Danfoss Guideline of Water Quality for copper brazed Plate Heat Exchangers User guide

Guideline of Water Quality for copper brazed Plate Heat Exchangers

0 Summary

Danfoss District Heating has prepared this guideline for the water quality of tap water and district heating water used in plate heat exchangers of stainless steel (1.4404, X2CrNiMo17-12-2 acc. to EN 10088-2:2005 ~ AISI 316L) brazed with pure copper.

The water flowing in these brazed plate heat exchangers (PHEX) varies a lot from application to application and corrosion can become a problem in some situations. This guideline is based on a comprehensive literature survey and on our experiences from many years using copper brazed stainless steel PHEX.

It is important to point out that this water specification is not a guarantee against corrosion, but must be considered as a tool to avoid the most critical water applications. A summary of the parameters and their recommended limits are listed in table 2 for water on the secondary side (tap water, drinking water) and table 3 for primary water (heat supply, district heating water). These limits are only valid for PHEX made of stainless steel 1.4404 brazed with pure copper.

1 Introduction

Danfoss District Heating has prepared this guideline for the water quality of tap water and district heating water used in heat exchangers of stainless steel (1.4404, X2CrNiMo17-12-2 acc. to EN 10088-2:2005 ~ AISI 316L) brazed with pure copper. Normally, tap water (drinking water) flows in the secondary side and a heating media (e.g. district heating water) flows in the primary side of the heat exchanger.

Surfaces in contact with water can be subject to two problems, scale formation and corrosion. Gases and salts being dissolved in the water play the major role; besides that, component design (e.g. design, materials used, fabrication processes) and operating conditions (e.g. temperature, flow conditions, times of stagnation) influence the risk for scaling and/or corrosion.

Furthermore it must be kept in mind that the reaction rate of chemical reactions, e.g. the corrosion rate, increases with increasing temperature. According to van’t Hoff’s rule, the increase is in the order of factor 2 to 3, for every 10 °C of temperature increase.

Knowing the chemical water composition and the operating conditions of a heating system, the risk for scaling and corrosion can be evaluated. Based on that, recommendations in order to avoid scaling and/or corrosion problems in components can be given. This is the intention of this water specification.

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1.1 Scale formation

Raw water used for the production of drinking water (tap water) contains more or less high amounts of dissolved gases and salts depending of the geological properties of the extraction area. These differences cause a different composition also in the finally produced drinking water. For the formation of scale, especially carbonate hardness (= content of hydrogen carbonate) and total hardness, i.e. sum of calciumand magnesium ions, are determinant; besides that, other ions like e.g. sulphate can have an influence.

From the compounds mentioned above, lime scale (boiler scale, calcium carbonate, CaCO3) can be formed under increasing temperatures and/or loss of carbon dioxide, e.g. by degassing). Further temperature increase might lead to deposition of different salts, e.g. gypsum (CaSO4).

Other compounds being able to cause blocking of components are iron containing deposits like „rust“, i.e. iron oxides and –hydroxides, or magnetite. These can be built in the PHEX itself, but can also be flushed in from other parts of the entire system, being formed due to corrosion processes elsewhere in the system.

1.2 Corrosion

Corrosion can be caused by different mechanisms resulting in many types of corrosion. Some of these can take place in a PHEX during service. Most of the corrosion mechanisms are caused chemically, whereas the chemical composition of the water influences the different materials differently.

Besides of the factors mentioned above (material, operating conditions…), oxygen content plays a major role in corrosion of metals. Furthermore, pH-value (acid concentration), acid capacity (buffering capacity), salt content, are important parameters for corrosion to occur. Insofar, knowledge of these is crucial for the evaluation of possible corrosion risks.

A detailed explanation of the different types of corrosion would go beyond the scope of this report; however, an overview of the most typical corrosion types is given in the following table 1.

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Table 1 Typical corrosion types in copper brazed stainless steel plate heat exchangers [12]

Corrosion type	Description
General corrosion	If general corrosion takes place in a PHEX it is typical copper
	that will corrode and not stainless steel. If the copper brazing
	corrodes it will result in loss of mechanical strength and
	possibly leaks in the heat exchanger.
Crevice corrosion	Normally the heat exchanger is free of crevices, but crevices
	can be formed under deposits from scaling and other kinds of
	deposits as well as imperfect brazing joints.
Galvanic corrosion	Metallic contact between copper and stainless steel in water of
	high electrical conductivity can initiate a corrosive attack of the
	more electronegative metal, in this case copper.
Stress corrosion cracking	Stress corrosion cracking (SCC) can occur in stainless steel if
	tensile stresses and a high amount of chloride are present. An
	increase in temperature will furthermore increase the risk of
	SCC; it will often take place at temperatures above 60 °C. [14]
Intergranular corrosion	Stainless steel can experience intergranular corrosion due to
	formation of chromium carbide in the grain boundaries during
	improper heat treatment. Areas with decreased chromium
	content will become sensible towards corrosion.
Liquid metal embrittlement	If the brazing process takes place at too high brazing
	temperatures, copper can diffuse into stainless steel and
	decrease the strength of the stainless steel plates.

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