Houston HVAC Dictionary


Acidize	When acid is used to remove mineral and iron deposits (scale) to restore heat transfer efficiency.
Air Balance Test	A test used to measure airflow across a tower or air handler system.
Air Balancing Hood	Measures airflow from grilles and diffusers.
Amp (Ampere)	The practical meter-kilogram-second unit of electric current that is equivalent to a flow of one coulomb per second or to the steady current produced by one volt applied across a resistance of one ohm.
Amp/Ohm/Volt Meter	Measures motor amperage and voltage test controls.
ARI	Air Conditioning and Refrigeration Institute

B
Baseline Data	Baseline (or historical) data is compiling past chiller logs for review.
Basin	Cold water reservoir in the cooling tower system.
Basin Temperature	The actual temperature of the cold water basin in the cooling tower system.
Bearing	A support or guide by means of which a moving part such as a shaft or axle is positioned with respect to the other parts of a mechanism.
Bearing Oil Reservoir	Excess oil used in the lubrication of the bearing system located in the compressor system.
Biocide	A substance (as DDT) that is destructive to many different organisms.
Blockage	Blockage is obstruction of flow, i.e., valve, debris, etc.
Boroscope	A fiber-optic camera or lens, used to inspect tube bundles or piping.
BTU	British thermal unit. The amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit.
Bypass Valve	A valve used to detour flow, i.e., to detour water from the hot deck to the tower basin to control tower basin temperature.

C
Carryover	The movement of a fluid from one area to another. Carryover of this liquid may contaminate another liquid, causing undesired results.
Chilled Water	Chilled water is the water in the closed loop (closed to the atmosphere) or evaporator system of the chiller.
Chiller (Centrifugal Chiller)	A heat exchanger using air, refrigerant, water and evaporation to transfer heat (BTUs) to produce air conditioning (measured in tons). A chiller is comprised of an evaporator, condenser and compressor system.
Chiller Barrel Passes	Many chillers are designed with more than one pass to maximize heat transfer. Baffels and gaskets are used to separate each pass to ensure design flow through the chiller barrel.
Chiller Design Tonnage	The design capacity of the chiller measured in tons produced in an hour.
Chiller Efficiency	Chiller efficiency is measured in Kilowatts per ton (Kw/Ton) of cooling produced. The higher the Kw/Ton, the lower the heat transfer efficiency and the higher the cost to produce a ton of cooling.
Chiller Log Sheet	A sheet on which specific chiller readings are documented.
Chiller Manufacturer	The company that built the chiller, i.e., York, Trane, Carrier, McQuay, etc. The manufacturer name is typically prominently displayed on the chiller control panel or technical documentation.
Chiller Stall	A chiller stalls when the refrigerant is no longer moving through the compressor and there is no cooling effect. All shaft work is being converted into heat in the compressor that may lead to permanent damage.
Chiller Startup	The chiller manufacturer’s recommendation on startup including pre-heating the compressor oil temperature prior to chiller startup.
Chiller Surge	When the refrigerant flows backwards through the compressor wheel every few seconds until the pressure builds up and the refrigerant moves forward again. This is even more dangerous than a stall because it reverses loads of thrust bearings in the compressor shaft.
Chiller Vein (Inlet Guide Veins)	Chiller veins are used to control the capacity of the compressor. As the inlet guide veins start to close, they change the gas entry angle to the impeller and reduce gas flow and compressor capacity. As the vanes near the closed position, they throttle the refrigerant flow.
Closed Loop	The evaporator side of the chiller system, closed to the atmosphere.
Compressor	A mechanical device where the refrigerant is compressed from a lower pressure and lower temperature to a higher pressure and higher temperature. The compressor maintains a low pressure in the evaporator by continually removing refrigerant vapors. This low pressure, low temperature vapor is then compressed into a higher pressurize, hot refrigerant vapor which leaves the compressor and travels to the condenser. The motor in the compressor is the main consumer of energy in the chiller system. The energy used by the compressor is dependent on the pressure increase. The head pressure divided by the suction pressure or the condenser pressure divided by the evaporator pressure expresses this. If the compressor operates out of its original design, it will effect the energy consumption.
Compressor Erosion/Corrosion	Damage to the bearings and impeller caused by poor lubrication, low oil levels or operating conditions such as liquid refrigerant carryover into the compressor system.
Compressor Motor Amps	The energy used to power the refrigerant cycle in a chiller system. The input energy is dependent upon the pressure increase in the compressor. The head pressure divided by the suction pressure or the condenser pressure divided by the evaporator pressure determines it.
Compressor Oil Temperature	Compressor oil temperature is the temperature of the oil in the compressor bearing lubrication system. This temperature is predetermined by manufacturers specifications.
Condensation	The process a vapor goes through to change phase from vapor to a liquid.
Condenser	Heat exchanger where the system heat is rejected and the refrigerant condenses into a liquid. The condenser is where hot pressurized refrigerant vapors in the compressor are cooled and liquefied by cooling tower water circulating through the tubes of the condenser. The condenser side is commonly referred to as the open recirculation system or open loop.
Condenser Delta P	The pressure drop between the suction side and the discharge side of a chiller barrel. This pressure drop can be measured against known values from tables/charts to determine water flow through the chiller barrel. Delta P is dependant on chiller tube ID and tube length.
Condenser Delta T	The temperature difference between the entering and leaving water through the chiller barrel. i.e., the entering temperature is 85°F and the leaving temperature is 95°F then the delta T would be 10.
Condenser Water GPM (Gallons Per Minute)	The velocity of the condenser water measured in gallons per minute. All chiller condensers have a manufacturers design GPM flow rate.
Condenser Pressure	The head pressure produced in the condenser dependent on the entering condenser water temperature and the saturated refrigerant temperature. The condenser pressure can affect the refrigerant cycle and heat transfer. The pressure correlates to saturated refrigerant temperature on a pressure/temperature chart for each refrigerant type.
Condenser Refrigerant Approach Temperature	The refrigerant temperature minus the leaving water temperature. The refrigerant temperature can be determined by locating the condenser (head) pressure on a pressure/temperature chart, or if the chiller control panel provides this information.
Condenser Water	The open recirculating system connects the chiller condenser to the cooling tower system and the atmosphere. Condenser water is subject to evaporation, debris/contamination, fouling/scaling and microbio growth.
Conditioned Fluid	The fluid being acted upon by the working fluid. Example: in an air conditioning system the air is conditioned by the refrigerant (refrigerant is the working fluid).
Constant Speed Drive	Constant speed drives are the most common type of chiller compressor motor. They have limited ability to adjust speed based on part load conditions.
Contamination	Any foreign object or substance not normally found in a specific location.
Cooling Tower	Where the process of heat exchange occurs by evaporation in the open recirculating system. The cooling tower system includes tower fans, plenum, drift eliminators, fill, levelers, hot deck and cold basin. All cooling towers have a design rated delta temperature.
Cooling Tower Approach Temperature	The difference between the leaving water temperature and the entering air wet bulb.
Cooling Tower Fan	Fan system designed to remove the heat generated by the open recirculating system through evaporation.
Cooling Tower Fill	Designed to brake up the circulating water into smaller particles or a thin film. This increases the surface area of the water to enhance evaporation and heat removal.
Cooling Tower/Condenser System	The cooling tower system includes tower fans, plenum, drift eliminators, fill, levelers, hot deck and cold basin combined with the condenser barrel and piping making up the open loop or open recirculating system.
Corrosion	The decay and loss of a metal due to a chemical reaction between the metal and its environment. It is a transformation process in which the metal passes from its elemental form to a combined (or compound) form.
Cost of Blowdown	The total cost of water sent to drain/sewer, typically determined on a per 1,000 gallon basis. This value can be found on the water and sewer utility bill or from the utility company.
Cost of Kw	The cost of electricity charged by the electric and is typically $0.04 to $0.15 per kilowatt depending on geographic location.
Cost of Makeup	The total cost of water added to the condenser/cooling tower system, typically determined on a per 1,000 gallon basis. This value can be found on the water and sewer utility bill or from the utility company.
CPLV – Calculated Part Load Value	The calculated Kw/Ton derived from the effect of part load and entering condenser water temperature on the chiller when compared to full load design.

D
Dead Leg	A static condition that exists over a period of time in equipment or system that has no water flow. This condition is usually associated with lay-up and can promote the growth of microbes and corrosion.
Debris	Anything the environment can introduce that is washed out by the tower operations, i.e., dirt, trash, chip scale “flash corrosion”, bugs, broken tower fill, broken tower wood, plastic, etc.
Delta P	The pressure drop between the discharge side and the suction side of a chiller barrel. This pressure drop can be measured against design values provided by the chiller manufacturer or previously developed charts by plant personnel to determine water flow through the chiller barrel.
Delta T	The temperature difference between the entering and leaving water through the chiller barrel, i.e., entering temperature is 85°F, leaving temperature is 95°F = delta T10°F. If the actual water flow is known, delta P could determine a problem if the actual delta P does not match the current water flow.
Design Amps (Full Load)	The maximum amp load on the chiller. This information can be found in the chiller technical documentation.
Design Condenser Delta T	The temperature difference between the entering and leaving water through the chiller barrel when the chiller is running at full load, i.e., the entering temperature is 85°F and the leaving temperature is 95°F then the delta T would be 10. Modern high efficiency chillers are designed to run at 9.4°F delta T at 3gpm/ton.
Design Condenser Refrigerant Approach Temperature	The difference between the condenser water out temperature and the condenser leaving refrigerant temperature at design full load. This information can be found in the chiller technical documentation.
Design Condenser Water GPM	Below is a list of design GPM ratings based on the chiller design delta T. - Entering Water Temperature: 85°F - Delta T Range: 10-20°F - Normal delta T: 10°F - 3.0 GPM/Ton @ 10°F delta T - 2.5 GPM/Ton @ 12°F delta T - 2.0 GPM/Ton @ 15°F delta T - 1.5 GPM/Ton @ 20°F delta T - 5,000 Btuh/GPM @ 10°F delta T - 6,000 Btuh/GPM @ 12° F delta T - 7,500 Btuh/GPM @ 15°F delta T - 10,000 Btuh/GPM @ 20°F delta T - ARI Condenser Fouling Factor: 0.00025 Btu/Hr.Ft2.°F
Design Entering Condenser Water Temperature	The design entering condenser water temperature is typically 85°F based on ARI standards for chillers commissioned after 1989. This information can be found in the chiller technical documentation.
Design Evaporator Chill Water GPM	Below is a list of design GPM ratings based on the chiller design delta T. - Leaving Water Temperature: 42-46°F - 10-20°F delta T - 2.4 GPM/Ton @ 10°F delta T - 2.0 GPM/Ton @ 12°F delta T - 1.5 GPM/Ton @ 16°F delta T - 1.2 GPM/Ton @ 20°F delta T - 5,000 Btuh/GPM @ 10°F delta T - 6,000 Btuh/GPM @ 12°F delta T - 8,000 Btuh/GPM @ 16°F delta T - 10,000 Btuh/GPM @ 20°F delta T - ARI Evaporator Fouling Factor: 0.00010 Btu/Hr.Ft2.°F - Chilled Water Flow Range: Chiller Design Flow ±10% - Chiller Tube Velocity for Variable Flow Chilled Water: - Minimum Flow: 3.0 FPS - Maximum Flow: 12.0 FPS
Design Evaporator Delta T	The temperature difference between the entering and leaving water through the chiller barrel when the chiller is running at full load, i.e., the entering temperature is 54°F and the leaving temperature is 42°F then the delta T would be 12.
Design Evaporator Refrigerant Approach Temperature	The difference between the evaporator chill water out temperature and the evaporator leaving refrigerant temperature at design full load. This information can be found in the chiller technical documentation.
Design Full Load	Design refers to full load conditions. Full load is a chiller running at 100% load capacity, 85°F ECWT, 42-46°F leaving chill water temperature and is the rating of the manufacturer.
Design Full Load Amps	The maximum amp load on the chiller. This information can be found in the chiller technical documentation.
Design Kw/Ton	The Kw used to produce one ton of cooling when the chiller is running at full load design (ex: 0.6). This information can be found in the chiller technical documentation.
Design Specifications	Manufacturer tested specification when determining design to actual operations. This includes the chiller, cooling tower, air handlers, etc. to ensure expected performance of the equipment.
Digital Manometer	Measures positive and negative air pressures in ducts, from room to room, and for taking traverses.
Distribution Holes	Holes in the hot deck of a cooling tower designed to evenly distribute the water flow over the tower fill below.
Drift	Entrained water droplets leaving the tower system. If the drift is severe, caused by missing or damaged drift eliminators, it can increase corrosion to the fan components.
Drift Eliminator	A device that removes entrained water droplets (drift) from air leaving the tower system.
Dry Bulb	The ambient outside temperature.

E
ECWT	Entering Condenser Water Temperature
Emulsification	The entrainment of one substance into another, i.e., oil in water or oil in refrigerant.
Energy Balance	Energy cannot be created or destroyed during heat transfer; therefore the amount of energy leaving the source must equal the amount reaching the sink.
Entering Condenser Water Temperature	Entering condenser water temperature is the temperature of water entering the condenser.
Enthalpy	The quantity of internal energy of a body plus the product of its volume and pressure.
Erosion	The group of natural processes, including weathering, dissolution, abrasion, corrosion, and transportation, by which material is worn away.
Evaporation	The process a liquid goes through to change phase from a liquid to a vapor.
Evaporator	Heat exchanger where the system heat is absorbed and the refrigerant evaporates into a gas. By continually pulling refrigerant vapor out of the evaporator headspace, low pressure can be maintained causing the refrigerant to evaporate rapidly. Evaporation cools the refrigerant. This cold refrigerant produces chilled water by heat transfer. The evaporator side is commonly referred to as the closed loop system (chill water).
Evaporator Chill Water GPM (Gallons per minute)	The gallons of chill water going through the evaporator per minute.
Evaporator Delta P	The pressure drop between the suction side and the discharge side of a chiller barrel. This pressure drop is measured against known values to determine the water flow GPM through the chiller barrel. Delta P is dependant on chiller tube ID and tube length.
Evaporator Delta T	The temperature difference between the entering and leaving water through the chiller barrel. i.e., the entering temperature is 54°F and the leaving temperature is 42°F then the delta T would be 12.
Evaporator Fouled and/or Scaled	A film or compound, which attaches to the internal tube surface, impeding heat transfer and lowering efficiency.
Evaporator Refrigerant Approach Temperature	The leaving water temperature minus the refrigerant temperature. The refrigerant temperature can be determined by locating the evaporator (suction) pressure on a pressure/temperature chart, or if the chiller control panel provides this information.
Evaporator Leaving Refrigerant Temperature	The temperature of the saturated leaving refrigerant is at prior to leaving the evaporator in the refrigerant cycle.
Evaporator Pressure	A vacuum measured in inches of Hg (mercury) for low-pressure chillers and a positive pressure (psig) for high-pressure chillers. This vacuum/pressure correlates to refrigerant temperature on a pressure/temperature chart for each refrigerant type.
Evaporator Refrigerant Level	The level of the liquid refrigerant in the evaporator barrel.

F
Fan	A device for producing a current of air in the cooling tower used to remove heat from the condenser tower system.
Flow Rate	The quantity of fluid in motion per a unit of time. Flow rate is expressed in mass per unit time or volume per unit time.
Flute	Allows the transfer of liquid refrigerant from the condenser back to the evaporator.
Flute Frosting	A problem associated with rapid movement of liquid refrigerant at low temperatures from the condenser to the evaporator, causing frost to form on the flute.
Fouled Tubes	Debris or substance which impedes flow or heat transfer. Fouling can be caused by trash, chip scale “flash corrosion”, microbio, etc.
Free Oil	“Free” or floating oil on top of a liquid with a higher specific gravity.
Full Load Design	Design refers to full load conditions. Full load is a chiller running at 100% load capacity, 85°F ECWT, 42-46°F leaving chill water temperature and is the rating of the manufacturer.

G
Gasket	Any of a wide variety of seals or packings used between matched machine parts or around pipe joints to prevent the escape of a gas or fluid.
Gauge	An instrument or device for measuring, indicating or comparing a physical characteristic, i.e., temperature or pressure.
GPM	Gallons Per Minute

H
Heat	When energy is added to an object or location, the motion of the molecules increases causing them to collide more frequently. The motion energy is converted into heat during the collisions. As more collisions occur more heat is released and the temperature of the object or location increases.
Heat Index	The heat index is the combination of actual outside air temperature plus the affect of relative humidity to give an apparent temperature.
Heat Exchanger	Device that provides a practical means for the working fluid to heat or cool the conditioned fluid efficiently with out the two fluids mixing.
Heat of Rejection (Heat of Expansion)	The amount of heat rejected by the refrigerant in the condenser, which includes compressor heat.
Heat Sink	Object or location that is at a lower temperature than the heat source and receives the energy from the source.
Heat Source	Object or location that is at a higher temperature than other objects or locations.
Heat Transfer	The movement of energy as heat moving from a heat source to a heat sink.
Heat Transfer Coefficient	A proportionality constant in the heat transfer rate equation derived from the conditions of the fluid motion, the tube or fin surface geometry, and other thermodynamic properties.
Heat Transfer Efficiency	The ability of heat to transfer from one substance to another.
Heat Transfer Fluid	Any gas or liquid used by heat exchangers to transfer heat.
Heat Transfer Rate	Amount of energy that is moved from the heat source to the sink per an amount of time. Usually stated as Btu/hr in the English measurement system and Watts (Joules per second) in the Metric system.
High Pressure Chiller	A chiller that operates in a positive pressure for both the evaporator and condenser (measured in psig). Some high-pressure refrigerants include R-12, R-22, R-134a and R500.
Hot Deck	The top deck of the cooling tower where the condenser water returns to the tower prior to be cooled by evaporation, also referred to as the hot basin.
Hot Deck Distribution Holes	Holes in the hot deck that distribute the flow of returning condenser water evenly across the tower fill.
Hot Gas Piping (Piping)	A means of recirculating hot discharge refrigerant back into the evaporator. The refrigerant must pass through a pressure-reducing device (hot gas bypass valve). The purpose of hot gas bypass is to maintain a minimum gas volume flow rate through the compressor to avoid surging or stalling during low load conditions. A disadvantage is that the work of compression on the recirculated refrigerant does not generate any refrigeration effect.
Hot Wire Anemometer	Measures air velocity in grilles, filters, coils and ducts.

I
IPLV – Integrated Part Load Value	A single number, part-load efficiency indicator calculated using the ARI method at standard rating conditions. Introduced in ARI Standard 550-1986, the definition of IPLV was changed in ARI Standard 550/590-1998 to more closely reflect actual operating experience found in the field for a single chiller.

K
Kw/ton	A kilowatt (Kw) is a measure of electrical energy. A ton is a measure of cooling and is defined as 12,000 BTU of cooling per hour.

L
Laminar Flow	Smooth undisturbed flow of a fluid.
Latent Heat	Heat given off or absorbed during phase change (condensation, evaporation, solidification, melting, or sublimation).
Lay-up	The process of winterizing a tower or condenser system when not in use.
Legionella	A bacterium of the genus legionella, especially pneumophila, that can cause Legionnaires’ disease - an acute, sometimes fatal respiratory disease caused by and characterized by severe pneumonia, headache, and a dry cough.
Liquid Refrigerant	The compressor coolant used in the refrigerant cycle of a chiller for heat transfer.
Liquid Refrigerant Stacking	This condition occurs can be caused by mechanical failure or low head pressure in the condenser and evaporator caused by low condenser water temperature.
Liquid Piping	Refrigerant piping from the condenser outlet to the evaporator inlet.
Load	Amp load is compared to full load and the percentage value equals the load of the chiller. Actual amps divided by full load amps is the percentage of full load design.
Load Swing	A large or radical change in cooling load requirements, i.e., weather or building operations requirements.
Log Mean Temperature Difference	A specialized average temperature difference used to determine the heat transfer rate.
Low Pressure Chiller	A chiller that operates the evaporator in a vacuum, measured in inches of Hg (mercury). In some cases relating to entering condenser water temperature, the condenser may also operate in a vacuum. Some low-pressure refrigerants include R-11, R-113, R-114, R-123.

M
Maintenance Practices	An organized schedule of chiller and plant maintenance.
Microbe	An organism of microscopic or ultramicroscopic size.

N
Non-Condensable Gasses (Air)	Air that enters the evaporator through a vacuum leak and migrates to the condenser (in low-pressure chillers only). This affects the condenser head pressure, condenser refrigerator approach temperature and condenser heat transfer efficiency.
NPLV – Non-standard Part Load Value	A single number, part-load efficiency indicator calculated using the ARI method referenced to rating conditions other than ARI standard. The 1998 standard adopted NPLV for situations when a single chiller is not intended to operate at standard ARI rating conditions.

O
Oil	Used as the lubricant in the compressor system to lubricate and protect bearings, shaft, etc.
Oil Analysis	Tests used to determine impurities and the ability of the oil to lubricate.
Oil Change Interval	The length of time between oil changes, determined by hours of operation, oil analysis, etc.
Oil Entrainment	Entrainment is another term for flow. This term is primarily used to reference the flow of oil through the system to the return.
Open Loop	The condenser/tower side of the chiller system, open to the atmosphere.
Operating Conditions	The values of temperature, flow rate, and pressure of the heat transfer fluids as they enter and leave the heat exchanger. Used to determine the heat transfer rate for the heat exchanger.

P
Part Load	Chiller load conditions below full load design. Most chillers operate at part load ~99% of the time.
Plate Exchanger	A heat exchanger used to create free cooling (not using a compressor or refrigerant to transfer heat) by running colder tower water over stainless steel plates which transfers heat between the closed loop to the open loop.
Power Factor	The ratio of actual power (Kw) to apparent power (kVA). Most centrifugal motors have a power factor between 0.87 and 0.91. Additional capacitors can be added to raise the power factor to a practical limit of 0.95. In the table below, the closer the length of the kVA line is the the kW line the more efficient the user of the energy.
Pressure Drop	Amount of reduction in the pressure of a fluid between the entering and leaving pressures. The pressure drop in a chiller is dependant upon the tube ID of the tube bundle and its length. Rough surfaces or objects that are in the flow path can cause reduced pressure.
Pressure Gauge	An instrument that measures pressure in psig, psid or psia depending on the circumstances.
Primary Surface Area	The area that contains the working heat transfer fluid. Usually the tubes of a finned/tube heat exchanger.
Pump	A device used to circulate fluid from one location to another.
Pump Curve	The design capacity of a pump’s ability to circulate fluid.
Pump Curve Calibration	A flow test to determine the capacity of a pump’s ability to circulate fluid.
Pressure	The application of force to something by something else in direct contact with it.
Pressure Drop	The reduction in pressure between a fluid entering and leaving a closed system.
Pretreatment	The removal of oil and grease from new piping and chillers to ensure maximum heat transfer. It should also lay down a passivating film to prevent flash corrosion and in some cases, white rust.
Pumpout System	Pumpout systems consist of a storage tank large enough to hold the chillers entire refrigerant charge and a refrigerant pump/compressor to move the refrigerant from the chiller to the pumpout tank and back again. It’s primary purpose is for servicing the chiller.
Purge Unit	Removes non-condensable gasses (air) from the condenser barrel of the chiller. Required on all low-pressure chillers only.

Q
Quality	The ratio of vapor mass to total mass of a substance at the substance's saturation temperature and pressure.

R
RAT	Refrigerant Approach Temperature
Refrigerant	The mechanism used by the chiller, which performs heat transfer by converting from liquid to gas and gas to liquid at various pressures and temperatures. Common refrigerants used in commercial HVAC are R-11, R-12, R-22, R-113, R-114, R-123, R-134a, R-500.
Refrigerant Analysis	Laboratory analysis of a refrigerant sample to determine contamination, typically oil or moisture.
Refrigeration Effect	The amount of heat absorbed by the refrigerant in the evaporator.
Restricted Flow	Restricted flow is caused by an obstruction (blockage) in system piping, chiller barrel or tower system.
Retrofit	A retrofit is a modification to a chiller system. For example, a new style compressor replacing an older, less efficient style or to use a different refrigerant to comply with new standards. The results can affect energy efficiency.
Rotating Vane Anemometer	Measures air velocity by use of a rotating vane or fan.

S
Sacrificial Anode	An anode that is made of a metal, typically zinc or magnesium, that is lower on the galvanic chart so it will be sacrificed to protect the mild steel tube sheet and end bells in the chiller from galvanic corrosion.
Seal	A seal is a tight and perfect closure (as against the passage of gas or water) or a device to prevent the passage or return of gas or air into a pipe or container.
Secondary Surface Area	The area that extends from the primary surface area into the fluid being conditioned to enhance the heat transfer.
Sensible Heat	Heat that causes a change in the temperature of an object or location.
Separation Gasket	Gasket used to separate a two or more pass chiller between the inlet and discharge of the chiller.
Specific Heat	The ratio of the quantity of heat required to raise the temperature of one pound of a substance one degree Fahrenheit to that required to raise the temperature of one pound of water one degree, Btu/lb. For the metric system, the unit of mass is kilogram and the temperature scale is Celsius.
SRB	Sulfate Reducing Bacteria. SRB can cause significant localized pitting corrosion and severe damage in the cooling tower system.
Strainer	Used to remove foreign material from the water flow. The mesh size determines the size of the material/debris being removed.
Superheat Test	A superheat test is the excess of the gas suction temperature above the gas saturation temperature The purpose of the superheat is to ensure that liquid refrigerant does not enter the compressor. Typically the standard is between 10 and 20°F. A high superheat value is an indication of low refrigerant levels and low superheat value is an indication of high refrigerant levels especially when the compressor is operating at full load.
Suction Piping	Refrigerant piping from the evaporator outlet to the compressor suction inlet.

T
Temperature	Degree of hotness or coldness of an object or location measured on a definite scale.
Temperature Difference	Degree of change between two temperatures.
Thermal Expansion	Pressure and temperature regulation valve, located in the liquid line, which is responsive to the superheat of the vapor leaving the evaporator coil.
Thermal Conductivity	Material property indicating how easily heat travels through material. It depends on the physical structure of matter at both the molecular and atomic level as well as the state of matter, solid, liquid, or gas.
Thermometer	Measures and evaluates temperature.
Transcritical Cooling	Cooling by using the subcritical and supercritical state of the refrigerant. The critical point of a substance is the temperature and pressure where the phase changes to what is called a "supercritical fluid" which shows properties of both a liquid and a vapor at the same time. Only a substance that has a critical point near the ambient temperature can be used, making CO2 the refrigerant of choice for this process.
Total Surface Area	The sum of the primary surface area and secondary surface area.
Tube	A pipe that water flows through to transfer heat.
Tube Bundle	A group of heat exchanging tubes.
Turbulent Flow	Disturbed, chaotic flow of a fluid. The velocity at a given point varies erratically in magnitude and direction.
Two Phase Flow	Change in phase (liquid to gas, gas to liquid), due to changes in pressure or temperature, that takes place while the fluid is circulating through the heat exchanger.

U
Ultrasonic Flow Meter	A device that is used to measure flow through piping.

V
Vacuum	A space partially exhausted by artificial means (i.e., suction measured in inches of Hg in a low pressure chiller).
Variable Frequency Drive (VFD)	A chiller with a variable frequency drive has a controller that monitors the operating conditions and uses a combination of inlet guide vanes and speed control. VFDs act as a soft-starter. VFDs can offer significant energy savings at part load conditions, but they typically do not run as efficiently at full load when compared to constant speed drives.
Vortex	A mass of fluid (as a liquid) with a whirling or circular motion that tends to form a cavity or vacuum in the center of the circle and to draw toward this cavity or vacuum bodies subject to its action.
Volts	The practical meter-kilogram-second unit of electrical potential difference and electromotive force equal to the difference of potential between two points in a conducting wire carrying a constant current of one ampere when the power dissipated between these two points is equal to one watt and equivalent to the potential difference across a resistance of one ohm when one ampere is flowing through it. Typical chiller voltages are 460, 480, 2400 or 4160. This information can be found in the chiller technical documentation.

W
Water Treatment Program	A water treatment program provides a biocide program that minimizes microbiological growth along with excellent scale/corrosion protection.
Wet Bulb	The combination of outside air temperature and relative humidity, affecting the ability of the tower to evaporate water into the atmosphere. The higher the relative humidity, the more difficult it is to evaporate additional moisture.
Working Fluid	The heat transfer fluid that changes the temperature.

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