Saturday, July 27, 2013

A GLOSSARY OF STEAM SYSTEM OPTIMISATION TERMINOLOGY

Steam systems comprise the generation of steam, its distribution to point of use, its deployment in various processes and the recovery of the condensate produced. The efficiency of steam systems is a complex subject area with lots of terminology, some of which can be confusing to those starting out with resource efficiency projects in this area. The attached glossary is meant to assist with the various terms you will come across. I will update it with additional terms over time, so you can use this as a reference.

Approach temperature
The temperature difference between the process fluid leaving a heat exchanger and the service fluid entering the heat exchanger. The smaller this difference, the greater the heat exchange area required and the higher the cost of the heat exchanger.
Backpressure turbine
A turbine that discharges steam at a pressure greater than atmospheric pressure. This type of turbine typically discharges into a steam header, with the steam then used for heating or other process uses. The turbine can be used to generate electricity or to drive a piece of equipment.
Balanced draft
When the air and flue gases in the boiler stack are maintained at a pressure equal to atmospheric pressure
Ball float steam trap
A mechanical type of steam trap that uses a ball that floats on the condensate in the trap. The ball is attached to a lever that is connected to a condensate release valve. The valve is closed when the level of condensate is reduced and the ball is lowered. The trap can also be fitted with a thermostatic air vent, which increases its capacity during start-up. Its biggest advantage is that since it does not rely on temperature, condensate is discharged under all conditions, as soon as it is formed. Hence both saturated and sub-cooled condensate are released through this trap.
Balanced pressure steam trap
A thermostatic type of steam trap comprising a capsule containing a fluid with a boiling point lower than that of water. In cold conditions, the capsule is relaxed and air and condensate can leave the system.  When warm condensate approaches, the fluid vaporises, closing the trap.  Cooling causes the trap to open again, and steam causes the cycle to repeat.
Bimetallic steam trap
A type of thermostatic steam trap that uses a bimetallic strip for its operation. The metals are dissimilar, and expand at different rates with increasing temperature. The deflection in the strip is used to close the condensate release valve at higher temperatures when steam is present, and open to release condensate at lower temperatures.
Blowdown
As steam and condensate are lost from the overall steam system, the concentration of salts in the boiler increases. If the concentration of these salts becomes too high, this can impact on boiler integrity and also cause scaling and heat transfer problems. It is therefore necessary to remove suspended and dissolved solids by draining a portion of the concentrated water and replacing this with fresh make-up water with low salt concentration. This process is called “blowing down”.
Blowdown flash tank
The water in the boiler is under pressure, and when it is exposed to a lower pressure, a portion of it will flash as steam. This can be achieved in a blowdown flash tank, which is simply a vessel large enough to allow the liquid and vapour streams to disengage. The flash steam can be recovered into a low pressure header, or used to heat make-up water (this is less preferable). Heat can also be recovered from the remaining liquid stream using a blowdown recovery heat exchanger.
Boiler
Device used to heat water and produce steam. The two main types of boilers are fire-tube boilers and water-tube boilers.
Boiler efficiency
The efficiency with which the energy contained in the boiler fuel is converted into energy contained in steam. It is important to distinguish between whether the higher heating value (HHV) or lower heating value (LHV) is being used for the calculation, and to be consistent when making comparisons.
Boiler feedwater
The water fed into the boiler to produce steam. This typically comprises a mixture of fresh make-up water (which is typically treated) and recovered condensate.
Boiler feedwater pump
The pump used to pump feedwater into the boiler. Since this pump has to overcome the pressure inside the boiler, it is often a multi-stage pump capable of generating significant head.
Bottom ash
Large non-combustible residues from the burning of solid fuels which typically are too heavy to be conveyed by the combustion gas stream and are hence removed from the bottom of the boiler.
Bottom blowdown
This blowdown removes solids that have settled in the boiler “mud drum” and in the bottom of the boiler tubes in the case of watertube boilers. Firetube boilers may also have a blowdown outlet near the bottom of the water level.
Cogeneration

Also known as combined heat and power (CHP). This is the simultaneous production of electricity and heat energy from a single fuel source. It could entail the recovery of heat from the exhaust gases of a gas turbine to produce steam, for example, or the production of steam with concomitant production of electricity in a manufacturing plant.
Combustion air pre-heater
A heat exchanger used to heat up combustion air using heat recovered from hot flue gases.
Condensate flash tank
A vessel used to allow condensate leaving a high-pressure system to flash as it encounters a lower pressure. The flash can then either be diverted into a lower-pressure steam header or condensed using a flash recovery heat exchanger.
Condensate tank
A vessel used to store recovered condensate. It may also serve as a boiler feedwater tank, in which case it would typically be combined with a deaerator.
Condensate polishing
The process of removing corrosion products and dissolved minerals from returning condensate. In very high pressure systems, demineralisation may be carried out using processes such as ion exchange.
Condensate recovery
The recovery of condensate from processes that condense steam and the return of this condensate to steam generation.
Condensing economiser
Heat exchanger used to heat up boiler feedwater using heat recovered from combustion gases that are cooled sufficiently to condense the water vapour in them, so that it gives up its latent heat. Condensing economisers are typically used with low-sulphur fuels which do not produce a strongly acidic condensate.
Condensing turbines
These turbines exhaust saturated steam (typically with a quality less than 1) at pressures below atmospheric pressure to a condenser, where the steam is condensed and the resulting condensate is typically returned to the boiler. They are typically used in power generation applications and to drive large pieces of equipment.
Condensate recovery
After steam gives up its latent heat it becomes a saturated liquid, and may cool down further to become sub-cooled condensate. The resulting water is of very high quality and also contains significant amount of thermal energy. It is hence a valuable commodity that can be recovered for reuse as boiler feedwater to produce steam.
Conductivity
A measure of an electrolytic solution’s ability to conduct electricity. Used to assess boiler feedwater and boiler water quality, in order to determine when a boiler should be blown down to reduce the build-up of ionic salts in the boiler. At high concentrations these salts could form scale or cause corrosion.
Deaerator
A vessel used to remove air (or more specifically, oxygen) from boiler feedwater, thereby limiting corrosion. Usually achieved using live steam and an arrangement which maximises the residence time of the water and the contact area between the steam and the water being deaerated e.g. a system of trays or a spray system. The vessel is fitted with a vent through which the air and some steam can escape.
Dealkalisation
Removal of carbonate and bicarbonate alkalinity. If they are not removed, these chemical species can cause priming, foaming and carryover in the boiler, and also decompose to form carbon dioxide, and carbonic acid, causing corrosion. Dealkalisation is usually done downstream of softening using a technique such as ion exchange.
Demineralisation
The removal of minerals, typically using ion exchange or reverse osmosis.
Desuperheating
The process whereby superheated steam is transformed into steam with fewer degrees of superheat or to saturated steam through the addition of water. This reduces the specific enthalpy of the steam, but increases the mass of steam.
Distillation column
A device used to separate materials on the basis of their boiling points, using a heat input (typically steam) and a heat exchanger.
Distribution
This is the process whereby steam produced in the boilers is distributed to point of use, using pipelines, valves and pressure reducing devices as appropriate.
End use
This is where the steam is actually employed, either for heating, direct injection or to drive turbines.
Enthalpy
The measure of the total energy content of a thermodynamic system. When analysing steam systems we typically use the specific enthalpy, which is simply the enthalpy per unit mass.
Excess air
A certain minimum amount of air (or more specifically, oxygen) is required in order to meet the stoichiometric requirements of combustion. A small excess is needed to account for imperfect mixing. Too much air will however lead to inefficiency, since the mass of hot flue gases leaving the boiler would be more than optimal for a given mass of fuel combusted. Control of excess air is therefore a critical aspect of boiler efficiency management.
Extraction turbine
With this type of steam turbine, steam is extracted from various stages of the turbine (after having been expanded) and used for process requirements.
Generation (steam)
This is the process whereby steam is produced using boilers. Boilers can use a number of fuels, or can even be powered by waste heat.
Generator (electrical)
A device that converts kinetic/mechanical energy to electrical energy. In the electricity generation process, a conductor is rotated inside a magnetic field, generating a flow of electrical current perpendicular to the field.
Feedwater economiser
A heat exchanger used to heat up boiler feedwater using heat recovered from the hot combustion gases leaving the boiler.
Fire tube boiler
A boiler type in which the hot combustion gases pass through the tubes and heat the water (in the “shell”) to produce steam.
Fly ash
Light particulate residues that are conveyed by the combustion gas stream and are removed from the flue gas stream using equipment such as bag filters or electrical precipitation units.
Forced draft
Control of the combustion zone pressure such that the pressure of the air and combustion gases in the stack is maintained at a level above atmospheric pressure.
Header
A steam pipeline carrying steam at a given pressure and distributing this steam to users.
Heat Exchanger
A device used to exchange heat between two or more fluids. The fluids do not come into physical contact with each other, but are separated by heat transfer surfaces, such as tubes or plates.
Higher Heating Value (HHV)
Also called “higher calorific value – HCV”. The amount of energy liberated upon combustion of a fuel, including that recovered by condensing of the water vapour formed during combustion.
Induced draft
Control of the pressure at the stack entrance such that the air and combustion gases are maintained at a pressure below atmospheric pressure.
Inverted bucket steam trap
A type of mechanical steam trap containing an inverted bucket attached by a lever to a condensate release valve. In the presence of condensate, the bucket sags and the valve remains open, releasing the condensate. When steam arrives, the bucket is buoyed by the steam, and the valve is closed. The bucket typically has a small vent/bleed hole through which air can escape. A small amount of steam is therefore lost through this vent.
Liquid expansion trap
A type of thermostatic steam trap that uses an oil-filled element which contracts when contacted with cooled condensate, and allows the condensate to vent, and expands when contacted with steam, causing it to expand and shut the release valve, trapping the steam. Since these traps tend to activate at a fixed temperature, they are best used to discharge condensate after a shutdown period. They are better at releasing sub-cooled rather than saturated condensate.
Log mean temperature difference
A means of expressing the driving force for heat transfer in a heat exchanger (remember that Q=UA ΔTLM). If A and B are the two ends of the heat exchanger, then LMTD = (ΔTA – ΔTB) / ln(ΔTA/ ΔTB). For crossflow and multi-pass heat exchangers a correction factor has to be applied to calculate the LMTD.
Loss on ignition
The proportion of unburnt carbon and other combustibles in the ash remaining after combustion of solid fuels.
Lower Heating Value (LHV)
Also called “lower calorific value (LCV)”. The amount of energy liberated during the complete combustion of a fuel, excluding heat that would be recovered by condensing water vapour in the flue gases.
Maximum Demand
The peak apparent power drawn by a site over the course of a month, measured in kVA. This may be of interest for sites wishing to generate power using their steam systems.
Mechanical steam traps
Steam traps that rely on mechanical means to operate. They typically exploit the density difference between steam and condensate.
Orifice plate steam flow meter
A plate with a hole in the middle of it through which the steam flows. The flow of steam is inferred from the pressure drop across the plate, the square of which is proportional to the velocity of the steam. A differential pressure measurement device such as a DP Cell is used to measure the pressure difference. The data can then be fed to a computer/PLC and used either on its own or together with additional measurements (such as temperature) to calculate the mass flow of steam.
Oxygen scavengers
Chemicals (such as sodium sulphite) used to remove oxygen from boiler feedwater. Deaerators cannot remove all of the oxygen, and even the low oxygen levels after deaeration may be enough to cause corrosion.
Pressure reducing station
Also called a “pressure reducing valve – PRV” or a “letdown station/valve”. These are systems comprising a control valve and pressure sensing which reduce the pressure of the steam from the pressure in the header to a reduced pressure, either in another header or for a specific individual user. They are always fitted with a bypass to allow for maintenance.
Reboiler
A heat exchanger used to heat distillation column bottoms. It is called a reboiler rather than a boiler since the bottoms are circulated continuously through the heat exchanger, and boiled over and over again.
Refractory
Heat-resistant material used to line high-temperature furnaces and boilers.
Saturated steam
Water vapour in equilibrium with liquid. When heat is removed from saturated steam, it immediately begins transforming back into the liquid phase to form condensate. Saturated steam has a pressure that is completely determined by its temperature and vice versa. The thermodynamic properties of saturated steam can be found from steam tables if either the temperature or pressure are known.
Steam ejector
A device which uses steam as a motive fluid to entrain another fluid (the suction fluid) using a venturi. The steam is injected into the throat of the venturi, where its velocity increases, decreasing its pressure. The lower pressure “sucks” in the fluid being transported and the steam-fluid mixture then enters the wider area of the throat, where the pressure increases again, mixing the steam and the suction fluid.  The word “ejector” implies that the fluids are discharged to the atmosphere, for example where steam is used to generate a vacuum.
Steam injector
This is a variant of an ejector, in that the same principle is used to entrain water with steam, and then mix the two to condense the steam and produce hot water at a high pressure that can be injected into a process or a boiler.
Steam quality
A number between 0 and 1 that reflects the fraction in the saturated mixture that is vapour, with the balance being liquid. At a steam quality of 1, all of the steam is saturated vapour, while at a quality level of 0 all of the steam is saturated liquid.
Steam reforming
The process of reacting steam with natural gas (methane) to produce hydrogen and carbon monoxide in the presence of a nickel catalyst. The carbon monoxide can be reacted with additional steam to produce yet more hydrogen, this time accompanied by the production of carbon dioxide.
Superheated steam
Steam that has been heated to a temperature above its saturation temperature. When heat is removed from superheated steam, it does not immediately begin to condense, but first cools to the saturation temperature corresponding to its pressure. Once it has become saturated, it will begin to condense if more heat is removed from it. The pressure and temperature of superheated steam are independent of each other and both have to be known to assess the thermodynamic properties of superheated steam from steam tables.
Thermodynamic steam trap
This trap works using a disc which either allows condensate to pass, or traps steam inside the steam system. The pressure exerted by cold condensate simply lifts the disc, allowing the condensate to flow. When hot condensate arrives and flashes, the high velocity creates a zone of low pressure underneath the disc, causing it to seat. At the same time, flash steam in the enclosed space above the disc is trapped, creating a positive pressure above the disc. Since force = pressure x area, and the area above the disc is larger than the area below the disc (by virtue of the design of the body of the trap), the flash steam above the disc applies a larger force than the steam below it. This force will remain higher until this flash steam condenses sufficiently for the force of the condensate below the disc to exceed it.  It is important for the top of this trap to be well insulated in order to prevent rapid condensing of steam above the disc, and hence too high an opening frequency.
Reheat turbine
With this type of steam turbine, steam is extracted from the turbine, sent to the boiler to be reheated and then returned to the turbine, from where it continues to expand.
Reverse osmosis
In natural osmosis, water moves from an area of low solute concentration to an area of high solute concentration through a semi-permeable membrane, until the solute concentration is equalised on both sides of the membrane. Increasing the pressure on the side of the membrane containing the high solute concentration can slow or oppose osmosis, and if this pressure is made high enough, water can be forced in the opposite/reverse direction. Reverse osmosis is used to purify water containing dissolved solids. 
Softening
The process of removing “hardness” from water (calcium, magnesium, iron and other metals). These form scale which impacts on heat transfer.
Soot blowing
The process of removing soot from the surfaces of boiler tubes, using either compressed air or steam. The reduction in fouling improves heat transfer rates.
Steam trap
A device used to “trap” steam inside a system until it condenses, after which it is released as condensate. Steam traps ensure that the latent heat of the steam is released, thereby allowing heat transfer equipment to work as designed. In addition to releasing condensate, they also release air and other non-condensable gases which could compromise the rate of heat transfer.
Steam turbines
Devices that convert the thermal energy in the steam into shaft work, which can be used to drive mechanical devices (e.g. pumps, compressors and the like) or to generate electricity using a generator.
Steam Stripping column
A column in which a liquid product containing volatile components is contacted with live steam, stripping the volatile materials out of the liquid. The column may contain packing or trays to maximise the surface area for mass transfer and the residence time of the material being stripped.
Surface blowdown
This blowdown removes concentrated water from the boiler, but generally does not remove solids. It is therefore carried out close to the surface of the liquid in the boiler.
Thermocompressor
A device used to compress low pressure steam using high pressure steam, resulting in a mixed steam stream at an intermediate pressure. The principle of operation is the same as that of a steam ejector i.e. a venturi is used.
Thermostatic steam traps
These traps operate on the basis of the temperature of the steam or condensate around them. Condensate tends to cool down, leading to a contraction of a component in these traps, which is leveraged as a means of releasing condensate.
Turndown ratio
The ratio of the boiler’s maximum capacity and the minimum capacity it can be operated at by virtue of its design. If a boiler is rated at a maximum capacity of 10 tons/hr of steam and a minimum capacity of 3 tons/hr, its turndown ratio would be 10/3 = 3.33.
Water tube boiler
A boiler type in which the water being heated passes through the tubes, with the hot combustion gases outside the tubes.

Copyright © 2013, Craig van Wyk, all rights reserved