RESEARCH AND DEVELOPMENT

Thermistance is the first Indian start-up completely devoted to develop advanced passive cooling technologies and its commercialization. Our world class team is continuously working to develop innovative micro and miniature cooling devices. We are also developing our research and development centre in Pune.

Cylindrical Evaporator Loop Heat Pipe

The need for a passive cooling technology was so loud that it led to the development of advanced passive cooling systems. One such invention is the Loop Heat Pipe. The spark that implored the refinement of the Heat Pipe technology is the problems that are faced by them in space, where there is zero gravity and limitation of weight and size. Loop Heat Pipes surpass Heat Pipes with their ability to work without gravity, reliability for heat transmission over long distances with a small temperature difference.

Structure of Loop Heat Pipe

The main disadvantage of Heat Pipes is the decreased heat transfer, especially when the working fluid has to be transported against gravity. To ensure maximum movement against gravity, the capillary head should be at a maximum. The capillary head depends on two variables: surface tension coefficient and the effective pore radius of the wick structure. Capillary head increases with increasing surface tension coefficient and with decreasing effective pore radius of the wick structure. Increasing the surface tension is not plausible, even with water. Decreasing the effective pore radius of the wick structure causes an exponential increase in the hydraulic pressure. Thus, maximizing the capillary head is obsolete in this case. Loop Heat Pipe technology surmounts all these inane solutions.

1. Evaporator: This is the part that receives the heat input. Most of the body of Loop Heat Pipe, including the evaporator, is made of stainless steel owing to the amenability to different kinds of vacuum-hermetic welding and high strength, but they have a low thermal conductivity. Copper can also be used for the construction.

2. Wick (capillary structure): The wick is restricted to that part of the evaporator that receives the heat input. It acts as a capillary pump and a heat exchanger. Wicks are made up of sintered nickel and titanium powders.
3. Working fluid: The ideal working fluid must have a temperature range, suited to the working environment. Ammonia is the most widely used working fluid of the temperature range of -20°C to +80°C.
4. Compensation chamber: This is fitted to the Loop Heat Pipes to accommodate the accumulated working fluid during the operation of the Loop Heat Pipe.

5. Condenser: This part is where the vapors condense dissipating heat, which is delivered to a heat sink.
6. Vapor removal channel: This is fitted to the evaporator end to remove the vapor of working fluid, direct it to the vapor lines which take them to the condenser.

7. Vapour line and liquid line: These are separate smooth-walled lines for vapor and liquid with a relatively smaller diameter.


Principles of Loop Heat Pipe

1. Fine-pored wicks: The advantages of using fine-pored wicks are: the creation of a high capillary pressure even with a low temperature of the working fluid, chemical compatibility with most of the working fluids.

2. Minimization of liquid motion distance in the wick: This reduction in the distance traveled by the working fluid comes at the expense of the wick design. The wick design, in the case of a Loop Heat Pipe, is restricted only to the evaporator zone and not the entire length of the tube as in the case of a Heat Pipe.

3. Organization of effective heat exchange during evaporation and condensation of the working fluid: During evaporation, effective heat exchange is ensured by the ramified system of vapor removal channels. The smooth surface of the condensation chamber and the film character which promotes film suction is responsible for maintaining effective heat exchange in the condensation end.

4. Minimization of pressure losses in the transportation (adiabatic) section: The main causes of pressure losses in the adiabatic region are the thermal and viscous interaction between the counterflows of vapor and liquid. To avoid these interactions and subsequent losses, this section of the Loop Heat Pipe is fitted with separate smooth-walled pipelines for the movement of working fluid, namely the vapor line and the liquid line.

Working of Loop Heat Pipe

When there is no heat input, the wick is saturated with the working fluid, there is a free surface of the working fluid in both the liquid line and the evaporator. The vapor line and condenser are also completely filled with vapor.

When the evaporator is subjected to a heat load, the working fluid evaporates in two sites: wick and compensation chamber. The wick, which is closer to the heat source, has a higher temperature and pressure compared to the compensation chamber. In addition to this, the wick also has the property of thermal resistance. Thus, the wick acts as a thermal lock, ensuring effective heat is exchanged.

High-temperature vapor cannot enter into the compensation chamber through the already saturated wick as the stronger capillary forces hold the liquid inside it. This way, the wick acts as a hydraulic lock.

A pressure difference is created, which causes the displacement of working fluid from the vapor line and condenser. Vapour condenses into the fluid, enters the liquid line. Capillary pressure enables the flow of working fluid through the channels into the wick to saturate it again. The excess enters the compensation chamber and the cycle repeats.

Advantages of Loop Heat Pipe

1. High power densities: The specialized design of the evaporator enables the Loop Heat Pipes to work under the conditions of high power densities.

2. Flexibility: The ability to alter the configuration of vapor and liquid lines gives the advantage of being flexibly mounted to electronic devices.

3. Compact size: The fine diameters of the transport lines have led to the compactness and miniature size of the Loop Heat Pipe.

4. Antigravity function: Loop Heat Pipes overcome one of the main drawbacks of Heat Pipe, that is, the ability to function in a microgravity environment and antigravitational orientation.

5. Two-phase: Loop Heat Pipes work on a two-phase cooling circuit.

Applications of Loop Heat Pipe

1. Spacecraft: Loop Heat Pipes are the best choice for thermoregulation in spacecraft. Their high heat transfer, low thermal resistance, mechanical flexibility, excellent adaptability, most importantly, the capability to operate in an anti-gravity regime.

2. Electronics and computers: Electronic gadgets are getting miniaturized, making the need for a miniature cooling technology more pronounced. Loop Heat Pipe can solve this problem and lead to better functioning of the devices.



The main disadvantage of Heat Pipes is the decreased heat transfer, especially when the working fluid has to be transported against gravity. To ensure maximum movement against gravity, the capillary head should be at a maximum. The capillary head depends on two variables: surface tension coefficient and the effective pore radius of the wick structure. Capillary head increases with increasing surface tension coefficient and with decreasing effective pore radius of the wick structure. Increasing the surface tension is not plausible, even with water. Decreasing the effective pore radius of the wick structure causes an exponential increase in the hydraulic pressure. Thus, maximizing the capillary head is obsolete in this case. Loop Heat Pipe technology surmounts all these inane solutions.

1. Evaporator: This is the part that receives the heat input. Most of the body of Loop Heat Pipe, including the evaporator, is made of stainless steel owing to the amenability to different kinds of vacuum-hermetic welding and high strength, but they have a low thermal conductivity. Copper can also be used for the construction.

2. Wick (capillary structure): The wick is restricted to that part of the evaporator that receives the heat input. It acts as a capillary pump and a heat exchanger. Wicks are made up of sintered nickel and titanium powders.
3. Working fluid: The ideal working fluid must have a temperature range, suited to the working environment. Ammonia is the most widely used working fluid of the temperature range of -20°C to +80°C.
4. Compensation chamber: This is fitted to the Loop Heat Pipes to accommodate the accumulated working fluid during the operation of the Loop Heat Pipe.

5. Condenser: This part is where the vapors condense dissipating heat, which is delivered to a heat sink.
6. Vapor removal channel: This is fitted to the evaporator end to remove the vapor of working fluid, direct it to the vapor lines which take them to the condenser.

7. Vapour line and liquid line: These are separate smooth-walled lines for vapor and liquid with a relatively smaller diameter.


1. Fine-pored wicks: The advantages of using fine-pored wicks are: the creation of a high capillary pressure even with a low temperature of the working fluid, chemical compatibility with most of the working fluids.

2. Minimization of liquid motion distance in the wick: This reduction in the distance traveled by the working fluid comes at the expense of the wick design. The wick design, in the case of a Loop Heat Pipe, is restricted only to the evaporator zone and not the entire length of the tube as in the case of a Heat Pipe.

3. Organization of effective heat exchange during evaporation and condensation of the working fluid: During evaporation, effective heat exchange is ensured by the ramified system of vapor removal channels. The smooth surface of the condensation chamber and the film character which promotes film suction is responsible for maintaining effective heat exchange in the condensation end.

4. Minimization of pressure losses in the transportation (adiabatic) section: The main causes of pressure losses in the adiabatic region are the thermal and viscous interaction between the counterflows of vapor and liquid. To avoid these interactions and subsequent losses, this section of the Loop Heat Pipe is fitted with separate smooth-walled pipelines for the movement of working fluid, namely the vapor line and the liquid line.


When there is no heat input, the wick is saturated with the working fluid, there is a free surface of the working fluid in both the liquid line and the evaporator. The vapor line and condenser are also completely filled with vapor.

When the evaporator is subjected to a heat load, the working fluid evaporates in two sites: wick and compensation chamber. The wick, which is closer to the heat source, has a higher temperature and pressure compared to the compensation chamber. In addition to this, the wick also has the property of thermal resistance. Thus, the wick acts as a thermal lock, ensuring effective heat is exchanged.

High-temperature vapor cannot enter into the compensation chamber through the already saturated wick as the stronger capillary forces hold the liquid inside it. This way, the wick acts as a hydraulic lock.

A pressure difference is created, which causes the displacement of working fluid from the vapor line and condenser. Vapour condenses into the fluid, enters the liquid line. Capillary pressure enables the flow of working fluid through the channels into the wick to saturate it again. The excess enters the compensation chamber and the cycle repeats.

1. High power densities: The specialized design of the evaporator enables the Loop Heat Pipes to work under the conditions of high power densities.

2. Flexibility: The ability to alter the configuration of vapor and liquid lines gives the advantage of being flexibly mounted to electronic devices.

3. Compact size: The fine diameters of the transport lines have led to the compactness and miniature size of the Loop Heat Pipe.

4. Antigravity function: Loop Heat Pipes overcome one of the main drawbacks of Heat Pipe, that is, the ability to function in a microgravity environment and antigravitational orientation.

5. Two-phase: Loop Heat Pipes work on a two-phase cooling circuit.

1. Spacecraft: Loop Heat Pipes are the best choice for thermoregulation in spacecraft. Their high heat transfer, low thermal resistance, mechanical flexibility, excellent adaptability, most importantly, the capability to operate in an anti-gravity regime.

2. Electronics and computers: Electronic gadgets are getting miniaturized, making the need for a miniature cooling technology more pronounced. Loop Heat Pipe can solve this problem and lead to better functioning of the devices.