Why Does the Expansion Valve Need a Temperature-Sensing Bulb? Can It Operate Without One?

I. The Core Role of the Temperature-Sensing Bulb

The temperature-sensing bulb is the heartbeat of the expansion valve’s flow regulation system. It must be properly secured to guarantee accurate temperature measurement—failure to do so will cripple the entire refrigeration system.
Filled with the same refrigerant as the system, the bulb clings tightly to the suction pipe at the evaporator outlet, continuously monitoring the pipe’s surface temperature. As temperature changes, the internal pressure of the bulb fluctuates. This pressure signal acts on the expansion valve’s diaphragm, adjusting the valve opening to control refrigerant flow. The goal? To maintain a stable "superheat" at the evaporator outlet—the difference between the temperature of superheated vapor and the saturation temperature of the refrigerant.

II. Direct Consequences of Not Securing the Temperature-Sensing Bulb

Skipping the bulb’s installation leads to three critical issues:

• Inaccurate Temperature Measurement
• System Instability
• Hidden Safety Risks
   

Let’s examine each problem in detail.

1. Inaccurate Temperature Measurement

• Poor Thermal Contact: Without securing the bulb, an air gap forms between it and the pipe. Air’s thermal conductivity (0.023 W/(m·K)) is negligible compared to metal (e.g., copper at 400 W/(m·K)), causing delayed temperature readings or interference from ambient conditions (e.g., air drafts).

     Example: 

• Ambient Temperature > Pipe Temperature: The bulb misreads "insufficient superheat," prompting the valve to open wider. Liquid refrigerant may flood the compressor, risking liquid slugging (a destructive hydraulic).
• Ambient Temperature < Pipe Temperature: The bulb misreads "excessive superheat," closing the valve and starving the evaporator of refrigerant. This plummets cooling efficiency.
• A too-closed valve triggers discharge temperature protection or high-pressure alarms.

2. System Instability

• Superheat Runaway: Without reliable temperature feedback, the expansion valve oscillates or fails to adjust, leading to:
• Uneven frost on the evaporator or localized liquid pooling, slashing heat transfer efficiency.
• Abnormal compressor suction temperatures (too hot or cold), causing overheat protection, oil carbonization, or liquid slugging damage.
• Energy Waste: The system operates inefficiently, forcing the compressor to work harder. Energy consumption can surge by 10%–30%.

3. Potential Safety Risks

• An unsecured bulb may vibrate loose or detach entirely, leaving the expansion valve uncontrollable. In severe cases, this causes high-pressure alarms, refrigerant leaks, or catastrophic equipment failure.

III. The Necessity of Proper Installation

1. Physical Securement Is Non-Negotiable

• Use metal clamps, nylon ties, or specialized fixtures to press the bulb tightly against the pipe—no gaps allowed. For large-diameter pipes (>25mm), multiple bulbs may be needed to cover the pipe’s circumference.

2. Thermal Insulation Is Critical

• Wrap the bulb and pipe together with rubber or foam insulation to block ambient temperature interference:
• In high-humidity environments (e.g., cold storage), lack of insulation causes condensation on the bulb, worsening heat transfer.
• In high-temperature settings (e.g., outdoor units), ambient heat absorbed by the bulb skews superheat calculations.

IV. Exceptions to the Rule?

While securing the bulb is standard practice, rare exceptions exist:

• Miniature Systems: Tiny capillary-throttled devices (e.g., some water dispensers, dehumidifiers) may skip expansion valves altogether, eliminating the need for a bulb.
• Integrated Sensors: A handful of modern electronic expansion valves have built-in temperature probes, but these still require direct pipe contact—they aren’t "unsecured."

V. Conclusion

Installing the temperature-sensing bulb is a make-or-break step in refrigeration system commissioning. Skipping it destroys the bulb’s ability to gather critical data, risking system failure. In any professional refrigeration project or maintenance task, follow the trio of rules: tight contact, secure fixation, and effective insulation. Cutting corners here could lead to costly damage or safety hazards—don’t let a simple oversight compromise your system’s reliability.