Dictionary of Terms


Absolute Humidity - The weight of water vapor in grains actually contained in one cubic foot of the mixture of air and moisture.

Absolute Pressure - Absolute pressure is the pressure above a perfect vacuum. This scale is used to eliminate the possibility of error when comparing pressures at different times or locations.

Absolute Temperature - The temperature of a sub-stance measured above absolute zero. To express a temperature as absolute temperature add 460° to the reading of a Fahrenheit thermometer or 273° to the reading of a Centigrade.

Absolute Zero - The temperature (-460°F. approx.) at which all molecular motion of a substance ceas-es, and at which the substance contains no heat.

Air - An elastic gas. It is a mechanical mixture of oxygen and nitrogen and slight traces of other gases. It may also contain moisture known as humidity. Dry air weighs 0.075 Ibs. per cu. ft. One Btu will raise the temperature of 55 cu. ft. of air one degree F. Air expands or contracts approximately 1/490 of its volume for each degree of rise or fall in temperature from 32° F.

Arc - The curved portion of tube rolling or pipe rolling bends.

Atmospheric Pressure - Atmospheric pressure is the force exerted by the atmosphere because of its weight. The valve for normal atmospheric pressure at sea level is 14.696 pounds per square inch. Atmospheric pressure decreases with increasing elevation above sea level. This must be taken into consideration in determining pressures at the pump suction. Atmospheric Pressure is the weight of a column of air, one square inch in cross section and extending from the earth to the upper level of the blanket of air surrounding the earth. This air exerts a pressure of 14.696 pounds per square inch at sea level, where water will boil at 212°F. High altitudes have lower atmospheric pressure with correspondingly lower boiling point temperatures.

Bevel - The type of end preparation of tube or pipe for bending / welding / other fabrication.

Blow Down Valve - Also referred to as a blowoff valve. A valve which permits a boiler control to be flushed out, and the function of same to be checked.

Boiler Feed Pump - A pump that is governed by a control that monitors the actual boiler water level; and only adds water to the boiler when the boiler needs it. The pump controller is mounted on the boiler.

Boiler Heating Surface - The area of the heat transmitting surfaces in contact with the water (or steam) in the boiler on one side and the fire or hot gases on the other.

Boiler Horse Power - The equivalent evaporation of 34.5 lbs of water per hour at 212 degrees F to steam at 212 degrees F. This is equal to a heat output of 33,475 BTU per hour, which is equal to approximately 140 sq. ft. of steam radiation (EDR).

British Thermal Unit (BTU) - The amount of heatrequired to raise the temperature of 1 pound of water 1 degree Fahrenheit.

Centerline Radius (CLR) - Distance in inches from the center of curvature to the centerline axis of the tube rolling or pipe rolling bends / coils. Abbreviated as CLR.

Coefficient of Heat Transmission (Overall) U-Factor - The amount of heat (BTU) transmitted from air to air in one hour per square foot of the wall, floor, roof, or ceiling for a difference in temperature of one degree Fahrenheit between the air on the inside and outside of the wall, floor, roof, or ceiling.

Condensate - In steam heating, the water forms when steam cools, as in a radiator or other heating unit. The capacity of traps, pumps, etc., is sometimes expressed in lbs. of condensate they will handle per hour. To convert lbs. of condensate or steam per hour, to sq. ft. EDR, multiply by 4.

Condensation - The process of changing a vapor into a liquid by extracting heat.

Conductance (Thermal) - The amount of heat (BTU) transmitted from surface to surface, in one hour through one square foot of a material or construction for the thickness or type under consideration for a difference in temperature of one degree Fahrenheit between the two surfaces.

Conduction (Thermal) - The transmission of heat through and by means of matter.

Conductivity (Thermal) - The amount of heat (BTU) transmitted in one hour through one square foot of a homogenous material one inch thick for a difference in temperature of one degree Fahrenheit between the two surfaces of the material.

Convection - The transmission of heat by the circulation of a liquid or a gas such as air. If natural, it is caused by the difference of weight of hotter and cooler fluid.

Dew-Point Temperature - The air temperature corresponding to saturation (100 percent relative humidity) for a given moisture content. It is the lowest temperature at which air can retain water vapor.

Domestic Hot Water - Hot water used for purposes other than house heating such as laundering, dish-washing, bathing, etc.

Dry-Bulb Temperature - The temperature of the air as determined by an ordinary thermometer.

EDR (Equivalent Direct Radiation) - EDR (Equivalent Direct Radiation). The amount of heating surface that will give off 240 BTU per hour when filled with a liquid that is heated to 215°F and surrounded by 70°F air. It may not have a direct relation to the actual surface area.

Extended Heating Surface - Heating surface consisting of ribs, fins, or enhanced surfaces which receive heat by conduction from the prime surface.

Fahrenheit - A thermometer scale at which the freezing point of water is 32° and its boiling point is 212° above zero.

Flash Steam - Reevaporation of condensate back into steam. It occurs when condensate flows into a reduced pressure area where the condensate's temperature is above boiling point for the reduced pressure. For example: If hot condensate is discharged by a trap into a low pressure return or into atmosphere, a certain percentage of water will be immediately transformed into steam. Flash steam usually has a white, puffy appearance.

Grade - Manufacturers' specification of material types of plate, pipe, tube, fittings. E.G. Type 304 Welded Stainless Steel Tube: SA-249 T-304/304L

Heat - The form of energy which transfers from one substance to another by virtue of the temperature difference which exists between the two substances.

Heat of the Liquid - The heat (BTU) contained in a liquid due to its temperature. The heat of the liquid for water is zero at 32 degrees F, and increases 1 BTU: approximately for every degree rise in temperature.

Heat Transmission Coefficients - Used in the calculation of heat transmission by conduction, convection, and radiation through various materials and structures.

Heat, Latent - A term used to express the energy involved in a change of state.

Heat, Sensible - A term used to to indicate any portion of heat which changes only the temperature of the substance involved.

Heating Medium - A substance such as water, steam, or air used to convey heat from the boiler, furnace, or other source of heat to the heating units from which the heat is dissipated.

Hot Water Heaters - Storage Heaters Storage heaters have a large tank and a steam coil in the tank, or a separate shell and tube heat exchanger with a circulating pump. A temperature regulator senses the tank temperature. Cold water is added directly into the storage tank. The tank usually has 10 to 20 minutes' reserve capacity, and temperature change occurs gradually. Systems with storage tanks may have a recirculating pump to provide continuous recirculation of system water.

Semi-Instantaneous Heaters - Semi-instantaneous heaters have a small tank and either a steam coil in the tank, or a separate shell and tube heat exchanger and a circulating pump. The buffer tank should have a minimum of one gallon of storage for every GPM of system draw rate. Larger tanks may be used where system storage is desired. Temperature changes can occur quickly. If system load changes, pneumatic regulators are recommended. Semi-instantaneous systems may have recirculating pumps to provide continuous recirculation of system water. A minimum recirculation of 20% of design flow rate is recommended.

Instantaneous Heaters - Instantaneous heaters do not have a storage tank. Water is heated directly in a heat exchanger and sent to the system. As hot water is drawn off in the system, it is replaced directly in the heat exchanger with cold water. This causes sudden temperature changes. Pneumatic regulators should be used where possible, as they have faster response time. When instantaneous heaters are used for domestic hot water, separate anti-scald protection should be provided. Instantaneous heaters should have recirculating pumps. The recirculating return line should be at least 50 feet long, and the pump should recirculate at least 20% of the system draw rate.

Humidity - The water vapor mixed with air.

I.D. - Inside diameter. Can refer to the size of the material, or to the dimension of the part being formed / fabricated.

Induced Vacuum - As steam condenses back into condensate, its volume shrinks. This reduction in volume can create pressures below atmospheric and is often referred to as induced vacuum. An induced vacuum inside a steam space can cause a negative differential pressure across the steam trap and allow condensate to build up. To assure positive differential pressure, a vacuum breaker should be installed in the steam coil or piping so air may enter and relieve the vacuum. Complete condensate drainage under all possible conditions must be provided. This should prevent water hammer and coil freeze-up if condensate is held in the coil by induced vacuum.

Latent Heat of Evaporization - The heat (BTU of pound) necessary to change 1 pound of liquid into vapor without raising its temperature. In round numbers, this is equal to 960 BTU per pound of water.

Latent Heat of Fusion - The heat necessary to melt one pound of a solid without raising the temperature of the resulting liquid. The latent heat of fusion of water (melting 1 pound of ice) is 144 Btu.

Low Pressure Steam - As defined by A.S.M.E., low pressure steam is 15 PSIG or less.

Minimum Tangent - The minimum straight on the end of pipe or tube bends required by the bending / coiling machine to form the bend.

Neutral Axis - The portion of the tube / pipe that is neither in compression or tension.

NPT - National Pipe Thread.

O.D. - Outsider diameter. Can refer to the size of the material, or to the dimension of the part being formed / fabricated.

Out of Plane - The deviation of the horizontal plane of a single pipe or tube bend between its tangent points, based on the theoretical centerline of the pipe or tube rolling bend.

Ovality - The distortion or flattening of pipe or tube from its normal round shape caused by the forming process.

Plain End - Square cuts made prior to pipe or tube bending with no intended exact tangent length.

Pressure Drop - Pressure drop is the term which expresses the fact that power is consumed in moving liquids through pipes, heating units, fittings, etc. Or, expressed in another way, pressure drop is the amount of pressure lost between any two points in a system.

For example, if the city water pressure at the inlet of a tankless type copper coil Indirect Water Heater is 50 pounds, and at the outlet 45 pounds, we can easily see that there is a 5 pound pressure drop through the heater. This 5 pound pressure drop is equivalent to an 11.5' pressure drop.

Friction causes a drop in pressure when liquids move through pipes or heating units.

Pressure drop is caused by the friction created between the inner walls of the conveyor and the moving liquid. In a horizontal pipe in which there is no flow, the pressure is equal at all points. The moment flow starts, friction is set up, which increases in direct proportion to the velocity of the flow.

To calculate the change in pressure drop when you have an increase or decrease in gallons per minute (GPM), this simple rule may be followed -- "Divide the final GPM by the initial GPM and square the result. Multiply this result by the initial pressure drop, and you now have the new pressure drop."

The following example shows the effect of increasing the GPM from 4 to 10 in a system with initial pressure drop of 5 lbs.

Final GPM = 10
Initial GPM = 4
Initial pressure drop = 5

New pressure drop = (10/4)² x 5
New pressure drop = 31.25 lbs

Velocity in feet per second may be substituted for GPM in this formula.

Therefore, in designing both service water heating systems and hot water space heating systems, pressure drop must be taken into consideration.

In either case, enough power must be available to overcome the effects of pressure drop before the desired results can be obtained. This means that the power consumption, or pressure drop, of each component part of a system must be known and a source of sufficient power prowided. In a forced hot water heating system this power is provided by the pump -- in a domestic water heating system, city water pressure is the source of power.

Manufacturers who publish pressure drop information on their equipment express the data either in pounds per square inch, in feet of water, or milinches.

These figures are easily interchangeable as follows: 1 lb. per sq. inch equals 2.3 feet of water
or
1 ft. of water equals .43 lb. per sq. inch
or
1 foot of water equals 12,000 milinches
Therefore, 1" of water equals 1000 milinches

Prime Surface - A heating surface with the heating medium on one side and air (or extended surface) on the other.

Roll Past - Small amount of arc bent beyond a specified degree.

Rought Cut - Ends cut with an acetylene torch or arc gauge; ends are not required to be straight.

Safety Factor - Additional surface area of a heat exchanger included in design to compensate for periods of irregular operation. Also refers to additional static head needed to compensate for higher pressures and greater steam consumption during warm-up periods.

Sensible Heat - Heat which increases the tempera-ture of objects as opposed to latent heat.

Specific Heat - In the foot-pound-second system, the amount of heat (Btu) required to raise one pound of a substance one degree Fahrenheit. In the centimeter-gram-second system, the amount of heat (cal.) required to raise one gram of a sub-stance one degree C. The specific heat of water is 1.

Square Cut - Cuts made square to the tangent of pipe or tube rolling bends after the forming process.

Static Pressure - Head Pressure should not be confused with Static Pressure, as they have no relationship. Static Pressure is created by the weight of water in the system. Static Pressure has no effect on pump capacity. If you will consider a hot water heating system as being an upright loop of water confined in a pipe, the Static Pressure in one of the vertical pipes of the loop is identical with the pressure at the same level in the opposite vertical pipe. The Static Pressure is equal to .43 pounds per sq. inch per foot of height above the gauge. For example, if the highest radiation is 20 feet above the gauge in the boiler, that Static Pressure at the gauge will be: 20 times .43 which equals 8.6 pounds per sq. in. At various elevations above the gauge, the Static Pressure becomes correspondingly less. At 10 feet, it is 4.3 pounds per sq. in., and at the top radiation, located 20 feet above the boiler, there is no pressure.

Steam - Water in the vapor phase. The vapor formed when water has been heated to its boiling point, corresponding to the pressure it is under.

Superheated Steam - Steam heated above the temperature corresponding to its pressure.

Tangent - The straight portion of material on either side of arc of rolling bends.

Tangent Point - The point at which a tube or pipe bend starts or ends.

Ton of Refrigeration - The heat which must be extracted from one ton (2,000 Ibs.) of water at 32° F. to change it into ice at 32°F. in 24 hours. It is equal to 288,000 Btu/24 hours, 12,000 Btu/hour, or 200 Btu/minute.

Total Heat - The latent heat of vaporization added to the heat of the liquid with which it is in contact.

Total Pressure - The sum of the static and velocity pressures. It is also used as the total static pressure over an entire area, that is, the unit pressure multiplied by the area on which it acts.

Tube Bundle - A single tube (pipe) formed into a tight array so as to present a large surface area in a small space.

Vapor - Any substance in the gaseous state.

Vapor Pressure - Vapor pressure is very important where liquids that are volatile are being pumped. Considerations must be made as to the vapor pressure of the liquid at the pumping temperature so that the pump can operate without the liquid flashing into a gas. Temperature of the liquid is also very important when a selection of pump construction is considered such as gaskets, seals and castings. The amount of pressure needed depends on the liquid and liquid temperature. It is for this reason we must have pressure available on the suction side of the pump above the flash point of the liquid.

Velocity Pressure - The pressure used to create the velocity of flow in a pipe. It is expressed as a unit pressure.

Ventilation - Air circulated through a room for venti-lating purposes. It may be mechanically circulated with a blower system or through circulation with an open window, etc.

Wall - The thickness in inches of tubular pipe or tube bending material.

Water Expansion - Figure 1 illustrates the rate at which water expands or contracts as its temperature rises or falls. As the water heats up so does the piping in the system. The curve labeled net Expansion of Water takes this into account and is used in sizing a tank. Since water is incompressible a closed system filled with water must have space for any increase in volume as the temperature rises. This is true even in a chilled water system since the chilled water return is higher than the fill temperature. To accommodate this fluctuation in volume a compression tank is commonly used.


FIGURE 1 Rate at which water expands or contracts as its temperature rises or falls.

Water Hammer - Water hammer is one of the chief causes of noise in steam heating systems a nd also the major cause of damage to thermostats and floats in traps. It is a wave transmitted through a pipe filled, or partially filled, with water. It may originate as waves set up by steam passing at a high velocity over condensate collected in piping.

Wet-Bulb Temperature - The lowest temperature which a water-wetted body will attain when exposed to an air current.

Wet Saturated Steam - Saturated steam containing some water particles in suspension.

Wrinkles - Waving or corrugation of pipe or tube rolling bends in the inner radius.




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