GLOSSARY OF TECHNICAL TERMS RELATED TO HEAT AND THERMOTECHNIQUE

TECHNICAL DATA

Glossary

Absolute pressure: Pressure measurement whose datum is a perfect vacuum. See also gauge pressure.

Adiabatic process: The adiabatic process occurs without the transfer of heat across the system boundary. The adiabatic flame temperature, for example, is that temperature which would be reached by the flame if there was no heat loss to the surroundings.

Acidity: Represents the amount of free carbon dioxide, mineral acids, and salts (especially sulfates or iron and aluminum) which hydrolyze to give hydrogen ions in water; is reported as mill equivalents per liter of acid, or ppm acidity as calcium carbonate, or pH, the measure of hydrogen ion concentration.

Alkalinity: The amount of carbonates, bicarbonates, hydroxides, and silicates or phosphates in the water; reported as grains per gallon, or parts per million as calcium carbonate.

Blowdown pit: A blowdown pit consists of a covered pit partially filled with water which performs the same function as a blowdown vessel. They are more commonly found in older boiler houses as blowdown vessels are now preferred.

Blowdown vessel: Water which is blowdown from a boiler in the form of dissolved solids (see: TDS), blowdown or bottom blowdown is at elevated pressure and temperature. Both the temperature and pressure have to be dissipated safely before the blowdown can be safely disposed of. This is done in a blowdown vessel, i.e. a pressure vessel partially filled with water and possessing a large vent to atmosphere. When the blowdown enters the vessel some steam is lashed off and vented to atmosphere. The remainder mixes with the cold water and is discharged to drain. If blowdown rates are very high, the flash steam may be collected and reused. Blowdown vessels are rated for pressure and are matched to the boiler house for capacity. An alternative to a blowdown vessel is a blowdown pit.

Boiler: A closed vessel in which water is heated, steam is generated, steam is superheated, or any combination thereof, under pressure or vacuum by the application of heat from combustible fuels, electricity, or nuclear energy. The term dose not include such facilities of an integral part of a continuous processing unit but does include fired units of heating or vaporizing liquids other than water where these units are separate from processing systems and are complete within themselves.

Boiler - Central Heating: A boiler in which no steam is generated and from which hot water is circulated for heating purposes and then returned to the boiler.

Boiler horsepower (American): A measure of the useful output of a steam boiler. 1 boiler horsepower is equivalent to the generation of 15.6 kg steam from and at 100 °C in one hour (9.8 kW).

Bottom blowdown: As a steam boiler exports steam, it concentrates solids contained in the feedwater. Some of the concentrated solids sink to the bottom of the boiler. If allowed to build up they would interfere with its normal operation and may pose a hazard. They are normally removed on a daily basis by opening a valve located at the bottom of the boiler. The pressure inside the boiler then forces the solids out along with a quantity of boiler water. The blowdown is normally sent to a blowdown vessel.

Burner: A device which combines fuel and air in proper proportions for combustion and which enables the fuel-air mixture to burn stably to give a specified flame size and shape.

Carryover: Droplets of liquid water, foam or solid particles are drawn up with the steam vapor into the distribution system.

Cavitation: A fluid close to or at its saturation temperature may contain bubbles of vapor. These frequently collapse of their own accord. The term for collapsing bubbles is “cavitation”. Cavitation is sometimes observed on the pressure side of a feed pump in boiler houses. If the feedwater is hot enough and there is not enough head, the water will be close to its saturation temperature and steam bubbles will be present. As the water passes through the pump, it moves from the low pressure side to the high pressure side. Due to the increase in pressure, the water is now unsaturated
and the steam bubbles collapse instantaneously creating shock waves. These shock waves are capable of causing significant damage to the pump.

Condensate: Condensate forms inside the steam mains due to heat loss through the pipe walls. It is removed at frequent intervals using steam traps to prevent water hammer and maintain the quality of the steam. Indirect processes only utilize the latent heat of the steam so all the steam condenses back to the liquid phase. Condensate is normally returned to the boiler house for reuse. In some systems however the condensate is not recycled. This is frequently due to unavoidable contamination making the condensate unsuitable for reuse.

Critical point: The point at which the liquid, vapor and gaseous phases of a substance are in thermodynamic equilibrium. For water this occurs at a pressure of 221.2 bara and a temperature of 374.15 °C (647.30 K).

Crown valve: The main valve attached to a boiler through which steam passes through to the distribution system. It derives its name from its location on the top, or crown, of the boiler.

Deadleg: A steam line which is connected to the distribution system and maintained at pressure but which no longer serves any consumer.

Degrees of superheat: Superheated steam is commonly defined in terms of its pressure and degrees of superheat. The degrees of superheat is the temperature difference between the actual temperature of the steam and the saturation temperature at the steam pressure. Thus superheated steam at 10 bara and a final temperature of 199.9 °C would possess 20 degrees of superheat. (The saturation temperature being 179.9 °C).

Desuperheater: A piece of equipment used to reduce the temperature of superheated steam to its saturation temperature for the pressure at which it exists. Common examples are small turbines and Venturi water injection.

Direct efficiency: The ratio of useful heat exported from the boiler to the net or gross heat input. Comprehensive efficiency calculations may include the electrical energy consumption as well as the fuel consumption; sub-systems may also be accounted for.

Dry fry: The period during a low-water event/condition when the burner is still firing and the metal of the furnace exposed to steam rather than water. This causes an unwanted and potentially hazardous rise in the metal temperature.

Dryback furnace: A furnace in which the reversal cell is sealed using a refractory plug. No useful heat transfer to the heat carrier takes place through the refractory.

Dryness fraction: The fraction of wet steam which, if separated out, would consist of steam vapor only. A wet steam mixture comprising 95% by mass steam vapor and 5% by mass of saturated liquid would have a dryness fraction of 0.95.

Economizer: A heat exchanger used to recover waste heat from the exhaust gases of a boiler and transfer that heat into the feedwater.

Enthalpy: A term for the energy of a substance. The enthalpy of a substance is made up of the addition of its internal energy and the energy it possesses by virtue of its pressure and specific volume hence the distinct name for it. Enthalpy cannot be directly measured, however the change in enthalpy can be determined if the change in properties such as pressure and volume are known. Enthalpy is referenced to an arbitrary point. For water this is the triple point at 0.01 °C (273.16 K) and 0.006112 bara.

Entropy: A measure of the disorder of a system. As work is performed the entropy of a system tends to increase.

Fire-tube boiler: A boiler in which the furnace and heat transfer tubes are submerged by the water being boiled. The exhaust gases pass through the boiler inside the furnace and subsequent heat transfer surfaces, usually banks of tubes known as convective passes. The furnace and heat transfer surfaces are surrounded by a shell of sufficient strength to withstand the internal pressure – also known as “smoketube” boilers, boilers multi-tubular horizontal steam boilers (MTHS).

Flash steam (first type): Steam which is generated as a result of liquid water coming into contact with a surface which is hotter than the saturation temperature of the water. If the temperature of the hot surface is great enough, a flash steam explosion may result.

Flash steam (second type): Steam which is generated as a result of a pressure reduction. Liquid water at high pressure has a greater enthalpy than water at a lower pressure. If a saturated liquid at elevated pressure is suddenly reduced in pressure, the difference in enthalpy is given up by the liquid causing a small fraction of the liquid to boil off forming flash steam. From and at 100 °C (F&A 100 °C): A measure of the useful heat output of a steam boiler – equivalent to the enthalpy of the change of phase of water at 100 °C and 1.013 bara. This measure is normally expressed in kg/hr. 1 kg/hr from and at 100 °C equates to 0.627 kW.

Gauge pressure: Pressure which is referenced to atmospheric pressure (i.e. 0 barg = atmospheric pressure at the time of measurement). Gauge pressure is the most common pressure measurement in steam systems. If required to be selected, the reference pressure used in instrumentation is frequently a fixed value chosen at the discretion of the end user, e.g. one standard atmosphere 1.01325 bara, or, more simply, 1.0 bara.

Gross Calorific Value (GCV): The total energy content per unit mass of a fuel.

Heat carrier: The working fluid of a boiler.

Hotwell: A tank of water maintained at around 80 °C which is used to supply water to the boiler. The water is maintained at temperature to reduce thermal shock to the boiler when it is injected via the feed pump and also to reduce the dissolved oxygen content of the feedwater.

Indirect efficiency: The determination of thermal performance by the assessment of the thermal losses and the measured thermal input or output.

Isenthalpic: A process in which there is no loss of enthalpy. In practice this is an idealized state. Therefore a device for which an isenthalpic efficiency is quoted is comparing its actual performance to that if it were perfectly efficient.

Isentropic: A process which takes place with constant entropy. In practice this is an idealized state. Therefore a device for which an isentropic efficiency is quoted is comparing its actual performance to that if it were perfectly efficient.

l-v (liquid–vapor) mixture: A substance which is composed of two phases in equilibrium, e.g. wet steam.

Latent heat: Heat which is added to or taken away from a fluid at constant temperature, which results in a change of phase of the fluid, e.g. wet steam condensing back into liquid water.

Low-water condition: Low-water conditions occur when the free surface of the water inside a steam boiler has dropped so that the heating surfaces are exposed to steam vapor rather than liquid water. The temperature of the gases on the fireside of the boiler can easily exceed 1000 °C and the surfaces exposed rapidly heat up. If they become hot enough, the steel loses its strength and is plastically deformed, i.e. pushed inwards by the pressure on the waterside. This situation is called a furnace collapse. In extreme cases it can cause a rupture in the pressure vessel resulting in an explosion.

Maximum continuous rating (MCR): The maximum capable output of a steam boiler under continuous operation, often expressed in terms of kg/hr from and at 100 °C.

Mollier chart or diagram: A Mollier chart is a graphical representation of the thermophysical properties of steam. The underlying data are the same as tabulated in steam tables. A Mollier chart plots the thermodynamic properties of a substance: entropy, enthalpy, temperature, pressure, specific volume and dryness fraction. If any two of the thermodynamic properties are known, then the others can be read from the diagram.

Net Calorific Value (NCV): The total energy content per unit mass of a fuel minus the energy content of the latent heat of evaporation of the water formed as a consequence of combustion.

Phase diagram: A diagram, chart or graph which presents the various states of a fluid divided into regions: solid, liquid, wet, vapor (superheated) and gaseous (supercritical). The boundaries between these regions are commonly
termed “phase lines”.

Priming: A situation in which the steam demand is so great that the free surface of the water inside the boiler rises up locally beneath the crown valve. It is strongly associated with carryover. In severe instances a slug of water can detach and enter the distribution system. Priming can also be known as swell.

Pressure reducing valve (PRV): Steam is distributed at a higher pressure than processes require it. Its pressure is educed close to the point requirement using a PRV to provide steam of the correct, controlled pressure for the downstream processes. The operation of a PRV may cause a small amount of superheat downstream of the valve on saturated steam systems.

Reverse flame boiler: In this type of boiler the furnace is blank ended. The flame passes through the centre of the furnace. The hot gases impinge on the blank end of the furnace and in doing so are reversed in direction, passing back along the furnace wall prior to exiting the furnace.

Reversal cell: A wetback or dryback boiler is constructed with a chamber at the end of the furnace which causes the flow of combustion products to be reversed. These combustion products that pass through a bank of tubes in which further useful heat is transferred from the gases to the waterside.

Safety valve: A mechanical valve fitted to pressure vessels intended to prevent overpressure events by venting fluid to atmosphere when its set point is reached. Safety valves are completely independent of other protection measures and are always fitted to steam boilers. They are also found downstream of pressure reducing valves in
steam distribution systems. Saturation point: The temperature and pressure at which a fluid cannot absorb or give up further heat without a phase change.

Saturated steam: Steam vapor which cannot absorb any additional heat without a consequent increase in temperature. Saturated steam is often called dry saturated steam. Saturated steam lies on the phase line between the wet steam and superheated steam regions of a phase diagram.

Saturated water: Liquid water which cannot absorb additional heat without a phase change occurring.

Sensible heat: Heat which is added to or taken away from a fluid which results in a change of temperature of the fluid. Water, for example, which is cooled from 50 °C to 30 °C has had sensible heat removed.

Smokebox: A chamber in which the burnt combustion gases are reversed in their direction of flow, normally not part of the pressure vessel.

Specific: A property of a substance which is defined per kilogramme of that substance, e.g. specific heat capacity.

Specific heat capacity: The amount of heat required to raise the temperature of 1 kg of a substance by 1 Kelvin [kJ/(kg K) or kJ/(kg °C)].

Specific energy consumption: The energy consumption of a process per unit of product or the units of steam produced per unit of fuel burnt (this is similar to miles per gallon [mpg] or liter per 100 km [l/100 km]).

Steam hammer: An overpressure event which is caused by the sudden opening or closing of a valve, for example. It can result in damage to the steam system.

Steam tables: The thermophysical properties of water and steam have been formulated by the International Association for the Properties of Water and Steam IAPWS (http://www.iapws.org). These formulations are frequently reproduced in the form of tables known collectively as steam tables although they include all the various phases of water. Simplified versions referenced to both bara and barg are produced by many organizations and companies. Those interested in further reading on this subject are directed to the IAPWS website.

Steam trap: A device for separating condensate from steam and removing it from the distribution system. There are a number of different types commercially available.

Sub-cooling: Usually applied to condensate that has lost some heat resulting in a fluid temperature less than the saturation temperature. Sub-cooled condensate is frequently associated with several forms of water hammer.

Superheated steam: Steam vapor which is above the saturation line but below the critical point. For example, if heat is added to saturated steam under isobaric conditions its temperature will increase and the steam becomes superheated. Superheated steam is a vapor, not a gas. It is very commonly generated in boilers for power generation or less commonly process applications. Superheated steam may also be found in unexpected situations, e.g. in a notionally saturated steam distribution system due to a sudden reduction in pressure.

Supercritical steam: Steam, the pressure and temperature of which is above the critical point. Supercritical steam is a gas, not a vapor.

Three-pass boiler: A boiler arranged with a furnace and two convective passes. The products of combustion pass hrough the furnace and their direction is reversed prior to entry to the first convective pass in a reversal cell. This is repeated prior to entry to the second convective pass (in a smokebox); hence the description “three pass”.

Total dissolved solids (TDS): As a steam boiler exports steam it concentrates the dissolved solids contained in the feedwater. Some of the concentrated solids remain dissolved. This concentration is often expressed in parts per million. Excessive TDS may cause foaming at the free surface of the water inside the boiler. This is undesirable as it promotes carryover and may negatively affect the operation of the level controls.

Total dissolved solids blowdown: The TDS naturally concentrates near the free surface of the water inside the boiler. It is controlled by opening a valve located just under the free surface. The pressure inside the boiler forces water along with the dissolved solids out of the boiler.

Vapor: Saturated and superheated steam are strictly a vapor, not a gas. A vapor is capable of being compressed back into the liquid phase under isothermal conditions.

Triple point: The temperature and pressure at which the solid, liquid and vapor phases of a substance are in thermodynamic equilibrium. For water the triple point is 273.16 K (0.01 °C) and 0.006112 bara.

Waste heat: Sensible heat in non-combustible gases.

Water hammer: There are several phenomena which are collectively termed “water hammer”: plug flow water hammer, condensate-induced water hammer and column closure water hammer.

Water-tube boiler: A boiler which consists of two pressure vessels connected by tubes with one pressure vessel below the other. The lower vessel contains liquid water and is often known as a “mud drum”. The upper vessel contains steam and is known as the “steam drum”. The furnace is external to this arrangement. As the water passes up through the tubes it boils to form steam. This type of boiler is capable of producing very high pressure steam with a large amount of superheat.

Water wedged: A boiler that has been taken out of service and completely filled with water to prevent corrosion of the internal surfaces of the pressure vessel.

Wetback furnace: A furnace in which the reversal cell is completely submerged under the surface of the water inside the boiler except for an access hatch. This typeof furnace increases the available surface area for heat transfer.

Wet steam: A liquid-vapor mixture of steam, usually but not always with a high dryness fraction when found in a distribution system during normal operation. Condensate return systems however have a much lower dryness fraction as their purpose is to return condensate to the boiler house.