENSITY - .075 pounds per cubic foot. Equivalent to dry air at 700 F and at sealevel pressure.

STATE CONDITION - Substances can exist in three states - solid, liquid or vapor.

STATIC PRESSURE - The normal force per unit area at a small hole in the wall of a duct.

STATIC TAP - A means by which static pressures of a duct system may be read directly, usuallyconsisting of a small diameter hole in the side of the duct connected to a manometer.

SUB COOLING - Cooling of a liquid, at a constant pressure, below the point at which it wascondensed.

SUBLIMATION - A condition where a substance changes from a solid to a gas without becominga liquid.

SUCTION LINE - A tube used to convey the refrigerant vapor from the evaporator outlet to thesuction inlet of compressor.

SUCTION LINE ACCUMULATOR - A device located in the suction line that intercepts quantitiesof a liquid refrigerant and thereby prevents damage to the compressor.

SUPERHEAT - Heat added to a vapor after all liquid has been vaporized.

TEMPERATURE - A measurement of heat intensity.




THERMISTOR - Basically a semiconductor which has electrical resistance that varies inverselywith temperature.

THERMOSTAT - A bimetal actuated switch to close and open a circuit to indicate or terminateoperation of a heating or air conditioning system.

THERMOSTATIC EXPANSION VALVE - Refrigerant control which monitors the flow rateaccording to the superheat at the evaporator outlet.

THERMOSTAT SUBBASE - When installed with a thermostat it permits selection of function forheating, cooling, automatic changeover and blower cycling or continuous operation.

TON OF REFRIGERATION - The amount of heat necessary to completely melt one ton of 320Fice in 24 hours. 200 BTUs per minute, 12,000 BTUs per hour, 288,000 BTUs in 24 hours. This isbased on the latent heat of fusion for ice which is 144 BTUs per pound.

TOTAL HEAT (Enthalpy) - Total heat energy in a substance. The sum of sensible and latent heat.

TOTAL PRESSURE - The sum of all partial pressures in a mixture of gases.

TRAP - A depression or dip in refrigerant piping in which oil will collect. A trap may be placed atthe base of a suction or hot gas riser to improve oil return up the riser.

VACUUM - Any pressure below atmospheric pressure.

VAPOR BARRIER - The term applied to an impervious layer of material superimposed upon alayer of insulation. Vapor barriers are always applied on the warm side of the insulation layer.

VAPOR PRESSURE - The pressure exerted by vapor.

VELOCITY PRESSURE - In a moving fluid, the pressure capable of causing an equivalent

velocity as applied to move the same fluid through an orifice such that all pressure energyexpanded is converted into kinetic energy.

WATER MANOMETER - Used to measure pressure in inches of water.

WET BULB TEMPERATURE - Temperature read with a thermometer whose bulb is encased in awetted wick.

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