Ice Breaker: Multiple Evacuations

January 11, 2010
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One contaminant that must be removed from a refrigeration system before refrigerant can be added is water vapor. If water vapor is left trapped in a system, it can lead to some serious problems. Technicians will typically reduce the system’s pressure to about 500 microns (or the pressure required by the system manufacturer) to remove this water vapor as well as other system containments.

Occasionally, a technician will come across a system that contains an abnormal amount of water vapor. Some refer to this as a wet system. When dealing with a wet system, there is an alternate method that can be used to evacuate it and get rid of this excessive water vapor.

The multiple evacuation method (sometimes referred to as the triple evacuation method) can be used to more effectively remove water vapor from a system. This method is similar to the standard single evacuation procedure except that several evacuations are run, along with nitrogen being introduced in the system between evacuations. The nitrogen is used to absorb water vapor contained in the system.

Below is a general procedure that can be used to perform the multiple evacuation method:

1. Attach the low and high side hoses from a manifold gauge to the low and high sides of the system.

2. Attach a tee to the center port of the manifold gauge. This will allow two hoses to be connected to the center port.

3. Attach one end of a vacuum hose to this teed center port and the other end of the hose to the inlet of a vacuum pump. If your vacuum pump does not have an isolation valve, then add an isolation valve in this line.

4. Attach one end of another hose to the teed center port on the manifold gauge and its other end to a nitrogen tank.

5. Attach a vacuum gauge to the system; the further away from the pump the better. If available, the receiver service valve is an excellent location.

6. Open the valves on the manifold gauges and the vacuum pump. Make sure the valve of the nitrogen tank is closed and the system is at atmospheric pressure.

7. Turn on the vacuum pump and run it until the vacuum gauge reads 1,000 microns.

8. Close the valve on the vacuum pump and shut it off.

9. Open the valve on the nitrogen tank and introduce nitrogen into the system until the compound manifold gauge reads about 5 to 10 inches of mercury.

10. Shut the valve on the nitrogen tank. Allow the nitrogen to absorb any water vapor in the system for a few minutes.

11. Open the valve on the vacuum pump and start the pump again until the micron gauge again reads 1,000 microns.

12. Close the valve on the vacuum pump and shut it off.

13. Open the valve on the nitrogen tank and introduce nitrogen into the system until the compound manifold gauge reads about 5 to 10 inches of mercury.

14. Shut the valve on the nitrogen tank. Allow the nitrogen to absorb any water vapor in the system for a few minutes.

15. Open the valve on the vacuum pump and start the pump again until this time the micron gauge reads 500 microns or a pressure recommended by the system manufacturer.

Although this method does take longer to complete, it will more effectively remove water vapor from a system and ensure that you have a dry system.

Publication date: 01/11/2010

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