Armacel l - Technical Ar t ic les Issue 4

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Key terms in low-temperature insulation: by Dipl.-Ing. Hubert Helms, Armacell GmbH

Part 4: WATER VAPOUR TRANSMISSION

In the case of low-temperature insulations there is the danger of moisture penetrating the insulation material.

The top priority when considering the design of low-temperature insulation is, therefore, not only to prevent condensation forming on the surface of the insulation material, but also to protect the material against the penetration of moisture.

If this danger is not eliminated, water and/or ice will form at those points in the insulation system where the temperature is below the dew-point temperature.

Water and ice must not penetrate the insulation system for the following reasons:

In the insulation material they reduce the insulation effect considerably, because water conducts heat around 20 times better than static air (λair ≈ 0.025 W/(m.K); (λwater ≈ 0.6 W/(m . K)). The thermal conductivity of ice is around 100 times higher. This not only leads to increas-ing energy losses, but in certain circum-stances also means that the insulation thickness determined in the dry state is no longer sufficient. This in turn results in additional condensation forming on the surface of the insulation material.

Water can cause corrosion on insulated plant and on the inside of any metal jackets. In the worst case this “creep-ing” corrosion can mean that the whole refrigerating plant has to be replaced.

It is also important not to underestimate the substantial weight gain due to water and ice, which can lead to static prob-lems – especially in combination with the corrosion processes mentioned.

Moisture transport through water vapour transmission

•

•

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How can moisture penetrate insula-tion materials?

Air is a mixture of several gases. At sea level, pure dry air contains approximately 78.1 vol. % of nitrogen, 20.9 vol. % of oxygen, 0.9 vol. % of argon, 0.03 vol. % of carbon dioxide, 0.01 vol. % of hydrogen and traces of further inert gases.

Apart from the substances mentioned, “normal” air also always contains a quan-tity of invisible water vapour which can be larger or smaller.

As already explained in the first part of this series, atmospheric (humid) air is a combi-nation of the two substances dry air and water vapour. Every gas of this compound generates a pressure which is also called partial pressure. Under normal ambient conditions, each individual gas in this gas mixture distributes itself as if to take up the whole of the space available – unhindered by the other gases. So the total pressure in a gas compound is calculated as the sum of all partial pressures. The following therefore applies for the total pressure (barometer reading) of humid air:

Only the partial water vapour pressure is of importance for processes concerning building physics.

There is a certain partial water vapour pressure (PD) for every temperature and relative humidity.

As we already know from Part 2 of this series, depending on its temperature air can only absorb a certain, limited amount of water vapour, i.e. depending on the temperature, the partial pressure of the water vapour can also only have a certain maximum value. The maximum partial pressure of the water vapour is known as the saturated water vapour pressure P

D.

If there are different temperatures and humidities on the two sides of the compo-nent/object, a vapour pressure difference arises as a result of the different water vapour partial pressures. Because pressures naturally endeavour to achieve a balance, the difference in pressure is the driving force behind water vapour transmission (see Model calculation and Figure 1). Water vapour transmission is the natural move-ment of the water vapour (the water vapour molecules) through building and insulation materials. Due to the temperature and par-tial pressure ratios, in refrigeration plants the diffusion current is generally directed at the insulated object. If the diffusing water vapour falls below the dew-point temperature, it condenses and builds up as moisture in the insulation material, with the possible consequences mentioned at the beginning of this article.

P = P

L+ P

D in Pa, hPa

(Pascal, Hectopascal)

PL = partial pressure of the dry air

PD = partial pressure of the water vapour

Model calculation for partial water vapor pressure:

Figure 1:

The driving force behind water vapour diffusion.

Temperature °C

Relativehumidity

Saturated water vapour pres-sure P

S

Partial water vapour pressure P

D

22

100 85

9.35 26.47

9.35 22.45

%

hPa

hPa

In the next issue:

Part 5 - Installing elastomeric insulation materials: Reliable bonds

6

Armacell-Tecnichal Articles | March 2006 | Page 2

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Every (building) material offers a differ-ent resistance to the migration of water vapour.

The most important parameters are:

. The water vapour diffusion coefficient δ (small delta)

. The resistance to water vapour diffusion factor μ

. The water vapour diffusion equivalent air layer thickness S

d

The water vapour diffusion coefficientThe water vapour diffusion coefficient indicates the amount of water vapour [kg] which diffuses through a layer of material which is 1 m thick and has an area of 1 m² at a partial water vapour pressure differ-ence of 1 Pa in 1 hour. (Figure 2)

The resistance to water vapour diffusion factor

The resistance to water vapour diffusion factor, also known as the µ-value for short, describes the ratio of the water vapour dif-fusion coefficient of the air δL to the value δmaterial of the building material in question (Figure 3).

The µ-value is a measure for the vapour tightness of a material. It indicates how many times greater the resistance to trans-mission of a layer of building material is compared to a static layer of air of the same thickness.

Water vapour diffusion equivalent air layer thickness

The following applies for the water vapour diffusion equivalent air layer thickness (sd-value) of a building material:

The Sd-value is the thickness of a static layer of air in metres, which displays the same resistance to water vapour transmission as the building material in the thickness s with the resistance to water vapour transmission value µ. As Figure 4 shows, the static layer of air would have to be 133 m thick to build up the same resistance to water vapour transmission as 19 mm AF/Armaflex.

As explained in this and the previous articles in the series, when insulating refrigerating plant it is essential to determine the correct insulation thickness in order to prevent condensation and to select an appropri-ate insulation material which will reliably protect the insulation against moisture penetration in the long term. However, good physical technical values are only one of several aspects when assessing and selecting a material. The best properties are no use if the insulation material is poorly installed. In the next part we will, therefore, present fundamental aspects of installation, taking elastomeric insulation materials as an example. Here adhesion will play a key role.

Water vapour diffusion Bulding equivalent air material layer thickness Mineral wool μ ≈3; s = 100 mm

Polyurethane μ ≈100; s = 100 mm

AF/Armaflexμ ≥ 7000; s = 19 mm

Sd

= 0.3 m

Table 1: Emision and absoption coefficients

of surfaces of insulation systems.

Issue 4: Water vapour transmission

Figure 2:

Difusion coefficient δ

Figure 3: Resistance to water vapour

sd = μ · s (m)

Sd

= 10 m

Sd = 133 m