8.2 TXV Behavior and Control

Thermostatic Expansion Valve (TXV) Fundamentals

Although flooded liquid overfeed systems dominate industrial ammonia refrigeration, the Thermostatic Expansion Valve (TXV) remains a key component in package chillers, compressor injection cooling lines, and smaller secondary cooling circuits. The primary function of a TXV is to regulate the flow of liquid refrigerant into the evaporator at a rate that matches the boiling rate, maintaining a constant superheat at the evaporator outlet.

The Three Operating Forces of a TXV

The operation of a TXV is governed by the mechanical interaction of three distinct pressures acting upon a flexible metal diaphragm inside the valve head. This diaphragm is connected via pushrods to a pin and seat assembly that modulates the flow area of the valve orifice.

          [ Sensing Bulb (Mounted on Suction Line) ]
                             |
                             v (Capillary Tube)
                    ===================
                   |   Bulb Pressure   |  <--- (Opening Force, Pb)
                    ===================
                   |====== Diaphragm ==|
                    ===================
                   |Evap/Spring Forces |  <--- (Closing Forces, Pe + Ps)
                    ===================
                             |
                             v (Push Rods)
                     [ Valve Needle ]
                     [  Valve Seat  ]

1. Bulb Pressure ($P_b$)

This pressure is exerted on the top side of the diaphragm and acts as the opening force. It is created by the thermal expansion of a refrigerant charge enclosed within the sensing bulb, which is mounted on the suction line at the evaporator outlet. As the suction line temperature rises (indicating higher superheat), the liquid in the bulb evaporates, increasing the pressure on top of the diaphragm and pushing the valve open.

2. Evaporator Pressure ($P_e$)

This pressure acts on the underside of the diaphragm and serves as a closing force. It is the suction pressure of the refrigerant inside the evaporator. As evaporator pressure increases, it pushes upward on the diaphragm, moving the needle toward the seat to restrict refrigerant flow.

3. Spring Pressure ($P_s$)

This is a mechanical spring force that acts on the underside of the diaphragm in the same direction as the evaporator pressure, functioning as a closing force. The spring provides a constant, adjustable mechanical force that prevents the valve from opening until the bulb pressure exceeds the evaporator pressure by a set amount.

The Force Balance Equation and Superheat Adjustment

When a TXV is operating in a stable state, the forces acting on the diaphragm are in equilibrium:

$P_b = P_e + P_s$

To change the superheat setting, the operator must alter the spring pressure ($P_s$) by turning the valve’s external adjustment stem:

• Clockwise Rotation (Tightening the Spring): Turning the stem clockwise increases the mechanical spring tension, raising the closing force ($P_s$). To overcome this, the valve requires a higher opening force ($P_b$), which can only be achieved if the suction vapor at the bulb location warms up to a higher temperature. Therefore, tightening the spring increases the operating superheat and starves the evaporator coil, reducing liquid flow.
• Counter-Clockwise Rotation (Loosening the Spring): Turning the stem counter-clockwise reduces spring tension ($P_s$). This decreases the required opening pressure, allowing the valve to open at a lower bulb temperature. This decreases the operating superheat, feeding more liquid refrigerant into the coil and bringing the system closer to a flooded condition (with a risk of liquid carryover).

Internal vs. External Equalization

TXVs are classified by how the evaporator pressure ($P_e$) is routed to the underside of the diaphragm:

Internally Equalized TXVs

An internally equalized valve has a passage inside the valve body that directs the pressure from the valve outlet (the inlet of the evaporator) directly to the underside of the diaphragm. This design assumes the pressure at the inlet of the evaporator is equal to the pressure at the outlet. This assumption is only valid for small coils with minimal pressure drop.

Externally Equalized TXVs

As refrigerant flows through a large industrial evaporator, it experiences friction, resulting in a pressure drop ($\Delta P$). If an internally equalized valve is used on a coil with a significant pressure drop, the valve will "see" the higher evaporator inlet pressure under the diaphragm, which forces the valve closed and starves the coil.

An externally equalized TXV isolates the underside of the diaphragm from the valve outlet. Instead, a small external line (the equalizer line) connects the chamber under the diaphragm to the suction line immediately downstream of the sensing bulb. This ensures the valve senses the actual pressure at the outlet of the evaporator, bypassing the coil's pressure drop.

• Rule of Thumb: An externally equalized TXV must be used if the pressure drop across the evaporator exceeds $1.5$ to $2 \text{ psi}$ (or a temperature equivalent of $1^\circ\text{F}$ to $2^\circ\text{F}$). In ammonia systems, pressure drops almost always exceed this threshold, making externally equalized valves the industrial standard.

Sensing Bulb Placement and Mounting Best Practices

To ensure accurate control, the TXV sensing bulb must be installed correctly on the suction line:

1. Horizontal Placement: The bulb must be mounted on a horizontal section of the suction line. It should never be mounted on a vertical line, as liquid refrigerant or oil can pool inside the pipe at that point, causing false, sluggish readings.
2. Clock Position:
   • For suction lines under $7/8 \text{ inches}$ in diameter, mount the bulb at the 12 o'clock (top) position of the pipe.
   • For suction lines $7/8 \text{ inches}$ and larger, mount the bulb at the 4 o'clock or 8 o'clock position. The bulb must never be mounted at the 6 o'clock (bottom) position, because liquid refrigerant and compressor oil flow along the bottom of the suction line, which would insulate the bulb from the vapor temperature and cause the valve to starve the coil.
3. Physical Contact & Insulation: The bulb must be strapped securely to clean, bare pipe using copper or steel straps (never wire ties or tape). After mounting, the bulb must be wrapped in water-resistant insulation to prevent it from being influenced by ambient room air temperature.

Safety and Maintenance of Ammonia TXVs

Ammonia TXVs require special maintenance and safety precautions:

• Toxic Refrigerant Charge: The sensing bulb and capillary tube of an ammonia TXV are typically charged with active ammonia refrigerant. If the capillary tube is kinked, bent, or cut during maintenance, it can rupture and discharge concentrated ammonia vapor directly into the operator's face.
• Required PPE: When adjusting or servicing TXVs, operators must wear safety glasses, gloves, and have a respirator readily available.
• Valving Out: Before attempting to replace a TXV, the liquid line solenoid, hand expansion valves, and suction stop valves must be closed, and the system pressure must be safely evacuated using a pump-down procedure.