Service & Maintenance / Extra Edition

Quick Facts: TXV Selection and Sensing Bulb Location

March 6, 2006
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TXV Selection

How do I select a TXV for a particular application?
In order to make an accurate thermostatic expansion valve (TXV) selection, it is necessary to match the flow rate (in tons) of the TXV to the tonnage rating of the evaporator coil. The following procedure is recommended:

  • Determine system refrigerant.

  • Determine tonnage rating of the evaporator coil at the operating temperature.

  • Determine liquid refrigerant temperature at the TXV inlet.

  • Calculate the pressure drop across the TXV by subtracting suction (low side) pressure from condensing (high side) pressure. Also subtract pressure drop from a distributor, if one is used. The difference is pressure drop across the TXV.

  • Find the proper extended capacity table in the catalog for the correct refrigerant at the proper evaporator temperature. Then locate the closest pressure drop column and find the nearest flow rate in tons (to the evaporator tonnage). Select the nominal tonnage rated valve.

  • You will have to recalculate the flow rate in tons using the "Liquid Correction Factor Table" if the actual liquid temperature is different from 100 degrees F used as the standard rating point.

    Sensing Bulb Location

    What is the correct position for the TXV sensing bulb?
    The placement of the remote sensing bulb of the TXV onto the suction line is critical to proper TXV performance. The important points to follow are:

  • Clean the suction line near the outlet of the evaporator.

  • The entire length of the sensing bulb must be in contact with the cleaned portion of the suction line.

  • The sensing bulb should be placed several inches downstream of the external equalizer connection.

  • The sensing bulb should be attached at 12 o'clock on any suction line of 7/8-inch diameter or smaller. On lines larger than 7/8-inch diameter, the bulb should be placed at either 4 or 8 o'clock. The bulb should never be placed at 6 o'clock.

  • Always insulate the sensing bulb after installation.

  • A sensing bulb can be installed on a vertical suction line if necessary, but never place the bulb downstream of a trap. Bulb placement before a trap (upstream) is recommended.

    Quick Facts is courtesy of the Flow Controls Division of Emerson Climate Technologies (formerly Alco Controls). For more information, visit

    Publication date: 03/06/2006

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