The wax-filled thermostatic element was invented in 1936 by Sergius Vernet (1899-1968). Its principal application was in automotive thermostats used in the engine cooling system. Wax thermostatic elements transform heat energy into mechanical energy using the thermal expansion of waxes when they melt. In addition to engine cooling systems, this wax motor principle also finds applications in heating system thermostatic radiator valves, plumbing, industrial, and agriculture. Today this technology is widely used across a broad spectrum of industries including aerospace & defense, most often for temperature control of various fluid systems.

Thermostatic Actuators

The operation of a wax-filled thermo-static actuator is based on the principal that there is a significant change in volume of a paraffin wax as it goes through a phase change from liquid-to-solid; solid-to-liquid as the wax temperature increases and decreases. The volume change is transduced into a linear, repeatable mechanical motion. Due to the non-compressible nature of the wax, this motion can also produce a significant amount of force. It has a very high-power density.

The basic elements of a thermostatic actuator include:

  • Wax – motion producing element.

  • Cup – contains the wax and other key compounds.

  • Diaphragm – seals in the wax and creates motion during expansion.

  • Guide – retains the Diaphragm creating a seal while guiding the Plug and Piston.

  • Plug – transmits and amplifies the wax expansion via the Diaphragm increasing stroke.

  • Anti-Chafing Disc – prevents the Plug from extruding around Piston when force is applied.

  • Piston – transmits the Plug's movement into useable stroke.

How the Technology Works

The Thermoloid® wax material that makes all of ThermOmegaTech's thermostatic valves operate is sealed inside the actuator, and as temperature increases above the melting point of the material, it expands in volume and pushes against a diaphragm, which in turn pushes on a piston. The piston acts as a valve stem, opening or closing a valve or other mechanical device. As the material cools below its melting point, its volume decreases and a spring or some other external applied force returns the piston and diaphragm to the “cold position”. The phase change, and resulting large volume change, produce motion over a narrow and highly predictable temperature range, and the temperature range at which the phase change occurs can be varied depending on the chemical composition of the Thermoloid material that is used.

The material operates at temperatures ranging from -150°F to 300°F (-101°C to 149°C). Direct Acting Valves are designed to be open when the actuator is in the “cold position” while Reverse Acting Valves are open in the “hot position”.

Because of the gradual transition of the phase change of the Thermoloid material, these valves act more like modulating valves, as opposed to on/off valves, and the precise motion of these thermal actuators can be used to operate a wide variety of devices, limited only by the imagination of the designer. No external power or signal is required, making these valves ideal for many hazardous and extreme environments.

Thermostatic Valves

Thermostatic, self-actuating, valves have the previously discussed thermostatic actuators as their primary element. These valves come in many shapes, sizes and temperature ranges based on the particular application, all designed to modulate the flow of a fluid through the valve based either on the fluid's temperature or ambient temperature.

The basic elements of a thermostatic valve include:

  • Thermal Actuator – produces linear motion, causing the valve to open/close based on increasing/decreasing fluid or ambient temperature.

  • Plug – interface between actuator and output port of valve; positioning of the plug on either side of the output orifice determines whether the valve is direct acting (flow decreases as fluid temperature increases) e.g. freeze protection valve; or reverse acting (flow decreases as fluid temperature decreases) e.g. scald protection valve.

  • Operating Spring – produces an opposing force on the actuator piston to ensure that piston fully retracts back into the actuator as the wax temperature (volume) decreases.

  • Valve body – housing of the valve that contains the thermal actuator, plug and spring. Provides the external connections for the fluid to flow into and out of the valve.

These thermostatic valves can be used in many applications including:

  • Freeze Protection – Protection of piping, valves, fittings, pumps, safety showers, condensate systems, fire lines, spray nozzles, or as backup protection on traced systems.

  • Scald Protection – Protection of overheating in safety shower/eyewash stations to protect plant personnel from being scalded.

  • High Fluid Temperature Shutoff – Protection of downstream analyzers, instruments and personnel in sampling systems from high temperature damage.

  • Pump Relief – Protection of pump and pump seals from over-temperature damage. These valves can be used for thermal relief of booster pumps; controlling cooling water outlet temperature; and controlling flow of cooling water, glycol or other cooling media in applications requiring economical removal of heat from equipment or a process.

  • Drain Tempering – Tempering of the high temperature discharge flow to a drain or sewer with cold water to meet the plumbing code requirements of temperatures less than 140°F going to the drain. Equipment such as industrial dishwashers, steam humidifiers, autoclaves, etc. can have effluent discharge in excess of 180°F and must be tempered to avoid costly consequences - failed inspections, fines, business interruptions, or injury and damage to personnel and equipment.

  • Enclosure Temperature Control – Controlling the temperature in an instrument or analyzer enclosure to keep equipment from freezing or overheating.

  • Domestic Hot Water Balancing – Balancing of hot water recirculation systems by automatically and continuously maintaining a set water temperature at the end of each domestic hot water supply. This solves the problem of delayed hot water delivery while eliminating the need for time consuming, expensive manual balancing procedures and equipment. By allowing only fluid that cools below the set point to flow to the return, the need for oversized recirculation pumps which lead to pipeline erosion and energy waste is eliminated.

  • Mixing/Diverting Valves – 3-way mixing or diverting applications. For mixing applications, the M/D will proportion the flow from two inlet ports to produce the desired outlet port temperature. For diverting applications, the M/D will divert or switch the inlet flow to either of two outlet ports depending on the fluid temperature.

Thermostatic Valve vs Electronic Sensing/Solenoid Valve for Temperature Control Solutions

The unique characteristics of a thermostatic valve are that temperature sensing, fluid control (open or closed loop) and valve position modulation are all inherently contained in the valve without the need for an external temperature transducer or external power. This has a tremendous advantage in a number of applications where the traditional solution requires a temperature sensor, amplifier, control electronics, solenoid valve and DC Power. The thermostatic valve is a highly reliable, repeatable, compact, low cost device for many fluid temperature control applications.

Aerospace & Defense Applications

Thermostatic actuators and valves have a long history of success meeting the tough demands of the aerospace & defense Industry. This includes attributes such as high reliability, compact size and weight, maintenance free, performance repeatability, self-actuated (no external power required) and many others. In addition to standard valve parts being machined with brass and stainless steel, valves can also be machined from other metals such as titanium, Inconel, Hastelloy, duplex stainless steel, plastics, etc.

Following are examples of applications where this technology's been used in the aerospace & defense industry:

  • Scald protection for commercial airplane galleys

  • Freeze protection for airplane lavatories

  • Freeze protection for cold water supply lines on military bases and naval shipyards

  • Electronics fluid cooling on military aircraft and land-based vehicles

  • Equipment enclosure temperature control

  • Electro-mechanical solenoid replacement in various fluid temperature control applications

  • Hydraulic fluid temperature control

  • Lube oil temperature control

  • Pump seal over-temperature relief

  • Thermal based permissive / safety latching mechanism

  • Temperature based fluid mixing applications

  • Linear (heated) actuator – supplying a high force in a small envelope

  • Actuators controlling various mechanisms based on temperature

  • Diverting fluid to one of two valve ports based on its temperature

Summary:

Thermostatic (paraffin wax) technology has been around since 1936. Over the years it has found its way into a broad range of industries. The aerospace & defense industry has adopted it because it checks most of the boxes required in this demanding industry – high reliability, compact size, very repeatable, maintenance free, self-powered, higher power to size ratio, few moving parts, and long service life.

This article was written by Tom Ruggiero, Program Manager of Aerospace, Defense and Government Facilities Division, ThermOmegaTech (Warminster, PA). For more information, visit here .


Aerospace & Defense Technology Magazine

This article first appeared in the October, 2018 issue of Aerospace & Defense Technology Magazine.

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