RTP Technical Bulletin

Following our discussions in RTP4 on oils and refrigerants in retrofit it should be obvious to you by now the old “clean” days of R12 servicing are finished. We are now in the era of multi refrigerant servicing (not necessarily using multi refrigerants but having to deal with systems that have assorted cocktail mixes in them). We must therefore adopt service procedures that best cater for these systems with respect to reliability and performance. The procedures outlined in this document are not necessarily more expensive or time consuming than those you are currently using - they are just a different way of doing things.

Additionally we are not suggesting the base line procedures you have been, or are currently using are obsolete. You may only choose to use the procedures and recommendations suggested in this Bulletin on some systems dependent upon your professional judgement as to what is actually needed for each particular system you work on.

This Bulletin will address three key areas: 1. Baseline Servicing Procedures 2. Flushing Procedures

3. Performance Testing Procedures

BASELINE SERVICE PROCEDURES

The basic procedures of servicing a system have not changed, but what is different is the importance of doing the job correctly particularly in the area of evacuation (vaccing down) and the use of sacrificial charges when necessary.

EVACUATION

System evacuation is without question the single most important aspect of basic system servicing yet we still have reports of 10 minute evacuation times in retrofit. There are some critical aspects of evacuation of which you must be aware:

♦ The use of alternatives and/or flushing agents in retrofit exaggerate the need for extended vacuum times in order to ensure complete removal of contaminants.

Professional Service Procedures in Retrofit

RTP Technical Bulletin

Category: RETROFITTING

REGISTERED TECHNICIANS PROGRAM Volume 1 Bulletin 4

♦ 1cc (1ml) of water under vacuum will boil off to produce in excess of 1000 litres of vapour that your vacuum pump must withdraw from the system.

♦ Therefore extended vacuum times are recommended for any system that has been open for extended periods of time.

♦ Any system that has been flushed is possibly moisture laden if the flushing procedure was not professional. ie Shop air was used as a propellant - not dry nitrogen or R134a as is recommended.

♦ Any system that has been empty as a result of a leak or system disconnection/decommissioning.

♦ Any system that has a low side leak where the “pulling in” of air/moisture is a possibility.

In addition to extended vacuum times:

♦ A sacrificial charge is strongly recommended - it should in fact be the standard service procedure - for systems that have been open or empty for extended periods.

The aforementioned points on extended vacuum times are second nature (or embarrassingly obvious) to the professionals of the industry but we get back to the dollar driven environment of retrofit we are in - and VASA gets numerous reports of what actually goes on - hence the content of this article - to highlight the effects on system performance and reliability if substandard procedures are practiced.

Vacuum Pumps - Performance and Capacity

There are 2 aspects to vacuum pump operation and system evacuation - pump performance and capacity.

Performance

Performance is the critical aspect of the ultimate vacuum the pump can pull (ie 40 microns). It is critical because unless we pull an acceptable vacuum we do not boil off the moisture in the system. (See Chart over.)

The photo above

clearly highlights

the need for

thorough

evacuation. It

shows residual

vapours (possibly

containing moisture

“boiling out” of the

oil drained from an

Accumulator (VP

BOILING POINT OF WATER UNDER A VACUUM

How do we measure the ultimate vacuum? We read it off our compound gauge of our manifold gauge set. This of course in real terms is a joke. The only way to measure real vacuum is with an accurate gauge (of which are available) and connect it to the vacuum pump or in line. The gauges may be LED (incremental step gauge), digital, or analogue (needle). For further information on these gauges, contact VASA on 08 8289 4260 (Fax).

In addition to the obvious problems of systems open or empty for extended periods we have the additional problems of:

♦ The hygroscopic nature of PAG’s and Esters

♦ Substandard retrofitting where some people are not even changing driers

♦ The extreme sensitivity of systems to moisture especially systems using high efficiency condensers and evaporators (ie genuine 134a systems).

Old style modine

High Efficiency

(Serpentine)

Parallel Flow

The chart opposite

clearly shows a

vacuum of at least

100.6 kPa is

required to “boil

off” system

moisture on a cool

day.

SYSTEM

VACUUM kPa

SYSTEM

VACUUM

INCHES

TEMPERATURE ºC

BOILING POINT

96.50

28.50

32.2

99.09

29.18

21.1

100.44

29.66

10.0

100.59

29.71

4.4

100.97

29.82

-6.7

-101.13

29.87

-15.0

Modern Parallel

Flow Condensers

(far right) are much

more critical to any

acid formation

(residual moisture

left in system) than

the “old” thick

walled modine type

In fact measuring the ultimate vacuum in a system is a more accurate way of ensuring complete moisture removal than the time based system we now use. By measuring the total system pressure, with an accurate gauge, we actually measure the presence of vapour therefore the dryness of the system. Remember our earlier statement that 1ml of water expands to over 1000 litres of vapour. If moisture is still present in the system (and still boiling off) it should be obvious that we will not achieve our desired vacuum. There is however one extremely important point to highlight in pressure analysing systems.

The reading on your vacuum gauge is always lower than your actual system vacuum due to molecular flow past the gauge. To accurately evaluate system pressure you must isolate your pump and allow the system to relax (settle out). A rise to above the recommended level (-100kPa) indicates vacuum is incomplete.

Have we got the picture yet? In fact what we are doing is evacuating by

PRESSURE not by TIME as has previously been the case. Providing our relaxed pressure is below the recommended level then we know:

⇒ We have achieved our acceptable vacuum

⇒ There is no moisture still present in the system (the gauges will rise if there is)

⇒ We have no leaks.

If the pressure does rise to above acceptable levels we need to differentiate between:

• Insufficient vacuum time to boil off the moisture

• Poor vacuum pump performance - (When did you last change your vacuum pump oil? - it is critical!!)

• Leaks causing a loss of vacuum.

Using this method what you are actually doing is evacuating for as long as it takes to remove the moisture and non condensables which will, given the factors (variables) previously mentioned, vary dramatically from system to system. A genuine 134a system of high integrity may only require a 10 minute evacuation if it has never been open to atmosphere - a retrofitted or empty system may take 2 hours or more. What we have previously done of course is evacuated for 30 minutes because some testing was done back in the dark ages to verify that this was long enough for a “normal” automotive service. Or perhaps it was a figure that was pulled out of the air!!

For systems with high moisture content a scavenge or sacrificial charge is recommended to reduce evacuation times (See next page).

The photo above

was taken with a

system “vaccing

down” - all looks

good. (-100 kPa)

However when the

system was isolated

it showed a pressure

rise to unacceptable

levels. (94 kPa) as

shown below.

Evacuation is

incomplete for this

Possibly the best example of evacuating by pressure is with “on line” factory fitted genuine systems. They are only evacuated to 100 kPa or better for a short period of time. WHY? Because the system has new componentry, the components were probably nitrogen headed and therefore the system is relatively free of moisture. Why vac down for 30 minutes when no moisture is present? They only vac down for as long as necessary. ie 3 minutes at 100kPa or better. They have pretested to verify adequate evacuation of these systems - given the near perfect conditions.

Capacity

Capacity relates to the size of the vacuum pump. ie 28, 56, 94 up to 300 litres per minute. How important is it? For larger systems a 75 to 100 litre/ min pump working through ¼" (5mm) service hoses will not be 3 times faster than a 28 litre pump simply because the capacity is really being governed by the restriction to flow offered by the hoses. It will be faster but not 3 times faster.

NOTE: For technicians working on plant and equipment using long service hoses it is strongly recommended you increase the vacuum service hose to 3/8" ID (or larger). This will maximise the benefit of using the larger vacuum pump. Workshop manifold vacuum systems require larger ID pipes depending upon the length and complexity of the manifolding system, with a pump capacity adequate to handle all outlets in operation.

Single or Two Stage Pumps.

Once again from a clinical point of view a two stage is superior but a well maintained single stage pump is adequate for “normal” automotive work. The critical point is that either pump be maintained to ensure its performance is retained. All vacuum pumps must be serviced in accordance with manufacturers recommendations. For maximum pump life the pump must be serviced it if is being “taken out of action” for an extended period of time.

Sacrificial (Scavenge) Charging

(Otherwise known as blotting or sweeping a system)

There is nothing new about this procedure - it has been around for years, except it was not widely practiced due to a lack of necessity. It is now considered standard practice on “wet systems” - systems that have been open OR on systems where flushing agents have been used to ensure the removal of residual flushing vapours.

The capacity of

large vacuum

pumps is often

reduced

dramatically by

using ¼" service

hoses. The vacuum

hose in particular

should be upgraded

to 3/8" if possible

Sacrificial Charging

has been an

accepted practice

for years (look up

an XE/XF

Workshop Manual).

It has not been used

through a perceived

lack of necessity.

By following the previously mentioned guidelines of “pulling a deep vacuum” and ensuring it is held it will become obvious that excessively long vacuum times will be required to facilitate total moisture removal on “wet” systems.. In practice this is messy (to pull a vacuum and check it holds repeatedly) it is simply too time consuming. To short cut the entire process you need to make the decision at the beginning of the evacuation “I’m going to do a sacrificial on this job because of the circumstances” (ie system has been flushed or open). What a sacrificial charge does is ensures adequate moisture and residual vapour removal without excessively long evacuation times.

E

VACUATION

P

ERFORMANCE

Examples below are for a wet system (system that has been open to atmosphere or empty for an extended time).

• System 1 Kg capacity

• Vacuum Pump pretested to verify performance.

Single Evacuation 90% 40 minute single evacuation Moisture Removal

Double Evacuation 20 minute initial evacuation. 97% 200g sacrificial charge. Moisture 5 minute rest and dump the 200g Removal sacrificial charge.

New drier and Revac 20 minutes. Charge system.

Triple Evacuation

20 minute initial evacuation 99% 200g sacrificial charge Moisture 5 minute rest and dump Removal New drier and Revac 20 minutes

200g sacrificial charge 5 minute rest and dump Revac 20 minutes Charge system.

The basic purpose

of using a sacrificial

charge is to shortcut

the evacuation

process on systems

that have been

flushed or open to

atmosphere.

When using

sacrificial charges

DO NOT

RECOVER the

sacrificial charge

(until legislation

governs otherwise).

It is very probably

laden with foreign

In approximate terms a double evacuation is equivalent to 24 hours on evacuation. A triple evacuation is equivalent to 48 hours on evacuation.

Changing The Drier/Accumulator

The next point of contention is when to change the receiver drier in service - particularly in retrofit. Without being too critical many technicians don’t give this point enough thought - and it is actually very important.

Let’s look at various case scenarios.

Clean Retrofit

No flush - System not previously open or empty.

The low foreign vapour component in the system means it is generally practiced that the receiver drier is changed during retrofit.

High Moisture Content Systems

System Open or Empty at time of Retrofit/Service.

If a double evacuation is practiced (incorporating a sacrificial charge) leave the old drier in during the initial evacuation and during the sacrificial charge and dump. By replacing the drier just before the final evacuation it is going into a relatively clean R134a system.

Flushed Systems

If the system is flushed (vapour contaminated) the process of changing the drier just before the final evacuation is strongly recommended. If the drier was removed for flushing the old drier (or a dummy) should be temporarily put into circuit for preliminary evacuation and sacrificial charging. Ideally the drier connections should be “bridged out” for this process. That way we don’t have to “pull all the junk” out of the old drier. Bridging out driers is not normally practiced with pad mounts and in difficult to access applications.

Nitrogen Purging/Pressure Testing

If your normal practice is to nitrogen purge and/or pressure test the system replacing the drier just prior to final evacuation is still the recommended practice if the system has been open (wet system) or flushed.

Nitrogen testing and sacrificial charges serve the same purpose with respect to system cleanliness - to set up a clean environment in the system for R134a to operate at its best.

Whenever a double

evacuation is

practiced or the

system is heavily

contaminated the

old drier should be

left in during the

initial evacuation -

otherwise we are

simply pulling all

the “junk” through

the new drier.

A better option is to

bridge out the drier

connections during

the initial stages.

Triple evacuations

are normally only

practiced on

expensive/large

capacity systems

when

optimum

cleanliness is

required. Those

technicians working

on large

earthmoving plant

and agricultural

equipment that has

been open should

practice this option.

SINGLE EVACUATION

METHOD

U

(

)

NORMAL SERVICING

.

N

.

L

.

C

R

F

N

D

S

E

C

S

DOUBLE EVACUATION

METHOD

R

,

.

H

H

.

R

R

R12

C

R

I

F

R

P

E

(O

D

B

P

)

A

200

R134

D

S

C

F

N

D

S