Seafrost recommissioning-vacuum pump specs

[Alii]

My Seafrost R-134a system has been dismantled in hot, humid tropical conditions for the past 5 months (to accommodate engine work). I will soon be recommissioning it DIY and am shopping for a vacuum pump. Should a single-stage 1.5CFM pump be adequate? I can leave it on for however long is recommended to get the best results.

[Richard Kollmann]

Originally Posted by Alii

My Seafrost R-134a system has been dismantled in hot, humid tropical conditions for the past 5 months (to accommodate engine work). I will soon be recommissioning it DIY and am shopping for a vacuum pump. Should a single-stage 1.5CFM pump be adequate? I can leave it on for however long is recommended to get the best results.

The CFM of a vacuum pump is not important for a do it yourselfer because pumping time is less important. A refrigerator vacuum pump you use should be rated at to 50 microns or lower most will indicate 25 microns. Even though pump micron rating my be low most gauge sets and hose connections prevent micron suction below 100. I am happy with 300 micron vacuum and 4 hours keeping complete system including evaporator and box at 100 degrees F. I assure you if your system has POE or PAG oil that has been exposed to air and it has not received the deep vacuum process you can expect future problems either ice blockage or over time interior corrosion.

{Copied From Refrigerant Charging section on my web site}

14. Vacuum Pumps Servicing
Today’s small icebox conversion refrigeration units are more sensitive to moisture and other contaminants than ever before. This is why a refrigerant deep vacuum pump is used any time refrigerant or its oil is exposed to air or contaminant gases. Moisture can cause ice crystals that will block refrigerant flow. Moisture mixed with refrigerant and oil and other chemicals will form sludge containing hydrochloric acids. Acids moving with refrigerants tend to cause aluminum evaporator corrosion and copper items to be electroplated to steel parts.
Polyester oil now used with 134a refrigerant absorbs moisture quickly and unlike previously used wax free Mineral oil refuses to release or boil off moisture to a gas.
A good vacuum pump, which reduces internal system pressures to the boiling point of water at normal temperatures, will convert moisture to vapor gas. Vacuum pumps will not extract liquid. Water must be vaporized to a gas before it can be removed. To reach a low pressure boiling point, which is necessary to extract moisture, a deep vacuum of as close to absolute vacuum is desirable. Low pressure (blue) refrigeration gauges reads in pressure and vacuum. With a standard day’s temperature 69º F at sea level atmosphere pressure is 29.30 inches of vacuum. If ambient temperature is colder than standard then a very deeper vacuum is required or heat.
.Most blue low pressure refrigerant gauge scales indicate 30 inches of vacuum so we assume that if while vacuuming gauge reads 30 inches moisture is boiled away. Accuracy of gauges is not to be relied upon because one inch of vacuum is equal to 25000 microns. To boil off moisture most technicians vacuum at less than 500 microns of vacuum.
My recommendation for dehydrating a small system confirmed to contain moisture and having ester oil is to warm complete system to 100º F. I prefer maximum vacuum pump vacuum and temperature be maintained for four hours

[Alii]

Thank you for this helpful information. How do you suggest I keep the temperature of the system at 100F? Ambient temperatures here are consistently 86-92F, 24 hours a day, although on a sunny day (rare during this monsoon season) I can get the interior of the boat up to 100F midday if I pull off the shade canopy.

When I disassembled the system I capped the compressor and kept it upright. How can I tell how much oil is now in the system, and whether it is the right amount or needs to be supplemented?

I appreciate your help,
Alii

[a64pilot]

Really should get a two stage, rotary vane pump if at all possible, CFM really doesn't matter much, but a single stage can't pull as deep a vacuum as a two stage can

[Richard Kollmann]

Originally Posted by a64pilot

Really should get a two stage, rotary vane pump if at all possible, CFM really doesn't matter much, but a single stage can't pull as deep a vacuum as a two stage can

My point is for non production DIY applications like a personal refrigerant Deep vacuum pump single stage is OK as long as it is rated at 50 or lower microns. Robinair has light weight models rated at 5Pa (35 microns).

In theory complete system temperatures of 70 degrees F is expectable but experiences with warmer temperatures 100 degrees F are more effective. These new oils are less likely to allow moisture mixed in oil to vaporize and vacuuming can not remove moisture only steam vapor. Check my opinions by searching this question on the web.

Hair dryers work well to warm complete system, do not use heat guns.

[pdf27]

Temperature is absolutely critical for getting water out of vacuum systems. See Water phase diagram - you're using the vacuum pump to turn water into gas, which it can then remove. Baking the system accelerates this process - ultra clean vacuum systems such as the ones I work on are regularly baked to very high temperatures (600F for mine), but I'm running at pressures down to 10^-8 Torr(!) - your target is much lower.