How does a capillary tube thermostat work？

A capillary tube thermostat is a temperature control system that makes use of a capillary tube filled with a fluid (either a gas or a liquid) that expands and contracts with temperature changes. This allows you to control the temperature of a system, like an air conditioner, refrigerator or heating unit.

How It Works:

1-Capillary Tube:

A capillary tube is a long, flexible tube that is connected to the sensing element (the actual temp detection portion) of a thermostat and a bulb at the end (the bulb can be in a location on the heat source or the particular environment you are controlling).

The capillary tube contains a fluid (liquid or gas) which will react to temperature changes through thermal expansion and contraction, so that as temperature increases, the fluid expands, and as temperature decreases, the fluid contracts.

 

2-Temperature Sensing:

As the temperature of the controlled environment (or system) changes, the fluid in the capillary tube expands or contracts.

If the temperature increases, the fluid in the capillary tube expands, which results in the forces acting on a diaphragm or mechanical mechanism that is part of a switch. If the forces are significant enough, the thermostat will not only stop the heating or cooling system, but in the event the switch was the normally closed type, it would also remove any load from the thermostat's electrical circuit (load = heating element or other resources).

If the temperature decreases and fluid contracts the open switch will have its pressure decreased by the diaphragm effect so that the system is initiated again. Thus the thermostat maintains the temperature set point.

 

3-Mechanical Action:

Generally inside the thermostat is a diaphragm or bellow connected to a switch, and as the fluid moves (expands or contracts) it will either open or close the switch and thereby activate the system (for example, turning off or on a heating element or compressor).
Some thermostats may have a dial that allows the user to select the temperature. When this is set, it establishes how far the diaphragm must move to change the switch position.

 

Main Parts:
Capillary Tube- A thin tube that has fluid in it, which is responsible for measuring a change in temperature.
Bulb- This is located at the sensing point, typically next to either a heat source or a place that requires temperature regulation.
Diaphragm or Bellow- A flexible surface that is impacted by the pressure from the expanding or contracting fluid, which moves the mechanical switch.
Switch- The switch controls the on/off status of a heating or cooling system.

 

Example of Use:

As an example, there is a capillary tube thermostat inside a refrigerator that detects the temperature within the refrigerator. If the refrigerator gets hot, the fluid within the capillary tube expands turning a switch on which activates the thermostat, and then the compressor is told to get to work and cool the refrigerator. Once the cooled air within the refrigerator reaches the desired temperature, the fluid in the capillary tube contracts thereby turning the system off to maintain the temperature.

Advantages:

Straightforward and Dependable: Capillary tube thermostats are mechanical devices and simply constructed, therefore they are generally more reliable, with fewer parts to fail.

Precise Control: Capillary tube thermostats exhibit good accuracy while maintaining a set temperature and are therefore ideal for use in heating and refrigeration.

Cost-Effective: Capillary tube thermostats are often less expensive and more robust than similar electronic thermostats, as well.

In summary, the capillary tube thermostat uses the expansion and contraction of a temperature-sensitive liquid contained inside the capillary tube to control the switch actuation (and to control the system temperature).