Valve, in particular expansion valve for refrigeration systems,

Details Valve, in particular expansion valve for refrigeration systems, Valve, in particular expansion valve for refrigeration systems,

1996-02-07

1998-09-22

Chambers, A. Michael

Valves and valve actuation

Valve

Materials

251356, F16K 5100

Patent

active

058103325

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a valve, in particular to an expansion valve for refrigeration systems, having a housing and at least one nozzle which are joined to one another by soldering; the invention also relates to a method for the manufacture of the valve.
Commercially available expansion valves have a housing of brass into which, in some cases, nozzles of copper are soldered. Brass housings of that kind become discoloured at the surface as a consequence of the so-called "patina". This phenomenon is undesirable in the food industry and in other applications. In addition, the visual impression suffers. For that reason, it is known to provide the surface with a layer of nickel. But this is said to lead to health problems, namely to nickel allergy, which is very widespread. In addition, a careful watch is kept on heavy metals in the natural-food chain, as nickel salts are suspected of causing cancer.
The invention is based on the problem of providing a valve of the kind described in the introduction which is better suited to practical applications.


SUMMARY OF THE INVENTION

This problem is solved according to the invention in that at least the housing and all nozzles are deep-drawn parts of stainless steel which has such a low carbon content that despite being subjected to the effects of heat during soldering it is practically insensitive to intercrystalline corrosion.
Stainless steel, as defined in DIN 17441, contains at least 12% chromium. Stainless steel does not tend to discolour. It is not harmful to the environment, neither does it give rise to health problems. It is, however, necessary to use a steel with a very low carbon content, because otherwise the heat treatment associated with the soldering process causes sensitisation to so-called intercrystalline corrosion (for example, through separating out of chromium carbide), which occurs in moist environments or environments containing water vapour and which ultimately affects the strength and seal of the valve.
If a low proportion of carbon is chosen, however, machining of the steel becomes more difficult and more expensive compared with steels containing larger proportions of carbon. Housings and nozzles are therefore in the form of deep-drawn parts. A valve (expansion valve, magnetic valve, check valve and so on) which is highly suitable not only for refrigeration systems but also for the food industry and for similar conditions of use is therefore produced.
The stainless steel may, in particular, contain chromium and nickel, and in particular be a chromium-nickel-molybdenum steel. Nickel improves the deep-drawing and soldering properties. Molybdenum counteracts corrosion associated with splitting and corrosion associated with stress.
The chromium carbides mentioned above are formed at temperatures between 500.degree. and 900.degree. C., the fastest rate of separating-out lying between 600.degree. and 700.degree. C. How long it is admissible to operate in these ranges depends on the carbon content of the steel. It is especially recommended for the stainless steel to contain less than 0.05% of carbon. A soldering time of 6 to 7 minutes is then admissible, which is a typical period for a soldering process between 600.degree. and 700.degree. C. If, however, soldering takes place at higher temperatures and if the critical temperature range is passed through more rapidly during cooling, then somewhat higher proportions of carbon, such as 0.055 or 0.06%, are also acceptable.
All in all, a stainless steel having almost the following composition is recommended: C.ltoreq.0.06%; Cr=12 to 22%; Ni=6 to 18%; Mo=0 to 6%; rest =Fe. To this may be added elements such as P, S, Si and/or Mn, in the usual small quantities.
In most cases it is preferable if the stainless steel contains less than 0.05% C.
The best results have been achieved with stainless steel having the following composition: C<0.06%; Cr =16 to 20%; Ni=8 to 15%; Mo=0 to 4%, rest =Fe.
It is advisable for the nozzle to have a flange that is soldered onto the outer surface

REFERENCES:
patent: 4357807 (1982-11-01), Aleksandrow
patent: 4633681 (1987-01-01), Webber
patent: 4951920 (1990-08-01), Tsuno

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