Glossary of Heat and Thermal Engineering Terms
A reference glossary with clear, concise definitions of 68 technical terms frequently used in thermal engineering, such as steam boiler, enthalpy, saturated steam, superheated steam, blowdown and deaerator.
Glossary of Technical Terms Used in Thermal Engineering
Manhole
A cover (manhole) used for inspecting and cleaning the water side of steam boilers.
Adiabatic process
Occurs with no heat or mass transfer across the system boundary. Adiabatic flame temperature is the temperature the flame would reach with no heat or mass loss to the surroundings.
Low water level
When the water level inside a steam boiler under operating conditions drops below the minimum level, the heating surfaces are exposed to steam instead of liquid water. The temperature of the boiler's flame tube (furnace) can easily exceed 1000°C. If it heats further, the steel loses its strength and deforms plastically, meaning the water pressure pushes the flame tube inward. This is called furnace collapse.
Fire-Tube Boiler
Combustion gases transfer their heat to the water by convection as they pass through the boiler's heat-transfer surfaces. Also known as "flue-tube" boilers, these are surrounded by a shell with sufficient strength to withstand internal pressure. In short, the flue gas passes through tubes on its way to the stack.
Alkalinity
The amount of carbonate, bicarbonate, hydroxide and silicate or phosphate in water, expressed in parts per million as calcium carbonate. In other words, it is water's capacity to resist becoming acidic.
Bottom blowdown
As water changes to the steam phase in a steam boiler, solids heavier than water in the feedwater settle to the bottom of the boiler. If allowed to accumulate, they can endanger the boiler's efficiency and operation. They are removed at regular intervals by opening a valve at the bottom of the boiler; the pressure inside the boiler expels the solids along with a certain amount of boiler water. With automated blowdown valves, this can be done without requiring an operator.
Rear Reversal Chamber
The section at the end of the boiler's furnace that reverses the direction of flow of the combustion gases.
Acidity
Represents the amount of free carbon dioxide, mineral acids and salts (particularly sulfates, or iron and aluminum) that hydrolyze in water to yield hydrogen ions; reported as milliequivalents per liter of acid, as ppm acidity in terms of calcium carbonate, or as pH, a measure of hydrogen ion concentration.
Waste heat
The sensible heat contained in non-combustible gases released to the atmosphere.
Pressure-reducing valve
In industrial facilities that use steam, different sections of the process often require different steam pressures. In areas where lower pressure is needed, a pressure-reducing valve is used to reduce the steam to the required pressure before use.
Feedwater Pump
Steam boilers continuously need feedwater to replace the water lost from the boiler body. The devices used to replace this lost water in the boiler are called feedwater pumps. Boiler feedwater pumps have sufficient pressure to overcome the boiler's steam pressure and sufficient capacity to meet the required water flow rate.
Blowdown Tank
Water discharged from the boiler as blowdown or bottom blowdown, carrying total dissolved solids (TDS), is at high pressure and temperature. For blowdown to be safely disposed of, both its temperature and pressure must be safely dissipated. This is done in a blowdown tank, a pressure vessel partly filled with water and with a large vent open to atmosphere. When water enters the tank, some flash steam is released to atmosphere; the remaining water mixes with cold water and is drained for filtering. If blowdown rates are very high, the flash steam can be captured and reused. Blowdown tanks are pressure-rated and sized to suit the boiler room's capacity.
Burner
A device that combines fuel and air in the correct proportions for combustion and ensures the fuel-air mixture burns stably to produce a flame of a specific size and shape.
Steam Dome (Steam Drum)
In water-tube steam boilers, a cylindrical, dished-head vessel used to store steam together with a certain amount of water. It stores the steam produced in the boiler and acts as a phase separator for the steam/water mixture.
Steam Boiler
A closed pressure vessel in which water is heated under pressure or vacuum, using heat energy from fuels, electricity or nuclear energy, to produce steam, superheat steam, or any combination of these.
Steam hammer (water hammer)
An excessive pressure event caused by a valve suddenly opening or closing; it can damage the steam system.
Desuperheater
A piece of equipment used to lower the temperature of superheated steam to the saturation temperature for its pressure. Common examples are small turbines and Venturi water injection.
Steam Table
A table of the thermophysical properties of water and steam.
Deaerator
Oxygen and carbon dioxide gases dissolved in boiler feedwater cause corrosion. Deaerators are used to remove these gases from boiler feedwater; they are beneficial because they free the boiler, its auxiliary equipment and piping components from gases that cause oxidation, rust or corrosion. Besides reducing corrosion damage, a deaerator raises the feedwater temperature before it enters the boiler, so less fuel is needed for the heat of vaporization. As a result, boiler efficiency increases while operating and maintenance costs decrease.
Direct boiler efficiency
The ratio of useful heat produced in the boiler to the heat input is called boiler efficiency. Comprehensive efficiency calculations can include electrical energy consumption as well as fuel consumption.
Indirect efficiency
Determining thermal performance by evaluating thermal losses and the measured thermal input or output.
Saturated steam
Steam containing no water droplets; also commonly 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 that cannot absorb additional heat without a phase change occurring.
Smoke Box
The pressure-free part of the boiler where combustion gases collect before being sent to the stack.
Sensible heat
Heat added to or removed from a fluid that results in a change in the fluid's temperature. For example, water cooled from 50°C to 30°C has had sensible heat removed.
Hand-hole
Two small covers at the front of a steam boiler, used for cleaning the boiler's water side.
Economizer
A heat exchanger that recovers waste heat from a boiler's exhaust gases and uses this heat to heat the feedwater or another area.
Safety valve
A valve that releases excess pressure from a pressure vessel to atmosphere once the set pressure is reached. There are two types: full-lift safety valves and proportional-lift safety valves.
Enthalpy
The enthalpy of a substance consists of the sum of its internal energy and the energy it possesses due to its pressure and specific volume. For water, this is measured from the triple point at 0.01°C (273.16 K) and 0.006112 bar.
Entropy
Referred to as the disorder of a system. A system's entropy tends to increase as work is done.
Phase diagram
A diagram, chart or graph showing the various states of a fluid, divided into regions: solid, liquid, wet, vapor (superheated) and gas (supercritical). The boundaries between these regions are generally called "phase lines."
Flash steam (first type)
Steam formed when liquid water contacts a surface hotter than the water's saturation temperature.
Flash steam (second type)
Steam formed as a result of a pressure drop. Liquid water at high pressure has a greater enthalpy than water at low pressure. If the pressure on a saturated liquid at high pressure is suddenly reduced, the difference in enthalpy causes a small portion of the liquid to boil off, forming flash steam. This is a measure of a steam boiler's useful heat output starting from 100°C; it is equivalent to the enthalpy of water's phase change at 100°C and 1.013 bar, and is normally expressed in kg/hour. 1 kg/hour at 100°C is equivalent to 0.627 kW.
Latent Heat
The heat a substance gives off to, or absorbs from, its surroundings during a phase change. Water must absorb heat from its surroundings to change into the steam phase.
Absolute pressure
A pressure measurement referenced to a total vacuum. Absolute pressure is measured as the difference between operating pressure and full vacuum (0 psia).
Gauge pressure
Pressure measured relative to atmospheric pressure (i.e., 0 barg equals atmospheric pressure at the time of measurement). Gauge pressure is the most commonly used pressure measurement in steam systems.
Heated Fluid
The type of fluid being heated inside a boiler or heat exchanger.
Wet Steam
A steam-water mixture that contains water droplets along with the steam, or that carries water along with steam and condensate as it travels through piping because it transfers its heat.
Isenthalpic
A process with no enthalpy loss. In practice this is an idealized condition; therefore, a device's isenthalpic efficiency is compared to its actual performance versus what perfect efficiency would achieve.
Isentropic
A process that occurs at constant entropy.
Heating Boiler (central heating boiler)
A boiler in which no steam is produced; instead, hot water is circulated for heating purposes and returned to the boiler.
Calorific Value
The total energy content per unit mass of a fuel.
Reverse-flame boiler
Thanks to reverse-pressure combustion, the burner flame is reversed within the same furnace. The returning hot gases are forced to meet the burner flame a second time, ensuring complete combustion and cleaner flue gases. The intense heat generated by the reversed flame is transferred to the water mainly through radiant heat transfer. Through a combination of radiation and convective heat transfer, complete heat transfer is achieved across the boiler's successive passes.
Boiler horsepower
A measure of a steam boiler's useful output. 1 boiler horsepower is equivalent to producing 15.6 kg of steam per hour (9.8 kW) at 100°C.
Boiler dry-firing
The event where the water in a steam boiler is depleted and not replenished, leaving the boiler without water. If the burner continues firing while this persists, the boiler metal is directly exposed to high heat, creating a hazardous condition for the boiler.
Boiler sweating
During the period from initial startup until a boiler, whether a steam boiler or a heating boiler, reaches operating conditions, hot combustion gases strike the boiler's cold surfaces, causing the water vapor they contain to condense and seep from the boiler doors.
Superheated steam
Water vapor above the saturation line but below the critical point. For example, under isobaric conditions, if heat is added to saturated steam its temperature will rise and the steam will become superheated. Superheated steam is vapor, not gas; it is very commonly produced in boilers for power generation or, less commonly, for process applications.
Degree of superheat
Superheated steam is generally described by its pressure and degree of superheat. Degree of superheat is the temperature difference between the steam's actual temperature and the saturation temperature at the steam's pressure. For example, superheated steam at 10 bar with a final temperature of 199.9°C would have 20 degrees of superheat (saturation temperature is 179.9°C).
Condensate Tank
Effectively using the condensate that remains after steam use in a facility is extremely important for energy efficiency and resource conservation. Condensate tanks collect and store the water formed as steam cools and turns back into water during boiler operation. These tanks allow the water to be returned to the boiler, so energy is used more efficiently.
Steam trap
A device that separates condensate from steam and removes it from the distribution system.
Critical point
The point at which a substance's liquid, vapor and gas phases are in thermodynamic equilibrium. For water, this occurs at 221.2 bar pressure and 374.15°C (647.30 K).
Main steam valve
The valve connected to a steam boiler's steam outlet line.
Dryness Fraction
Steam produced inside a steam boiler contains water particles. A wet steam mixture that is 98% steam by mass and 2% saturated liquid by mass has a dryness fraction of 0.98.
Mollier diagram
A graphical representation of steam's thermophysical properties. The underlying data is the same as in steam tables. A Mollier chart shows a substance's thermodynamic properties: entropy, enthalpy, temperature, pressure, specific volume and dryness fraction. If any two thermodynamic properties are known, the others can be read from the diagram.
Net Calorific Value
The value obtained by subtracting the latent heat of vaporization of the water formed by combustion from a fuel's total energy content per unit mass.
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)].
Explosion door
A spring-loaded door opening directly into the furnace, used to release excess gas pressure that can build up in the furnace during the burner's initial ignition.
Pump Cavitation
A liquid at or near saturation temperature can contain vapor bubbles, which often collapse spontaneously. The term used for these collapsing bubbles is "cavitation." Cavitation is sometimes observed on the discharge side of the feed pump in boiler rooms. If the feedwater is hot enough and pressure is insufficient, the water will be close to saturation temperature and vapor bubbles will form. As water passes through the pump from the low-pressure side to the high-pressure side, the increase in pressure means the water is no longer saturated, and the vapor bubbles collapse instantly, generating shock waves. These shock waves can cause significant damage to the pump.
Specific energy consumption
Energy consumption per unit of product, or steam produced per unit of fuel burned.
Specific
A property defined per kilogram of a substance, such as specific heat capacity.
Water-Tube Boiler
Combustion products transfer their heat by convection to the water inside the tubes, from outside the tube. Unlike a fire-tube boiler, water moves inside the tubes. A steam dome is positioned above to accumulate the steam produced.
Water Carryover
The movement of liquid water droplets, foam or solid particles along with steam through the system piping.
Supercritical steam
Steam whose pressure and temperature are above the critical point. Supercritical steam is a gas, not steam.
Three-pass boiler
A boiler type designed with one furnace (flame tube) and two convective passes. Combustion takes place in the furnace. The hot gases produced by combustion pass from the furnace to the rear reversal chamber, then to the second-pass tubes, reaching the front smoke box; from there they travel in the reverse direction through the third-pass tubes back to the smoke box, and then to the stack.
Total dissolved solids (TDS)
As a steam boiler produces steam, it concentrates the dissolved solids present in the feedwater. Some of the concentrated solids remain dissolved; this concentration is generally expressed in parts per million. Excessive TDS can cause foaming on the free surface of the water inside the boiler. This is undesirable because it increases corrosion and can adversely affect the operation of level controls.
Triple point
The temperature and pressure at which a substance's solid, liquid and vapor phases are in thermodynamic equilibrium. For water, the triple point is 273.16 K (0.01°C) and 0.006112 bar.
Surface (top) blowdown
Certain chemical substances in the water cannot pass into the steam during evaporation and remain suspended within the steam generator body. Being lighter than water, these substances accumulate on the surface of the water, increasing in concentration. Surface (top) blowdown must be performed to keep these dissolved or suspended solids within defined limits.
Condensation
The formation of condensate. In piping that carries steam, when heat loss through the pipe walls lowers the steam's heat, some of the steam changes phase and condenses into water. To prevent water hammer and preserve steam quality, steam traps are used to return the condensed water to the condensate tank. Steam-based processes use only the latent heat of the steam, so that all the steam changes back to the liquid phase. Condensate is normally returned to the boiler room for reuse; however, in some systems, particularly businesses that use open steam, such as concrete and food processing, the condensate is not recovered.