By Bruce I. Nelson, P.E., President, Colmac Coil Manufacturing, Inc.
ADX™ Ammonia System Evacuation
When water is present in a DX ammonia system, it changes the characteristics of the refrigerant and the operation
of the system. Because a DX evaporator coil boils off 100% of the refrigerant prior to the end of the circuit, a small
amount of water in the system will create a high water content at the coil exit. This will adversely affect the
superheat and refrigerant feed. Recirculated ammonia systems do not utilize 100% evaporation, so any water
accumulates on the low side, unnoticed, until the plant efficiency degrades from lower required suction pressures.
Please refer to the Colmac DX Ammonia Piping Handbook for a more thorough explanation.
Residual water left over from the construction period is the primary initial source of water in a refrigeration system.
Most ASME pressure vessels are pressure tested with water and they are merely drained after testing.
Refrigeration pipes are commonly left open to the atmosphere during construction. The normal daily ambient
temperature range will cause condensate to form in the pipes overnight. While these quantities of water are small,
collectively they can add up to a significant amount. A teaspoon of water at the exit of a coil circuit will falsify the
superheat reading by more than the TD of the coil. This completely overwhelms the superheat control, causing the
coil to overfeed.
Evacuation Comments
Evacuating a system prior to commissioning accomplishes two things. It removes most of the non-condensables
from the system. This allows efficient condenser operation immediately upon start-up. Secondly, it removes water
from the system. As the system pressure is reduced, water’s boiling temperature will gradually reduce from 212⁰F
(atmospheric pressure - 760,000 microns) to 32⁰F at 4580 microns. The residual heat of the pipe and vessel walls
will evaporate this residual water….if, it is spread out in the system. If it collects in a small low spot with little metal
volume, it may never absorb enough heat to evaporate, especially if the pipes are insulated. Note: a pound of steel
pipe giving up 20⁰F will only evaporate ¼ oz of water (2 teaspoons). Also, when that ¼ ounce of water evaporates
at 5,000 microns it generates 46 ft3 of water vapor which significantly slows the rate of evacuation.
Vacuum Pump Selection
The nominal capacity of a vacuum pump is measured at atmospheric pressure. When the vacuum reaches 13,200
(60⁰F) to 25,000 (78⁰F) microns, the range where water is evaporating, its cfm capacity is only 4% of the nominal
rating. Example a 10 cfm vacuum pump will spend most of the evacuation time pumping 0.4 cfm. That is 11
minutes of pumping for the 2 teaspoon example above. A small system may have 150 ft3 of internal volume and a
large system could have 1,500 ft3 or more internal volume. For a guideline; a 10 cfm vacuum pump will evacuate a
575 ft3 dry system to 5,000 microns in approximately 24 hours. The presence of water will extend this time.
Evacuation Steps
1. After a successful pressure test, blow off the pressure in stages from all of the valved low points in the
system. This will push any pooled liquid out of the system.
2. Re-install any safety relief valves that were removed for pressure testing and ensure that all parts of the
system are ready to accept an ammonia charge and are open to the vacuum pump.
3. Connect a digital vacuum gauge to the system such as a “Blu Vac” gauge that is designed to accurately
measure and display a deep vacuum in microns.
Page 1 of 2 4. Source a vacuum pump(s) of at least 8 cfm capacity and charge it with fresh vacuum pump oil that is water
free. (Water contaminated oil will flash and degrade the pumps vacuum pulling ability)
5. Dead head the suction side of the vacuum pump to the vacuum gauge and prove that it can pump down to
5,000 microns. If not, get a different pump.
6. Connect the system to the vacuum pump through the largest diameter and shortest, deep vacuum rated
hoses available and practical.
7. Operate the pump until a vacuum of 5160 microns (35⁰F) is achieved. There may be a vacuum level where
the vacuum appears to level off rather than drop. This is most likely water evaporating. When all of the
water is vaporized, the pressure will resume falling. A wet system may require an intermediate pump oil
change to achieve a full vacuum.
8. At this point all of the water should have been evaporated.
9. Break the vacuum with dry nitrogen until the system returns to about 760,000 microns, no vacuum or
pressure.
10. Change the oil in the vacuum pump.
11. Pull another vacuum down to 5,160 microns.
12. Valve off the pump and ensure that the vacuum stabilizes and then holds steady for 24 hours. This
confirms that there are no leaks or water remaining in the system.
13. Break this vacuum with refrigerant grade (99.98% pure, < 150 PPM H2O) ammonia.
14. Continue to charge the system with the prescribed amount of liquid ammonia.
For more information contact Colmac Coil Manufacturing, Inc.
www.colmaccoil.com | P: 800.845.6778 or 509.684.2595
PO Box 571 | Colville WA. 99114-0571
Copyright© 2017 Colmac Coil Manufacturing, Inc.
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