WO1999046569A1

WO1999046569A1 - Leakproof thermostat

Info

Publication number: WO1999046569A1
Application number: PCT/EP1999/000814
Authority: WO
Inventors: Giuliano Toffoli; Erio Carlin; Fausto Ginelli
Original assignee: Ge Procond Elettronica S.P.A.
Priority date: 1998-03-12
Filing date: 1999-02-08
Publication date: 1999-09-16
Also published as: ITPN980019U1; ITPN980019V0; IT245304Y1

Abstract

The thermostat includes a variable-volume sealed chamber (15) that contains a working fluid with a high coefficient of thermal expansion, wherein said chamber is defined by expansible bellows means (4) communicating with a capillary tube (6). The capillary tube is attached to a cap (2) by means of a soldered joint (10) that is covered by a mass of hardened resin (14) adapted to prevent any working fluid from leaking from said sealed chamber (15).

Description

LEAKPROOF THERMOSTAT

DESCRIPTION

The present invention refers to an improved thermostat, in particular a thermostat of the type adapted for use m conjunction with refiigeration appliances

Thermostats of the above cited kind aie genei lly known to usually comprise a load-bearing casing that is closed by means of a metal covei on whose inner surface there is attached a bellows-like member communicating with an open end of a capillary tube made of copper or similar material, the opposite end of which is on the contrary sealed Said bellows element and the capillary tube form a sealed variable-volume chambei that is filled with a working fluid featuring a high coefficient of theimal expansion

In response to the temperatuie being detected by said capillary tube the working fluid m said chambei expands or contracts, thereby causing the bellows element to expand oi to shπnk, accordingly In coirespondence of a pre-determined expansion/conti action position, the bellows element is adapted to actuate at least an electric oi similar switch to open or close the respective circuit

Near its open end, the capillary tube is attached to the covei by means of a soldered joint that may foi instance be earned out with the use of a tin-lead alloy

For the theimostat to operate m an adequate mannei , the variable-volume chambei should of couise be peifectly sealed, le fully tight against any possible leakage of woikmg fluid contained theiem This has been hitherto achieved by using, as a working fluid some CFC-based medium such as Fieon or the like, le

-1- a substance that does not give nse to any substantial problem in connection with possible leakages through microcrackmgs and/or micropores as they may be present for instance in correspondence of the soldered joint of the capillary tube, le a particular point of the thermostat commonly known for its relative cnhcalness and fragility

The current need for all CFC-containing media and substances to be phased out and eliminated has led to Freons being replaced with propane gas, which, however, is generally known to be such as to easily give nse to problems of related working fluid leaking in correspondence of said soldered joint, thereby making the entire thermostat substantially unreliable

Such a problem is particularly aggravated m the case of thermostats intended for use, for instance, m refrigeration appliances, owing not only to the low working temperatures prevailing m such applications, but also, and mainly, to the fact that no use of lead is allowed in soldering alloys for use m applications involving handling and/or storage of foodstuffs In these cases, therefore, tin-lead alloys are replaced with a different alloy, such as for instance a tin-silver alloy, which is generally known to be less amalgamating and therefore favours the formation of porosity (with resulting osmosis processes) in correspondence of the soldered joint Furthermore said soldered joint turns out to be undesirably fragile, so that propane, which is an inflammable substance, can easily percolate, le escape from the variable-volume chamber, thereby giving rise not only to situations of unreliablity of the entire thermostat, but also to dangerous situations for the user of the related appliance

It therefore is a purpose of the present invention to provide a simple improved thermostat m which, while doing m any case away with the use of CFC-containing or anyway polluting media, all such drawbacks connected with pnor-art solutions are effectively I emoved

-2- In a more precise manner, it is a purpose of the present invention to provide a thermostat of the above cited kind which is particularly reliable and substantially free from problems arising from working fluid leaking therefrom

Yet another purpose of the present invention is to provide a thermostat of the above cited kind which is capable of being manufactured in a simple, quick and economic manner in a large-scale industrial production system

According to the present invention, such aims are reached m an improved thermostat embodying the features and characteristics as substantially recited in the appended claims

The features and advantages of the present invention will anyway be more readily and clearly understood from the description that is given below by way of non-limiting example with refeience to the accompanying drawing, in which the sole Figure appearing there is a schematical, partially cross-sectional view of a preferred embodiment of the thermostat according to the present invention

With leference to the above cited Figure, the thermostat may for instance be of the type adapted for use m refrigeiation appliances and mainly comprises a box-like load-bearing casing 1 with a closuie element or covering cap 2 that is preferably made of tinned iron

The cap 2 is piovided with an opening that is prefeiably made m correspondence of a I ecess 3 m the same cap

The thermostat further includes at least an expansible membei 4, prefeiably in the form of a bellows-like member and made for instance of such a material as bronze, which is attached m an an -tight manner to the inner surface of the cap 2 by means of at least a soldered joint 5 An appropriate soldering alloy for said soldered joint 5 is prefeiably constituted by a tin-silver alloy, but it may also be a tin-lead alloy or the like The thermostat also comprises at least a tubular temperature sensor 6, or the like, which may for example be constituted by a capillary tube of soft tinned copper Said sensor 6 has a sealed end 7 and an open end 8, wherein the latter is inserted m the opening provided in the cap 2 To this purpose, near the end 8 of the capillary tube there is provided a bulge 9 adapted to abut against the outer surface of the cap 2 The capillary tube 6 is attached in an air-tight manner to the cap 2, m correspondence of its open end 8, by means of at least a soldered joint 10 Even in this case an appropriate soldering alloy for said soldered joint 10 is preferably a tm-silver alloy, but it may also be a tin-lead or similar alloy

The bellows 4 communicate with the capillary tube 6, with which it so forms a sealed variable-volume chamber 15 containing a working fluid that has a high coefficient of thermal expansion This working fluid may for example be a Freon medium, but it prefeiably compnses propane or a similar CFC-free medium

In a per se known manner, said working fluid expands or contracts m accordance with the temperature being detected by the above cited temperature sensor 6, thereby causing the bellows 4 to expand or contract, accordingly In correspondence of a predetermined expansion/contraction position, the bellows 4 are therefoie adapted to actuate at least an electric switch 11 , or the like, so as to cause it to open oi close the associated electric circuit, wherein said switch is housed in the afoi e cited box-like casing and is associated to appropriate electric terminals 1 , 13 piotmdmg in an an -tight manner fiom the same box-like casing

According to the present invention, at least the soldered joint 10 of the capillary tube 6 is substantially covered by a mass 14 of hardened resm, which preferably includes Locate 662 (legisteied trademark)

Said Loctite 662 I esin is largely known to be usually employed in such simple applications as bonding and sealing of electronic component so as to protect them against watei moisture and dust Conversely in the improved thermostat according to the piesent invention such a resm may be deposited onto the soldered jomt 10 and be for example hardened in a quick and affective manner by means of the application of UN rays In this manner the surprising, experimentally verified result is obtained that the so hardened resm not only is effective m preventing any propane gas from leaking from said sealed chamber 15 in the case of microcracks or porosity coming to exist in the soldering alloy used for said soldered joint 10, but also substantially improves the mechanical strength in correspondence of the same joint

As a result, although said soldered joint 10 is carried out with the use of a soldering alloy that does not contain lead (and therefore is compatible with foodstuffs, but less amalgamating than usual alloys), and although propane (or similar medium nt containing any CFC, but exibitmg a marked tendency to leaking through microcracks or pores) is used as a working fluid, any leakage of working fluid from the sealed chambei 15 is effectively and advantageously prevented

The hardened resm mass 14 may of course be of any other type adapted to answer the particular purpose In particular, it may be any other type of resm (le. a substance that is normally in a semisohd, le viscous state with a good wetting property) featuring an adequate sealing power, mechanical strength, an adequate resistance to working temperatures, further to the capability of being used m a simple, convenient manner m industrial manufacturing environments In a preferred manner, such a resm should of couise be free from emissions that might affect the final testing of the thermostat

As it has also been found experimentally, an alternative lesin 14 capable to meet the above stated requirements may for instance include the Loctite 151869

(registered trademark) or Loctite 3501 (registered trademark) materials which find usually application as sealant for electronic components and adhesive for simple bonding applications, respectively

Anyway, in view of an easiei and more reliable application on an industrial scale, the mass 14 of hardened resm is preferably applied to cover the soldered joint 10 in coi respondence of the outei surface of the cap 2 and can be most conveniently deposited into the recess 3 of the same cap It will be appreciated that the above described thermostat may be the subject of any of a number of variants and modifications without departing from the scope of the present invention. For instance, in order to further eliminate any possibility for the working fluid to leak from the chamber 15, even the soldered joint 5 may be substantially covered with a mass of hardened resin 16 similar to the mass of resin 14.

-6-

Claims

1. Thermostat comprising at least a vaiiable-volume sealed chamber containing a working fluid with a substantially high coefficient of thermal expansion and defined by at least an expansible member arranged to communicate with a tubular temperature sensor or the hke, said tubular sensor being attached to a covermg member by means of at least a soldered joint, characterized m that said soldered joint (10) is subtantially covered by a mass of hardened resm (14) adapted to prevent working fluid from leaking from said sealed chamber (15)

2 Thermostat according to claim 1 , characterized in that said mass of resm (14) substantially covers the soldered joint (10) m coi respondence of the outer surface of said covermg member (2)

3 Thermostat accoidmg to claim 2, wheiem the temperature sensor is attached to the covermg member in coi respondence of a recess provided in the same covermg member characterized in that said resm is deposited m correspondence of said recess (3) in the covermg member (2)

4 Thermostat according to claim 1 , characterized m that said hardened resm (14) is a substance having a high sealing power, a high mechanical strength, is resistant to the woikmg temperatures of the thermostat, and is substantially free from emissions

5. Thermostat according to claim 1 , characterized in that said resm (14) comprises Loctite 662 (registered trademark)

6. Thermostat according to claim 1 , characterized in that said resm (14) comprises Loctite 151869 (registered trademark)

7. Thermostat according to claim 1 , characterized in that said resm (14) compnses Loctite 3501 (registered trademark).

8. Thermostat according to claim 1, wherem said expansible member is attached to the inner surface of said covermg member by means of at least a further soldered jomt, characterized in that even said further soldered jomt (5) is substantially covered by a mass of hardened resm (16) capable of preventing working fluid from leaking from said sealed chamber (15).