Circuit water

Circulating water is an aqueous liquid that is constantly used in a process and is usually pumped in a cycle . In technology, circulating water is used as a cooling medium for cooling, i.e. to dissipate thermal energy, in plant cultivation (especially in hydroponics ) to reuse the added fertilizers and nutrients in the irrigation and seepage water .

A distinction is made between a closed circuit, where the circuit water never comes into direct contact with the atmosphere or the process itself, so that an exchange of substances can occur, and an open circuit, where an exchange of substances is desired. Often additives are added to the circulating water.

Obtaining the circulating water

As circulating water, the water can either be taken directly from a holder or it is cleaned beforehand in a water treatment facility. This is often followed by conditioning with additives. Depending on the application, the finished water is called process water or fill and make-up water or, in steam boiler technology, feed water .

Treatment and conditioning of the circulating water

The conditioning is carried out in accordance with the process engineering or equipment requirements in order to prevent one or more of the following properties of the circuit water:

If necessary, the following procedural processes must be carried out in order to achieve the properties of the circuit water:

Effects of incorrect processing

If the circuit water is not or correctly processed, boiler scale formation very often occurs with heat generators .

Effects of improper conditioning

Sodium hydroxide induced stress corrosion cracking

When using fully demineralized water with low conductivity (<50 µS / cm), stress corrosion cracking can occur after conditioning with caustic soda alone, which is used to raise the pH value . In shell boilers, we therefore advise against the sole use of sodium or potassium hydroxide as an alkalizing agent and instead recommend trisodium phosphate.

Circulating water guidelines

Continuous water analysis station for make-up and circulating water in a power plant. The pH and conductivity are measured

There are different guidelines depending on the application:

For steam boilers and hot water boilers, see the guidelines for boiler and feed water .
For circuit water in closed heating systems up to a flow temperature of max. 100 ° C, ÖNORM H 5195-1 applies.
There are several guidelines for district heating systems , those of the VGB of the AGFW and those of TRD 612. According to TRD, monitoring according to internal instructions is required either continuously and in a register or discontinuously. The hot water systems can be operated with low-salt or high-salt circulating water, whereby many years of operating experience have shown that low-salt water offers advantages, since problems caused by oxygen-related corrosion are reduced. In this way, when operating with low-salt circuit water, the conditioning can be simplified and the use of oxygen binding agents can be largely dispensed with. Filling and top-up water should be treated so that it is free from alkaline earths (hardness). The circuit water must be kept free of alkaline earths (guide value <0.02 mmol / l) and be clear and free of suspended substances.

Guide values for circuit water in hot water boilers	unit	low in salt	salty	salty
Electrical conductivity at 25 ° C	µS / cm	≤ 30	> 30-100	> 100-1500
pH value at 25 ° C		9.0-10.4 ^*	9.0-10.5 ^*	9.0-10.5
Oxygen (O ₂ )	mg / l	<0.1	<0.05	<0.02

^∗ In shell boilers, the pH value should primarily be adjusted with trisodium phosphate and sodium hydroxide solution should only be used if the desired pH value cannot be achieved with trisodium phosphate.

application

literature

VGB guideline M 410 N: Quality requirements for district heating water ( content )
AGFW guideline, FW 510 Requirements for the circulation water of industrial and district heating systems, as well as instructions for their operation

Individual evidence

[1] VGB guideline leaflet on quality requirements for district heating water, Chapter 3.3

[2] ttps://www.vgb.org/en/dt_2012_vortraege_grubmah_mai-dfid-44815.html

[3] TRD 611 Technical rules for steam boiler operation of feed water and boiler water from group IV steam generators

[4] Heating water , accessed on November 20, 2016.