Design of cold insulation to prevent formation of condensation on the surface

 

POWER PLANT

Design of cold insulation to prevent

                 formation of condensation on the  surface

1.       Introduction

 

The most frequent requirement in the design (determination of insulation thickness) of cold insulations is the prevention of condensation forming on the surface.

The whole discussion in this document is based upon the recommendations of Technical Letter No. 7 „Principles of cold insulation“, which describes the physical principles leading to the formation of condensation when tem­perature differences occur in gases carrying water vapour. Rules for the construction, which result from these physical principles and are independent of the conditions of the individual design, are discussed there.

 2.       Condensation on insulation surfaces

 

Condensation on the surface of cold insulation systems generally does not impair the effect of the insulation. Nevertheless, the prevention of condensation on the surface of insulation is a frequent criterion for the determination of the insulation thickness, since condensation is undesirable for several reasons:

-      Dripping condensed water could for example adversely affect neighbouring production areas.

-      Condensation water could lead to corrosion at the surface of the insulation and subse­quently at other parts of the installation reached by it.

-      The formation of condensation leads to general dirtiness of the installation.

 3- Relevant factors

 

To calculate the insulation thickness sufficient to prevent condensation formation on the surface, the factors given in Table 1 must either be known, be assumed, or agreed with the operator of the installation

Table 1: Checklist - Information needed for the design of cold insulations

Responsibilities of the client and the contractor

 

				Angabe Unit	AG CL	AN CO	Bemerkungen Remarks
1	Mediumtemperatur Normalbetrieb 1)	konstant constant		°C	X
	Medium tempera­ture	wechselnd/ changing/	Min.	°C	X
	(normal service conditions) 1)	gleitend flexible	Max.	°C	X
2	Umgebungsluft	Temperatur Temperature		°C	X
	Ambient air	Relative Feuchte Relative humidity		%	X
3	Dämmstoff Insulation material	Art/Produkt Type/product			X	B A
		Nennwert der Wärmeleitfähigkeit gem. VDI 2055 Declared thermal conductivity acc. to VDI 2055		W/(m . K)		X
4	Wärmeübergang Surface heat	Geometrie/Zeichnung Geometry/drawing			X
	transfer	Konvektion/Strahlung	Wind 2) Wind 2)	m/s	X	B A
		Convection/radiation	Ummantelungsmaterial/-beschichtung Casing material/-coating		X	B A
			Randeinflüsse 3) Other factors 3)		X	B A
5	Isolierschichtdicke Insulation thick­ness			mm		X
6	Brandschutz Fire protection	Anforderungen: Requirement:			X	B A
7	Schallschutz Sound insulation	Anforderungen: Requirement:			X	B A
8	Hygiene Hygiene	Anforderungen: Requirement:			X	B A
AG = Auftraggeber CL = Client/Customer		X = gibt an;                                       X = provides;		1) Störfallsicherheit ist ggf. gesondert zu ver-    einbaren
		ý Verantwortlichkeiten í ý Responsibilities        í		1) Safety in case of accidents needs an ad-    ditional agreement, when required
AN = Auftragnehmer CO = Contractor		B = berät                                       A = advises		2) in geschlossenen Räumen: freie Kon-    version
				2) in buildings:  free convection 3) z. B. benachbarte strahlende Flächen (siehe    auch Technischer Brief Nr. 5, Abschn. 2) 3) e. g. neighbouring radiating surfaces (see    also Technical Letter No. 5, section 2)

 

The table shows which data are the respective responsibilities of contractor and client (X = provides) and where the contractor is sup­posed to advise the client (A = advises).

3.1     Medium temperature

 

The medium temperature must be known to allow for the selection of the insulation mate­rial. For variable service conditions and for service conditions with changing temperatures, the respective minima and maxima must be given.

 3.2     Ambient air

 

The information concerning the condition of the ambient air is especially important. The insulation needs to be calculated so as to prevent condensation formation on the surface of the insulation provided the agreed conditions of the ambient air prevails.

The relative humidity and temperature of the ambient air, upon which the design shall be based, must be decided by the client and agreed upon in the contract. Whilst taking that decision it must be remembered that a relative humidity of > 85% leads to very high and thereby uneconomic insulation thicknesses and should, therefore, not be assumed as a design condition.

If - as is recommended - the design condition is not based upon extreme ambient condi­tions, it will be temporarily exceeded during the (actual) operation of the installation. This may lead to condensation on the surface of the insulation. Normally, this is not critical in the open air.

However, if in buildings the dripping of condensed water on products or other instal­lations must be prevented under any circum­stances, this can only be achieved by taking additional measures (see section 4).

3.3       Insulation materials

 

The insulation material is selected on technical and economic grounds. Where the client decides on a specific insulant, the contractor has to be consulted regarding the suitability of that insulant.

The determination of the declared thermal conductivity is according to VDI 2055 (available in English). The design values of thermal conductivity are to be calculated by the contractor

3.4     Surface heat transfer

 

The surface heat transfer from the ambient air onto the insulation surface is expressed nu­merically in the form of the surface coefficient of heat transfer a. In determining a, the follow­ing must be considered:

-      for the calculation of insulation thicknesses, designed to maintain a given surface tem­perature, it is of decisive influence and

-      smaller surface coefficients of heat transfer lead to larger insulation thicknesses.

 Therefore smaller surface coefficients of heat transfer must be selected to be on the safe side.

To gauge the surface heat transfer conditions, knowledge of the geometry of the installation and the positioning of its different components are required

Assumptions must be made especially regard­ing the radiation conditions of cold insulation surfaces in radiation

 exchange with other ra­diating surfaces and regarding the influence of convection

It must be taken into account that close spac­ing and neighbouring cold surfaces consid­erably decrease the

 surface heat transfer

Reliable conclusions are possible only in rare cases, even if the entire installation geometry is known precisely.

In many specifications for external installations, it is required to determine the surface coefficient of heat transfer at a wind speed of 5 m/s. This applies - following the above reasoning - to heat loss calculations; for the calculation of surface temperatures and the consequential insulation thicknesses, still air and free convection should be assumed

3.4.1  Convection - radiation

The surface heat transfer from the ambient air to the insulation surface consists of the com­ponents a_C and a_R for radiation.

a_total = a_C + a_R

 

Convection is the movement of air by which heat is transferred onto the installation. This movement can be caused by wind or artificial ventilation or it can occur naturally as free convection, as colder and therefore heavier air in the immediate vicinity of the insulation flows downward.

If the air movement is hampered through closed spacing conditions, e. g. in lowered ceilings and room corners, the surface heat transfer through convection decreases.

The thermal radiation absorbed by the insula­tion surface depends upon its absorption coef­ficient a. A black surface with a » 1 absorbs the major part of the oncoming radiation, whilst a bright surface with a << 1 reflects the major portion.

Surfaces with low absorptivity, therefore, pos­sess a lower surface heat transfer. This leads to high-temperature differences, and too large insulation thicknesses required.

The absorption coefficient a is rarely men­tioned in the literature. However, in In many cases, the rule applies that the absorption co­efficient a equals the emissivity e which is given for insulation surfaces in VDI 2055, table 6 and copied in Table 2 below.

3.4.2  Total surface coefficient of heat transfer a total

 Table 2 shows the total surface coefficients of heat transfer for different casing materials. Underlying is the assumption of a convection part of the surface coefficient of heat transfer of a_C = 2 W/(m^{2 .} K), which applies to horizon­tal pipes with a temperature difference of

4,5 K.

Table 2: Surface coefficients of heat transfer

 

Ummantelung Casing	e	aStr aR W/(m2 . K)	agesamt atotal W/(m2 . K)	Nr. im Diagramm No. in diagram
Aluminium, blank Aluminium bright	0,05	0,27	2,27	1
Aluminium, oxidiert, nichtrost. aust. Stahl Aluminium, oxidised, stainless austenitic steel	0,13	0,70	2,70	2
Stahl, verzinkt, blank Steel, galvanised, bright	0,26	1,50	3,50	3
Stahl, verzinkt, verstaubt Steel, galvanised, dusty	0,44	2,50	4,50	4
farbbeschichtetes Blech, Schaumglas, Elastomer- schaum, Kunststoff- ummantelung Paint-coated sheet metal, cellular glass, flexible elastomeric foam, plastic casings	0,90	5,00	7,00	5

One can see that the total surface coefficient of heat transfer increases with increasing emissivity. This means that the temperature difference between the ambient and the sur­face of the insulation decreases, and that a smaller insulation thickness suffices.

The consequence of the varying radiation behaviours of these surface materials for the insulation thickness required for condensation prevention is shown in the diagram overleaf depending on the prevailing relative humidity.

 3.5     Insulation thickness

 

The discussion above leads to the important result:

 

The insulation thickness required for condensation prevention is dependent upon the radiation conditions at the insulation surface and thereby upon the casing material chosen.

This is shown in an exemplary way in the diagram below:

 Diagram: Insulation thickness to prevent condensation formation

 

-      If using galvanised steel sheet or paint-coated steel sheet, lower insulation thicknesses are required than if using aluminium casings.

-      If the insulation of cellular glass or flexible elastomeric foam is cased with an uncoated sheet metal, the insulation thicknesses required are increasing.

-      In some cases, condensation on an insu­lation surface with sheet-metal casing can be prevented by the later application of paint. Each non-metallic paint (e. g. not sil­ver bronze) can be used, including white.

 It may appear strange that even white shows a high emissivity since on the one hand a high absorptivity is required for this, on the other white is characterised by the fact that it does not absorb incoming light, but instead almost totally reflects it.

The explanation for this is that white paint - in the same way as snow, ice or haw frost - be­haves differently in the area of visible light and the area of infrared radiation, which is decisive for surface heat transfer. Whilst in the area of visible light, the radiation is reflected, the infra­red radiation is absorbed almost completely, so that white and also other colours behave in this area almost like a black surface.

As a general conclusion, it can be summa­rised:

-      The brighter a surface, the „colder“ it is.

Paint coatings, oxidation layers, dust and dirt result in „warmer“ surfaces

3.6     Fire protection

In cases where the cold insulation needs to be installed in fire-risk areas or similar safety zones, for example:

-      fire safety zones in nuclear power plants,

-      control rooms in chemical plants,

-      escape routes in public buildings,

-      mines,

-      marine and off-shore installations

fire protection takes precedence over lower thermal insulation thickness or insulation costs

 Not only must the fire classification of the insulation material proper, e. g. according to DIN 4102, be heeded, but additionally the behaviour of the entire cold insulation system and its contribution to the total fire load, and possibly its fire resistance class. Details to be taken from the relevant fire protection directives.

Points for the consideration of fire behaviour are:

-      insulation material (building material classes A1, A2, B1, B2 acc. to DIN 4102),

-      casing material (metallic materials, plastics, gypsum, mastic),

-      adhesives (gap fillers, sealing compounds, erection aids),

-      vapour retarder, abrasion protection and other coatings,

-      mechanical fastenings (wire, strings, bands, adhesive pins),

-      stockpiling on the building site (increasing the fire load through ignitable components).

     

 Only when all components mentioned are brought into harmony in fire-protection aspects, and when all safety directives and re­quirements have been met, can the selection of the insulation material and with that the calculation of the insulation thickness required for condensation prevention takes place.

3.7        Sound insulation

Additional requirements of sound insulation are critical for cold insulations. The closed-cell insulation material necessary for prime thermal reasons (see FESI Document 08) possess bad acoustically protective properties

Examples for the composition of cold insulation systems with simultaneous acoustic-protective requirements are given in DIN 4140 and in AGI working document Q 03 (available in English).

3.8       Hygiene

For insulated installations, e. g. in the food industry, special requirements for hygiene may be required. Especially were parts of the installation need to be cleaned or disinfected, the compatibility of the casing with cleaning agents used is of decisive importance for the operating safety of the installation. Special attention is needed for the sealing of the cas­ing where wet cleaning, e. g. with high-pres­sure cleaning machines, occurs, to keep the danger of moisture ingress into the insulation as low as possible

Materials used must not be toxic. For plastic materials, paints, metals or alloys, special directives apply. Prior to their employment, manufacturers must be consulted.

Materials must be resistant to disinfecting agents and against corrosion. It is therefore recommended, e. g. for milk-processing op­erations, only to use casings of stainless aus­tenitic steel with designation codes 1.4541 or 1.4571.

4.       Additional measurements when leaving service conditions assumed for the design

These additional measurements must be taken in cases where the conditions assumed have been left „unfavourably“, which means in direc­tion of increasing condensation formation danger.

4.1     Gutters and troughs

Gutters can take the dripping water along the length of piping. They require a minimum inclination of 10 mm/m and an exit to an exist­ing drainage. Axes of pipe and gutter must be in alignment. The width of the gutter should equal the diameter of the casing. The mini­mum should be 2/3 of that diameter.

To receive dripping water in critical areas, troughs are employed below the dripping points. They need drainage.

Gutters and troughs should have a distance of at least 50 mm from the lower surface of the insulation

When installing gutters and troughs, the accessibility, e. g. of operating appliances, must be taken into account.

4.2     Enforced ventilation

Wherever gutters and troughs cannot be em­ployed, enforced ventilation is an alternative to prevent the formation of condensation when the design conditions are being left.

Enforced ventilation is only effective in a lim­ited area. Heating the ventilator air can im­prove the drying effect.

4.3     Additional insulation

If one realises during operation that the condi­tions assumed are being left permanently, it must be taken into consideration to increase the existing insulation thickness

Whilst applying an additional insulation layer, it must be determined whether the vapour retarder and the metal casing need to be dis­assembled to arrive at defined insulation conditions. The additional insulation needs to be equipped with a vapour retarder in any case and if needed also with an mechanical protection