HVAC Terms & Definitions

 

A's

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Absorption Chiller -

A chiller which creates a cooling effect using a chemical reaction instead

of mechanical compression of refrigerant. Common Absorption chillers use a Lithium-Bromide

Solution as the "refrigerant" and a source of heat to power the chiller. The heat source can be

waste heat in the form of steam or hot water or the chiller can be direct fired. Typical absorption

chillers are Water Cooled Chillers, requiring a cooling tower for the condenser.

 

Air Conditioner -

Device used to control temperature, humidity, cleanliness and movement of

air in a confined space.

 

Air Conditioning -- Control of the temperature, humidity, air movement and cleaning of air in a

confined space.

 

Air Cooled Chiller -- A chiller that utilizes an Air Cooled Condenser for rejection of heat; Air

Cooled Chillers are typically designed for outdoor use, but smaller units may be used indoors in

manufacturing facilities and shops.

 

Air Cooled Condenser -- A condenser that rejects heat to the surrounding environment by

moving air across a coil to condense the refrigerant moving through the coil as part of the

mechanical refrigeration cycle.

 

Ambient -- The surrounding atmosphere; encompassing on all sides; the environment surrounding

a body but undisturbed or unaffected by it.

 

 

B's

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Boiler -- A device for generating steam for power, processing, or heating purposes or for producing

hot water for heating purposes or hot water supply. Heat from an external combustion source is

transmitted to a fluid contained within the tubes in a boiler shell, a closed vessel in which water is

converted to pressurized steam. This fluid is delivered to an end-use at a desired pressure,

temperature, and quality.

 

BTU -- a unit of heat equal to the amount of heat required to raise one pound of water one degree

Fahrenheit at one atmosphere pressure

 

BTUH (British Thermal Unit Per Hour) -- Establishes a time reference to btu input or output rates.

A BTUH is how many BTUs are used per hour.

C's

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Celsius -- A temperature scale based on the freezing (0 degrees) and boiling (100 degrees) points

of water. Abbreviated as C in second and subsequent references in text. Formerly known as Centigrade.

To convert Celsius to Fahrenheit, multiply the number by 9, divide by 5, and add 32. For example: 10

degrees Celsius x 9 = 90; 90 / 5 = 12; 18 32 = 50 degrees Fahrenheit.

 

CFM (Cubic Feet per Minute) -- A standard measurement of airflow that indicates how many cubic

feet of air pass by a stationary point in one minute. The higher the number, the more air is being forced

through the system. A typical system produces 400 CFM per ton of air conditioning. A measure of

air-flow. Volume X Air Changes per hour / 60 minutes.

 

Chiller -- A device that produces chilled water to provide air conditioning for large buildings or cooling

for process applications. A chiller is a mechanical refrigeration device like an air conditioner except that

it cools a fluid (usually water) instead of cooling air. When a large air conditioner is required it is sometimes

more feasible to use one large chiller instead of many small air conditioners. Chillers are also used

wherever there is a need for cooling a fluid such as a chemical process or for plastics molding. To simplify

the concept of a chiller you should compare it to a drinking fountain where you get cold "chilled" water.

Chillers are primarily used in applications requiring LWT, (Leaving Water Temperature), of 65 F and lower.

 

Chiller Ton -- One Chiller Ton = 12,000 BTU's. A quick rule of thumb is: GPM's multiplied by /\T (Delta T)

divided by 24. This will calculate the actual heat removal in Tons. Chillers are usually nominally rated with

LWT (Leaving Water Temperature) of about 45F.

 

Compressor -- The heart of an air conditioning or heat pump system., as well as an integral component

of a chiller. The large part in the condenser (outdoor unit) that pumps refrigerant. The pump of a refrigerating

mechanism which draws a low pressure on cooling side of refrigerant cycle and squeezes or compresses

the gas into the high pressure or condensing side of the cycle. The compressor maintains adequate

pressure to cause refrigerant to flow in sufficient quantities to meet the cooling requirements of the system.

 

Condenser -- This is the unit that will sit outside and is part of a split system, it contains the compressor

which is the heart of your air conditioner or Heat Pump, it pumps the refrigerant through your system.

Some people call the condenser the compressor, but the compressor is a component of the condenser along

with the fan motor and condenser coil. Coil or outdoor coil dissipates heat from the refrigerant, changing the

refrigerant from vapor to liquid.

 

Cooling tower -- A heat transfer device, which cools warm water using outside air. Cooling towers are

used in conjunction with Water Cooled Chiller, but have many stand-alone uses as well. Cooling generally

are used when LWT (Leaving Water Temperature) requirements are 75F or higher. The term cooling tower

may be used to describe both open and closed circuit cooling towers, (see Evaporative Condenser). An

example of an open system is a cooling tower condenser loop to a water cooled chiller. The circuit is open

on the cooling tower end in that water enters the cooling tower at for example 95?F, is cooled to 85?F by

falling through the fill with air circulating through it, and is returned to the chiller condenser to be heated to

95?F and begin the cycle again. An example of a closed circuit system is an evaporative condenser used

in conjunction with a particular process cooling a chemical or fluid other than water. The liquid would pass

through a series tubes within the structure of tower. The tower would still utilize water recirculated within

the tower to reject the heat, only the liquid is within the tubes, closed to the environment. Make-up water is

required for both applications, since evaporation will take place in either case.

 

Cooling Tower Ton -- One Cooling Tower Ton = 12,000 Btu's. A quick rule of thumb is: GPM's multiplied

by /\T (Delta T) divided by 30. This will calculate the actual heat removal in Tons. Cooling Towers are usually

nominally rated with LWT (Leaving Water Temperature) of about 85?F.

 

 

 

D's

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Delta T - /\T -- A difference in temperature. In Chiller applications, the ?T is used for the difference in

temperature between the EWT, (Entering Water Temperature), and the LWT, (Leaving Water Temperature)

of the chilled water, either through the chiller or through the process.

 

Dewpoint -- Is the temperature at which air becomes saturated with water and begins to condense

- forming a dew. Therefore at 100 % RH the ambient or process temperature equals the dewpoint

temperature. The more negative the dewpoint temperature is from the ambient temperature the less the

risk of condensation and the drier the gas or air stream.

 

Direct Expansion (Refrigeration) -- Any system that, in operation between an environment where heat

is absorbed (heat source), and an environment into which unwanted heat is directed (heat sink) at two

different temperatures, is able to absorb heat from the heat source at the lower temperature and reject

heat to the heat sink at the higher temperature. The cooling effect is obtained directly from a fluid called

a refrigerant that absorbs heat at a low temperature and pressure, and transfers heat at a higher

temperature and higher pressure.

 

Dry Bulb Temperature (DB) -- The temperature measured by a standard thermometer. A measure of the

sensible temperature of air.

 

 

E's

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Entering Water Temperature -- The temperature of the water or fluid when entering chiller, cooling tower,

or other similar refrigeration device. Used with LWT, (Leaving Water Temperature), and flow rate in

calculating /\T (Delta T) for a refrigeration device. May also be used in reverse with LWT and flow rate to

calculate for a process or piece of equipment's rise in /\T.

 

Enthalpy -- Enthalpy is a measurement of energy of air. It consists of both sensible (temperature) and

latent (moisture content) heat of air. The amount of Enthalpy of a given condition can be found on the

Psychometric Chart. The quantity of heat necessary to raise the temperature of a substance from one

point to a higher temperature. The quantity of heat includes both latent and sensible.

 

Evaporative Cooler -- A type of cooling equipment that turns air into moist, cool air by saturating the air

with water vapor. It does not cool air by use of a refrigeration unit. This type of equipment is commonly

used in warm, dry climates.

 

 

 

F's

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Fahrenheit (F) -- A temperature scale in which the boiling point of water is 212 degrees and its freezing

point is 32 degrees. To convert Fahrenheit to Celsius, subtract 32, multiply by 5, and divide the product by

9. For example: 100 degrees Fahrenheit - 32 = 68; 68 x 5 = 340; 340 / 9 = 37.77 degrees Celsius.

 

 

 

G's

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Gallons Per Minute -- Flow rate used in refrigeration formulas. Gallons per hour divided by sixty.

GPM -- see Gallons per Minute

 

 

H's

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Heat Transfer -- Flow of heat energy induced by a temperature difference. Heat flow through a building

envelope typically flows from a heated, or hot area to a cooled, or cold area.

 

Horsepower (HP) -- A unit for measuring the rate of doing work. One horsepower equals about

three-fourths of a kilowatt (745.7 watts).

 

Humidifier -- A device that adds moisture to warm air being circulated or directed into a space. This

adds necessary moisture to protect your furnishings and reduce static electricity

 

Humidity -- The amount of moisture in the air. Air conditioners remove moisture for added comfort.

 

 

 

I's

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IWC (Inches of water column) -- IWC (Inches of water column) Commonly used in the USA

 

 

J's

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Joule -- A unit of work or energy. It takes ~ 1,000 joules to equal a British thermal unit. It typically

takes ~ 1 million joules to make a pot of coffee.

 

 

K's

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kW (kilowatt or kW) -- A kilowatt equals one thousand (1,000) watts. A unit of measure of the

amount of electric power production needed to operate given equipment. On a hot summer afternoon

a typical home, with central air conditioning and other equipment in use, might have a demand of four

kW. An electric power capacity of one kW is sufficient to power ten 100-watt light bulbs.

 

 

 

L's

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Latent Heat -- Heat, that when added or removed, causes a change in state - but no change in

temperature. A change in the heat content that occurs without a corresponding change in temperature,

usually accompanied by a change of state (as from liquid to vapor during evaporation)

 

Leaving Water Temperature - LWT -- The temperature of the water or fluid when leaving chiller,

cooling tower, or other similar refrigeration device. Used with EWT, (Entering Water Temperature),

and flow rate in calculating /\T (Delta T) for a refrigeration device. May also be used in reverse with

EWT and flow rate to calculate for a process or piece of equipment's rise in /\T.

 

Lithium-Bromide -- A chemical solution commonly used in absorption refrigeration systems

 

LWT - Leaving Water Temperature -- See Leaving Water Temperature

 

 

 

M's

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Megawatt (MW) -- One thousand kilowatts (1,000 kW) or one million (1,000,000) watts. One megawatt

is enough energy to power 1,000 average California homes.

 

 

 

N's

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Natural Ventilation -- The supply of outdoor air through passive flow from windows, chimneys, doors,

and other infiltration.

 

 

 

O's

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Ohm -- A unit of measure of electrical resistance. One volt can produce a current of one ampere through

a resistance of one ohm.

 

 

P's

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Package Unit or Package System -- A self-contained unit or system that has the Air Handler

 

Psychometric Chart -- A chart that was developed to graphically display the properties of various mixtures

of air and water vapor. It graphically plots the following: Dry Bulb Temperatures, Wet Bulb Temperatures,

Enthalpy, Relative Humidity, Specific Humidity, Dew Point and Vapor Pressure.

 

 

 

R's

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Refrigerant -- Substance used in refrigerating mechanism. A substance that produces a refrigerating

effect while expanding or vaporizing. Air conditioning systems use Refrigerant in the Evaporator Coil to

cool air as it passes by. Refrigerants absorb heat in evaporator by change of state from a liquid to a gas,

and releases its heat in a condenser as the substance returns from the gaseous state back to a liquid state.

 

Relative Humidity (RH) -- Relative humidity is defined as the ratio of the water vapor pressure to the

saturation vapor pressure (over water) at the temperature of the gas. RH = Pw/Pws * 100

 

 

S's

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Sensible Heat -- Heat, that when added or removed, causes a change in temperature but not in state.

Heat that results in a temperature change.

 

 

T's

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Temperature -- Degree of hotness or coldness measured on one of several arbitrary scales based on

some observable phenomenon (such as the expansion).

 

Temperature Differential -- The difference between the entering and leaving temperature for a given

fluid. For example, a 10-degree evaporator temperature differential for a chiller would describe an operating

condition where the entering water temperature is 54 degrees and the leaving is 44 degrees.

 

Ton or "Ton of Cooling" -- A cooling unit of measure. Each ton equals the cooling effect of 12,000 Btuh.

Heat pumps and air conditioners are generally sized in tons. Typical sizes for single family residences are

between two and five tons. It is important to note that actual capacity is not constant and will change based

on outdoor or indoor temperatures. The published capacity rating of air conditioners and heat pumps is based

on performance at the ARI standard temperature levels of 95 F outside, 80 F inside, and 50% relative humidity.

The number of tons a system has is the total BTU capacity of the system. The size of the area to be cooled will

determine the correct size of the system in tons. While an air conditioner may be called a three ton unit, it may

not produce 36,000 Btu/h in cooling. There is a wide variety of actual capacities that are called "three tons."

 

 

U's

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Unconditioned Space -- A space that is neither directly nor indirectly conditioned space, which can be isolated

from conditioned space by partitions and/or closeable doors.

 

 

V's

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VFD (Variable Frequency Drive) -- Electronic speed control for motors.

 

volt -- equivalent to the potential difference across a resistance of 1 ohm when 1 ampere of current flows through it

 

 

W's

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W (Watt) -- A watt is a unit of electricity. A unit of measure of electric power at a point in time, as capacity or

demand. It is the rate of energy transfer equivalent to 1 ampere flowing under a pressure of 1 volt at unity power

factor. One watt of power maintained over time is equal to one joule per second. Some Christmas tree lights use

one watt. The watt is named after Scottish inventor James Watt and is capitalized when shortened to w and

used with other abbreviations, as in kWh.

 

Water Cooled Chiller -- A chiller that utilizes a Water Cooled Condenser for rejection of heat; Water Cooled

Chillers are typically designed for indoor use, with the condenser piped to a cooling tower or other source of

cooling water. The heat is ultimately rejected to the environment through the cooling tower or discharge water.

 

Wet Bulb Temperature -- The temperature at which water, by evaporating into air, can bring the air to

saturation at the same temperature. Wet-bulb temperature is measured by a wet-bulb psychrometer. Traditionally

this was the temperature indicated by a thermometer whose bulb is wrapped in a wet sheath. The wet bulb

temperature and the dry bulb temperature (i.e air temperature) would then be used to calculate relative humidity

or dewpoint. Alternatively charts or tables can be used.

 

Wet-bulb Temperature -- When a wet wick is placed over a standard thermometer and air is blown across

the surface, the water evaporates and cools the thermometer below the dry-bulb temperature. This cooler

temperature (called the wet-bulb temperature) depends on how much moisture is in the air.