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  • Craig Migliacco

Filter Drier, Solenoid Valve and Liquid Line Sight Locations in a Refrigeration System

Updated: Apr 23, 2020

The filter drier, solenoid valve, and liquid line sight have unique responsibilities in an automatic pump down refrigeration system. To use each of these components to the fullest measure and for the greatest lifespan of the system, they must be placed in a specific order. These components are located after the receiver tank and before the TXV on the liquid line.

The filter drier:

This traps and stores any debris and water that may be inside the refrigerant circuit. Some filter driers also trap acid. This component should be placed first after their receiver king valve prior to the solenoid. The filter drier must not be installed after the solenoid or the steel filter drier will freeze and sweat on the outside when the refrigerant pressure lowers during the pump down. This could cause premature component failure due to rusting.

The solenoid:

This electrically activated valve allows liquid refrigerant through while in the open position. It also stops the flow of liquid refrigerant when in the closed position during an automatic pump down. This component should be placed after the filter drier but before the sight glass.

The liquid line sight (sight glass):

While the system is running, this component provides a visual way to determine if the refrigerant is in the fully liquid state or in the saturated state as the refrigerant passes through. If the refrigerant is in the fully liquid state, the refrigerant is subcooled and no bubbles are present. If bubbles are present, the refrigerant is in the saturated state. When the automatic pump down starts and after the solenoid closes, the liquid line sight can be viewed until the bubbles are gone and the sight glass becomes clear. This means that only vapor is present at the sight glass location.

Some sight glasses show if there is acid in the line due to a color change.

The sight glass should be placed after the filter drier and solenoid but prior to the TXV. By viewing the sight glass, it is possible to determine if there is a restriction in the liquid line such as a clogged filter. This would be seen as bubbles in the sight glass even if the liquid entering the filter drier is subcooled. By viewing the sight glass, it can also be determined if the solenoid is in the open position or closed position. If the solenoid is supposed to be in the closed position but is partially leaking, refrigerant bubbles will continue to be seen at the solenoid during operation.

For these reasons, the order should be the following:

After the receiver, first is the filter drier, second is the solenoid, and third is the liquid line sight. Following the liquid line sight, is the TXV metering device.

Check out this video for more information:

Published: 1/28/2020 Author: Craig Migliaccio

About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 17 years and holds an NJ HVACR Master License. AC Service Tech LLC creates educational HVACR articles and videos which are posted at &



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