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“Cap” Tubes: Temperature Sensing Elements

Temperature sensing controls using a capillary tube, or capillary tube and bulb, are being replaced in many instances with electronic controls using RTD’s as the sensing element.  There are many devices still using the capillary tube as the sensing element.  These capillary devices, such as non-electric self-contained valves, TXV’s, and household refrigerator controls, have been around for many years and will continue to be used for many more years.  Their “niche” in the marketplace has been well established.

Understanding how the capillary tube temperature control works will aid in selecting the proper control for a particular installation.

The temperature responsive capillary tube type element is basically a closed pressure vessel.  It consists of a bellows that can expand and contract, a capillary tube that acts as a connecting link between the bellows and a bulb, and usually a bulb that is the sensing element.  Sometimes the bulb is omitted and the capillary tube is used as the sensing area.  The cap tube may even be formed so as to function as a bulb when needed.

Cap tube elements are broadly classified into two types; vapor filled and liquid filled. 

In vapor filled elements, the force generated to operate the control mechanism is vapor pressure change in response to temperature change.

Vapor filled elements are further classified into three more types:

Limited Vapor - Control applications where the sensing point is the coldest portion of the element system

Cross Ambient - The bulb temperature, which is always the sensing point, may be above or below the temperature of the rest of the element system.

Fail Safe - This is a special type of element designed to prevent operation if a leak in the element system should occur.

Figure 1 shows the capillary tube and capillary tube and bulb sensing element systems.

Figure 1.

In limited vapor elements, a measured volume of liquid is charged into the element system so as to completely vaporize just above the highest operating temperature of the control.  This is known as the “break point.”  There is always liquid and vapor present in the element below the break point.  As the temperature increases, more of the liquid vaporizes, which raises the internal pressure in the element, causing the bellows to expand.  (The bellows is mechanically linked to contacts that “make” or “break’ an electrical circuit or circuits.)  As long as liquid is in the element, pressure will change in a definite relationship to temperature.  If a temperature is reached where all the liquid has vaporized, the vapor becomes superheated, and only a slight increase in pressure will take place.  The amount of fill is kept to a measured minimum so that the element can withstand higher temperatures without damage occurring.

Limited vapor elements are commonly used on controls for home refrigerators, freezers, water coolers, etc.—anywhere control of a system is desired from the evaporator coil temperature only.  It is usually at the coldest point of a system.  Figure 2 shows common configurations of the Limited Vapor Filled Element.

Figure 2.

Many TXV elements are limited vapor charged.  The “MOP” (Maximum Operating Pressure) rating of a TXV is the “break point” of the limited vapor charge, that is; the point at which all liquid in the remote bulb has evaporated.

Important points to remember when installing limited vapor charged element controls:

Make sure the bulb, or if no bulb at least six inches of capillary, is in good contact with the device being sensed.  Do not let the capillary tube touch any point along its length that may become colder than the point being sensed, otherwise control from the desired sensing point will be lost.

Remember, Limited Vapor Filled Elements control from the coldest point in the element system.  (This “feature” can be put to use in certain applications, such as freeze protection for air conditioning coils where a long cap tube is positioned over the face of the coil.)

Cross Ambient Elements.  Control is always from the bulb in this type of vapor filled element.  The principle is based on evaporation of the fill always taking place in the bulb.  This is accomplished by determining the size of the bulb and amount of fill, so that liquid is always present in the bulb.

Figure 3 shows what happens when the temperature sensed by the bulb changes from below to above the ambient temperature at the control, or bellows head.

Figure 3.

Figure 3-B shows the temperature of the bulb approaching ambient.  Some of the liquid in the bulb boils, creating a higher pressure in the capillary system.  In this example, the vapor pressure would be equal to 39,5°F.

Figure 3-C shows that the temperature of the bulb is now slightly higher than ambient at the bellows.  The boiling liquid forces some liquid into the capillary.  Vapor in the capillary condenses at the bellows.

Figure 3-D shows the bulb operating at a higher temperature than ambient.  The capillary and head of the bellows are now full of liquid.  The bulb still has enough liquid in it to provide a liquid to vapor interface above the end of the cap tube in the bulb.

Mounting of the cross-ambient bulb is critical.

One type is illustrated in Figure 4.  This type has the fill tube (a short sealed off tube) coming out of the same end as the capillary tube.  The word “top” is stamped on the end of the bulb nearest the capillary.  This bulb may be mounted either vertically or horizontally as long as the word “top” is positioned as shown in Figure 4.  This ensures that the internal end of the capillary is always below the liquid level in the bulb.

Figure 4.

Another type of bulb is shown in Figure 5.  The fill tube is coming out of the opposite end of the bulb from the capillary tube.

Figure 5.

This type of bulb is intended for vertical mounting only.  The top of the bulb should not be more than 65° from vertical, so that liquid covers the capillary end at all operating temperatures.  Should this bulb be mounted with the fill tube pointing down, a very slow response will result with a wide differential.

Should a condition exist where a section of the capillary will be exposed to very hot or cold temperatures as compared to the bellows or bulb, a liquid filled element should be used.

Liquid filled elements depend on volumetric expansion and contraction of their fill.  When high temperature settings, wide range, and a narrow, on-off differential are required, the liquid filled element is recommended.  This element is completely filled with liquid.

The liquid filled element operates by hydraulic action in response to rise and fall in temperatures.  The internal pressure of liquid filled elements is high, and consequently, movement resulting from temperature change is nearly constant throughout the control range.  The liquid expands and contracts at an almost constant rate with temperature change.  The liquid fill depends primarily on the volume of liquid in the bulb.  Less bulb volume produces less bellows movement per degree F, therefore a wider differential, greater range of adjustment, and wider tolerances on calibration.  More volume results in slower response to temperature changes, but higher accuracy of settings.  At fast rates of temperature changes, the liquid filled element will have a tendency to lag in response, appearing to have a wider operating differential when compared to a vapor filled element.  At rates of temperature change normally encountered in HVAC & R applications, the liquid filled element will respond as well as the vapor filled element.  In fact, in some cases, this lag can be beneficial in preventing short cycling of oversized equipment in some poorly designed systems.

The liquid filled element, such as found on the A19 Johnson Series of controls, predominate in the HVAC & R industry.

Due to proven technology, ease of installation, and low cost, the capillary tube temperature control will be around for a long time.

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