Kuehner - Glossary

Corrosion

The word corrosion stems from the Latin word corrodere that means as much as “to eat away and gnaw away.” Corrosion is a change to the material resulting from temporal and environmental factors. Corrosion can occur in various materials. The best known type of interaction occurs with iron and oxygen and in everyday life is known as “rusting”. In the worst case, corrosion may lead to components becoming unusable. For this reason, significant attention is given to corrosion protection.

The different types of corrosion can also be found in DIN EN ISO 8044 “Corrosion of Metals and Alloys – Basic Principles and Definitions” amongst other places.

References:
http://de.wikipedia.org/wiki/Korrosion (As of 07.03.2011 – 1200 hrs)

European Pressure Equipment Directive

(Pressure equipment directive PED 97/23/EG)

The European Pressure Equipment Directive 97/23/EG from 29 May 1997 regulates the requirements regarding pressure equipment and its introduction to the market.
The individual Member States must incorporate these directives into their national legislation. In Germany this is done through the German Equipment and Product Safety Act (GPSG) and the corresponding Ordinance on the German Equipment and Product Safety Act (GPSGV).

Different standards, regulations and specifications may be used to meet the requirements in accordance with the Pressure Equipment Directive. For example, Regulation AD-2000 or the corresponding DIN EN 13 445 standard may be applied to unfired pressure vessels. In doing so, the selection of the particular system is up to you.

References:
http://de.wikipedia.org/wiki/Druckger%C3%A4terichtlinie (As of 07.03.2011 – 1200 hrs)
http://www.vdtuev.de/publikationen/ad2000 (As of 07.03.2011 – 1200 hrs)

Evaporation / Condensation

Evaporation is the process when a substance changes from a liquid into its gaseous state. Energy in the form of heat is normally required for this. At the initial boiling point an isothermal process takes place during which the liquid evaporates. If all of the liquid evaporates, the temperature of the fluid will increase again when further heat is supplied.

During the phase transition (liquid – gaseous) the volume of the fluid increases dramatically, which is based on a change in the separation of the molecules. The energy supplied in the form of heat is needed here to overcome the existing cohesive forces.

Condensation is the opposite process, thus the process of change from a gaseous into a liquid state.

References:
1. A. Frohn: Einführung in die Technische Thermodynamik (Introduction to Technical Thermodynamics),
3rd Edition 1998, Verlag Konrad Wittwer GmbH, Stuttgart, ISBN 3-87919-263-42.
W. Wagner: Wärmeübertragung (Heat Transfer), 6th Revised and Extended Edition 2004,
Vogel Buchverlag, Würzburg, ISBN: 3-8023-1974-53.
http://de.wikipedia.org/wiki/Verdampfen (As of 07.03.2011 – 1200 hrs)
http://de.wikipedia.org/wiki/Kondensation (As of 07.03.2011 – 1200 hrs)

Heat capacity

A specific substance can store thermal energy based on a change in temperature. The heat capacity gives a quantative statement of the storage capacity / the property of the respective substance at a corresponding temperature and the state of aggregation.
Heat capacity equation: C = dQ/dT [J/K]

Generally the heat capacity of a substance is indicated in terms of its mass. It is known as its specific heat capacity. The unit normally used here is [J/(kgK)].Often two variables are indicated because of different dependencies of the property of this substance:

Cp – specific heat capacity at a constant pressure

Cv – specific heat capacity at a constant volume

The two specific heat capacities vary little from each other, generally Cp >= Cv applies.
Cp ~ Cv may be assumed in the case of solid and liquid substances in particular.

References:
VDI-Wärmeatlas (VDI Heat Atlas): Berechnungsblätter für den Wärmeübergang
(Heat Transfer Calculation Sheets), Prod. Association of German Engineers,
VDI Society for Chemical and Process Engineering (GVC), 9th Edition 2002,
Springer Verlag, Berlin, Heidelberg, New York, Barcelona, Hong Kong, London,
Milan, Paris and Tokyo, ISBN 3-540-41201-82.
A. Frohn: Einführung in die Technische Thermodynamik
(Introduction to Technical Thermodynamics), 3rd Edition 1998,
Verlag Konrad Wittwer GmbH, Stuttgart, ISBN 3-87919-263-43.
W. Wagner: Wärmeaustauscher (Heat Exchangers), 3rd Edition 2005,
Vogel Buchverlag, Würzburg, ISBN-10: 3-8343-3026-44.
W. Wagner: Wärmeübertragung (Heat Transfer), 6th Revised and Extended Edition 2004,
Vogel Buchverlag, Würzburg, ISBN: 3-8023-1974-55.
http://de.wikipedia.org/wiki/W%C3%A4rmekapazit%C3%A4t (As of 07.03.2011 – 1200 hrs)

Heat transfer media

Heat transfer media are used with indirect heating and cooling circuits. Here the media convey heat from one source (e.g. a water heater) to a sink (e.g. a consumer). Generally heat transfer media are used in a liquid or gaseous state. Gaseous systems are generally more expensive to operate, which is why liquid media are often preferred. The decision, however, is dependent on each individual case.

The following demands are placed on a heat transfer medium:

Good heat transfer properties, such as a high specific heat capacity,
excellent heat transfer, good heat conductivity,…
Late initial boiling point
Low solidification temperature
Constant and low viscosity over the entire temperature range
Thermal stability
Lowest possible corrosion potential
Inert behaviour / insensitive to other substances (such as air, …)
Lowest possible risk potential (health, environment, flammability…)
Easy to dispose of
As pressureless as possible during operation

An ideal heat transfer medium that has excellent properties in all areas has not yet been discovered or developed. Therefore, a good heat transfer medium must be selected according to each individual area of application. Often compromises must be made in this selection.

Choose from the following:

Water
Heat transfer oils
Gases (e.g. air)
Molten salts
Liquid metals

Water and steam:

This is actually the best heat transfer medium. With a heat capacity of approx. 4.2 kJ/(kgK), a high specific enthalpy of evaporation of approx. 2,100 kJ/kg and a heat conductivity of approx. 0.65 W/(mK), water is an excellent heat transfer medium and can also be used in most situations. In addition, water normally does not pose any danger to the environment. Only at high temperatures does water need high pressure to remain in its liquid state of aggregation, which leads to high plant and safety complexity.

Other disadvantages with water and steam are the tendency to corrode and become incrusted, which mean a certain element of cost for water and steam treatment.

Attention must be drawn to frost protection with systems that are operated using water or steam. There is a huge risk of frost if systems with these operating media stand idle during winter for any reason. This often results in significant damage to the system, such as burst heat exchangers. Therefore, close attention should be paid to appropriate frost protection.Note: antifreeze may have a negative effect on the positive heat transfer properties, but improve corrosion protection.

Water is therefore normally used at up to approx. 150°C.

Steam is generally used in temperature ranges up to 200°C.

Heat transfer oils:

The advantages of organic heat exchangers lie in their high temperature range of approx. 0°C to 400°C at an extremely low operating pressure. In this way, for example, “pressureless” systems with an operating temperature of up to 350°C can be achieved. A further advantage is their good corrosion behaviour compared to traditionally used substances. In addition, no significant expansion of the medium is likely with frost.

Organic heat transfer media can be divided into mineral oil-based and synthetic heat transfer oils. As a result of their different structure and chemical composition, properties such as thermal stability, deterioration and solubility of gases differ.

The reduction in the size of the molecules and the formation of larger molecules are the key decomposition variants with heat transfer oils. The reduction in the size of the molecules mostly results in an undesirable drop in the boiling temperature. The formation of larger molecules mainly leads to coking of the system.

When using heat transfer oils, in each case it is vital to coordinate closely with the oil supplier to check that the oils are suitable for their particular applications. Organic and inorganic heat transfer media have limitations of use and if these are exceeded, not only are the systems likely to be seriously damaged but operational safety may also be significantly jeopardised.

Gases:

Gases generally have a wide temperature range. Gases can definitely be used at up to 1,000°C.

Different gases have their own properties and availability. For example, helium generally has good heat transfer properties, a heat capacity of approx. 5.0 kJ/(kgK) and heat conductivity of approx. 0.3 W/(mK). In contrast, air, for example, has relatively poor heat transfer properties, a heat capacity of approx. 1.0 kJ/(kgK) and heat conductivity of approx. 0.03 W/(mK). Air is normally readily available and can generally also be used in open systems without any problems.

Molten salts:

Molten salts can be used in a temperature range of up to approx. 550°C. Decomposition of the molten mass as well as oxidation to surfaces that come into contact with oxygen may occur when operating. The latter can be avoided, for example, by covering the collecting container with inert gas.

Generally more care is required with molten salts with regard to the chosen material.

Liquid metals:

These are popular in temperature ranges of up to 800°C. In individual cases 800°C may also be exceeded. High boiling points and excellent heat conductivity tend to result in good properties for heat transfer.

It is crucial that the environmental impact and the required safety precautions are taken into consideration when using liquid metals. The risk of an explosion is a given when using sodium. Overall, heat transfer media in the form of liquid metals have high operating costs.

Mass flow rate

A mass that passes the cross-section of a flow (length x, width y) based on a unit of time.

References:
H. Ebertshäuser / S. Helduser: Fluidtechnik von A bis Z (Liquid Technology from A to Z),
2nd Revised Edition 1995, Vereinigte Fachverlage, Mainz, ISBN 3-8023-0420-92.
http://de.wikipedia.org/wiki/Massenstrom (As of 07.03.2011 – 1200 hrs)

Nusselt number

Also known as the non-dimensional heat transfer coefficient. The Nusselt number indicates the relationship of the thermal boundary layer to the characteristic dimensions of the heat transfer medium.

References:
VDI-Wärmeatlas (VDI Heat Atlas): Berechnungsblätter für den Wärmeübergang
(Heat Transfer Calculation Sheets), Prod. Association of German Engineers,
VDI Society for Chemical and Process Engineering (GVC), 9th Edition 2002,
Springer Verlag, Berlin, Heidelberg, New York, Barcelona, Hong Kong, London,
Milan, Paris and Tokyo, ISBN 3-540-41201-82.
W. Wagner: Wärmeaustauscher (Heat Exchangers), 3rd Edition 2005,
Vogel Buchverlag, Würzburg, ISBN-10: 3-8343-3026-43.
W. Wagner: Wärmeübertragung (Heat Transfer), 6th Revised and Extended Edition 2004,
Vogel Buchverlag, Würzburg, ISBN: 3-8023-1974-54.
http://de.wikipedia.org/wiki/Nusselt-Zahl (As of 07.03.2011 – 1200 hrs)

Prandtl number

A non-dimensional ratio between the kinematic viscosity and the thermal diffusivity of a liquid. The Prandtl number can be regarded as a measure of the thickness ratio of flow and thermal boundary layer.

References:
VDI-Wärmeatlas (VDI Heat Atlas): Berechnungsblätter für den Wärmeübergang
(Heat Transfer Calculation Sheets),Prod. Association of German Engineers,
VDI Society for Chemical and Process Engineering (GVC), 9th Edition 2002,
Springer Verlag, Berlin, Heidelberg, New York, Barcelona, Hong Kong, London,
Milan, Paris and Tokyo, ISBN 3-540-41201-82.
W. Wagner: Wärmeaustauscher (Heat Exchangers), 3rd Edition 2005,
Vogel Buchverlag, Würzburg, ISBN-10: 3-8343-3026-43.
W. Wagner: Wärmeübertragung (Heat Transfer), 6th Revised and Extended Edition 2004,
Vogel Buchverlag, Würzburg, ISBN: 3-8023-1974-54.
http://de.wikipedia.org/wiki/Prandtl-Zahl (As of 07.03.2011 – 1200 hrs)

Reference temperature

This is the temperature to which the specified flow rate refers. The density of liquids changes with temperature and this influences the resulting mass flow rate. The unit is usually in [°C].

Reynolds number

A non-dimensional number that helps answer the question of the turbulence of a flow. A literature value for the critical Reynolds number is Rekrit=2300. This means the flow from the laminar flow passes into a transition area as there is no abrupt transition from laminar to turbulent flow. A Reynolds number of > 10,000 generally represents a (highly) turbulent flow. The Reynolds number indicates the ratio of the inertial force to the viscous force.

References:
VDI-Wärmeatlas (VDI Heat Atlas): Berechnungsblätter für den Wärmeübergang
(Heat Transfer Calculation Sheets), Prod. Association of German Engineers,
VDI Society for Chemical and Process Engineering (GVC), 9th Edition 2002,
Springer Verlag, Berlin, Heidelberg, New York, Barcelona, Hong Kong, London,
Milan, Paris and Tokyo, ISBN 3-540-41201-82.
W. Wagner: Wärmeaustauscher (Heat Exchangers), 3rd Edition 2005,
Vogel Buchverlag, Würzburg, ISBN-10: 3-8343-3026-43.
W. Wagner: Wärmeübertragung (Heat Transfer), 6th Revised and Extended Edition 2004,
Vogel Buchverlag, Würzburg, ISBN: 3-8023-1974-54.
http://de.wikipedia.org/wiki/Reynolds-Zahl (As of 07.03.2011 – 1200 hrs)

Thermal boundary layer

The thermal boundary layer is the part of a flowing fluid that is affected, for example, by the influences of two fluid temperatures or a fluid temperature and a wall temperature.

If the flow in the tube is examined then the fluid has a specific fluid temperature and the surrounding tube has a specific wall temperature. The area where the transition takes place from one temperature to the other is the area of the thermodynamic boundary layer.

References:
1. W. Wagner: Wärmeaustauscher (Heat Exchangers), 3rd Edition 2005,
Vogel Buchverlag, Würzburg, ISBN-10: 3-8343-3026-42.
W. Wagner: Wärmeübertragung (Heat Transfer), 6th Revised and Extended Edition 2004,
Vogel Buchverlag, Würzburg, ISBN: 3-8023-1974-53.
http://de.wikipedia.org/wiki/Thermische_Grenzschicht (As of 07.03.2011 – 1200 hrs)

Tube types

Different types of tubes may be used to build heat exchangers, depending on the particular application. The following are normally involved when it comes to finned tube and plain tube heat exchangers:

Plain tubes
Lamellar tubes
Spiral or bimetal finned tube

Volumetric flow rate

The volume of a fluid that passes the cross-section of a flow (length x width) based on a unit
of time.

You don’t need to know everything. If you know where to look it up.