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Purge Units & Leak Control — EPA 608 Practice Questions

35 questions · 25% of the Type III — Low Pressure exam

Worked questions

1. A high efficiency purge unit on an R-123 chiller has a small chamber with a drain fitted to it. What routine service does that chamber need?

  • AIt has to be topped up with fresh oil, because the purge compressor draws its lubricant from it

    Purge compressors carry their own oil charge, and no purge is designed to be lubricated from a water separator.

  • It has to be drained of the water the purge has condensed out of the air it removes
  • CIt has to be filled with dry nitrogen, because that is what carries the non-condensable gas out

    Nothing is added to a purge unit to carry gas out. The purge compressor moves the gas, and the separator only collects what condenses.

  • DIt has to be pumped down with the recovery unit, because it holds the machine's spare charge

    The chamber holds water, not a reserve of refrigerant, and a chiller does not store spare charge inside its purge unit.

Why B is correct

The purge chills the gas it removes so refrigerant condenses and can be returned to the machine. Water vapor that came in with the air condenses at the same time and collects in a separator. Draining it is part of routine purge maintenance, and the rate at which it fills is a direct measure of how much moist air the machine has been drinking.

What this question is testing

This tests understanding of what actually comes out of a machine that has been drawing air in, and of the maintenance that follows from it. The exam links moisture to air ingress repeatedly, because the acid damage that follows is the reason leaks on these machines are urgent.

On the job

Technicians who service chillers regularly open that drain on every visit and note how much comes out, because the volume is one of the cleanest indicators of how much air the machine is taking in. On a tight machine there is almost nothing. On a machine with a bad shaft seal it can be a cup a week, and every drop of it represents moisture that spent time in contact with the refrigerant and the oil before it was caught.

Memory technique

The purge's drain is a rain gauge for your leak.

Exam tip

Air brings water. Anything that removes air from a chiller ends up with water to get rid of.

Where to look it up

Moist air drawn into a chiller condenses in the purge unit, and the water it produces collects in a separator that has to be drained.

2. A chiller has a leaking evaporator tube and the waterboxes have been removed so the tube sheet is exposed. How does the technician identify the individual tube that is leaking?

  • Bring the refrigerant side up with nitrogen inside the limit and check each tube end for escaping gas
  • BFill each tube with chilled water in turn and watch the shell sight glass for a change in level

    Filling tubes one at a time is unworkable on a bundle with hundreds of tubes, and the shell level would not respond usefully to a single tube being wetted.

  • CRun a light down the tubes and look for the one whose inside surface shows a bright scale line

    Scale and pitting look much the same on a sound tube and a failed one, and a visible defect is not proof of the leak path.

  • DMeasure the water flow through each tube and identify the one carrying the lowest flow rate

    Flow measurement through individual tubes is not practical, and a leaking tube does not necessarily carry less water than its neighbors.

Why A is correct

The leak path runs from the shell into the tube, so putting pressure on the shell side sends gas out through the failed tube and into the waterbox where it can be detected at the tube sheet. The pressure has to stay under the machine's limit, which is why the nitrogen goes through a regulator with a relief device set below that limit.

What this question is testing

This tests a practical procedure that follows directly from the pressure limit and from the direction of the leak path. The exam ties tube failures to the same low pressure ceiling that governs any other leak test on these machines.

On the job

This is a slow, methodical job: the technician works across the tube sheet with a detector probe, marking candidates, then confirms with soap. On a large bundle it can take most of a day. Once the tube is identified it gets tapered plugs driven into both ends, which is a temporary measure limited by how many tubes the manufacturer allows to be plugged. The machine is then evacuated and dried before the charge goes back in, because water has been inside the shell.

Memory technique

Pressure in the shell, detector at the tube end. The leak tells you which tube it is.

Exam tip

Any tube leak test on a chiller still has to obey the shell pressure ceiling. The method changes; the limit does not.

Where to look it up

A leaking tube is found by pressurizing the refrigerant side with dry nitrogen, within the appliance's pressure limit, and checking the exposed tube ends.

3. Beyond logging purge run time, what routine laboratory check tells a chiller owner that air and moisture have been getting into the machine?

  • AA hardness test of the condenser water, showing how much scale is forming inside the tubes

    Water side hardness tells you about scale and water treatment. It says nothing about what has entered the refrigerant side.

  • A sample of the refrigerant and oil analyzed for moisture content and acid formation
  • CA vibration survey of the compressor, showing whether the bearings have begun to wear

    Vibration analysis catches mechanical wear, some of which is caused by acid attack, but it reports the consequence rather than the moisture that produced it.

  • DA power reading at the starter, showing how far the machine has drifted from its design draw

    Power draw rises with non-condensable gas as well as with fouling and load, so it cannot distinguish air ingress from several other causes.

Why B is correct

Laboratory analysis of the refrigerant and the oil measures the two things air ingress actually leaves behind: moisture and acid. Because the results are numbers rather than impressions, they can be trended across years, which turns a vague concern about a leaking machine into evidence an owner can act on and budget for.

What this question is testing

This tests knowledge of the evidence trail around a leaking low-pressure machine. The exam is checking that the candidate connects air ingress to moisture and acid rather than treating a leak only as a loss of refrigerant.

On the job

Large plants put chiller oil and refrigerant analysis into the annual maintenance contract, and the report is one page of numbers with a trend line. It is the single most useful document for arguing that a leak has to be fixed now rather than next year, because the owner can see acid climbing. After a repair and an oil change, the following year's sample is what proves the machine is clean again.

Memory technique

The oil remembers every gallon of air the machine ever drank. Send it to the lab and ask.

Exam tip

Air in means water in, and water in means acid. Anything that measures moisture and acid is measuring the leak.

Where to look it up

Annual refrigerant and oil analysis for moisture and acid documents air ingress into a low-pressure machine.

4. A purge unit on a low-pressure chiller cycles and discharges through a line to the outdoors. What is in that discharge?

  • AOil vapor carried up from the compressor sump by the rising condenser temperature

    Oil is not volatile at these temperatures and does not travel to the top of the condenser as a gas.

  • BRefrigerant vapor that the machine has no room for at current condenser pressure

    A condenser does not run out of room for refrigerant, and a purge is not a pressure relief device.

  • CChilled water vapor that has crossed the tube walls into the refrigerant space

    Water crossing the tube walls means a tube has failed, which is a fault rather than a description of normal purge discharge.

  • Air and other non-condensable gas, with water vapor and a trace of refrigerant

Why D is correct

The purge exhaust is mostly atmospheric air that leaked into the machine, plus the water vapor that came with it. Refrigerant is meant to be separated out and returned, so a small trace is all that should leave. Understanding the composition explains both why the discharge is not treated as venting and why a poor purge costs the owner refrigerant.

What this question is testing

This tests understanding of the purge's contents rather than its function. The exam separates the two because the composition of the discharge is what determines whether it is a permitted release and how much refrigerant the owner is losing.

On the job

Operators watching a purge cycle for the first time almost always assume the machine is blowing off refrigerant, and the conversation that follows is a good chance to explain why purge run time is worth logging. On a tight machine with a modern purge, the annual refrigerant loss through the exhaust is small. On a leaking machine with an old thermal purge, it can be a significant share of the charge, and the operator's log is the evidence that pays for the repair.

Memory technique

Whatever leaked in, leaks back out through the purge, and it should take as little refrigerant with it as possible.

Exam tip

The purge discharge is the air the machine drank, plus its water, plus a little refrigerant. The last part is what a good purge minimizes.

Where to look it up

Purge exhaust consists of non-condensable gas drawn into the machine, with water vapor and a small amount of refrigerant.

5. A running low-pressure chiller has a leak at a joint on its condenser. What is happening at that joint while the machine is loaded?

  • ANothing moves either way, because the condenser sits exactly at atmospheric pressure at load

    The condenser is above atmospheric at load rather than at it, which is exactly why a leak there behaves differently from one on the evaporator.

  • BAir is drawn in, because every part of a low-pressure machine is under a vacuum at all times

    The evaporator is under a vacuum; the condenser is not. Treating the whole machine as a vacuum vessel is the most common error on these questions.

  • Refrigerant is escaping, because the condenser runs slightly above atmospheric pressure at load
  • DWater vapor is drawn in, because the condenser is the coldest surface on the machine at load

    The condenser is the hottest part of the machine at load, not the coldest, and nothing is drawing water vapor into it.

Why C is correct

The pressure profile across a running machine decides the direction of every leak on it. The condenser is above atmospheric at load, so a leak there pushes refrigerant out and can be found with an ordinary electronic detector. The same joint reverses direction on shutdown, which is why these machines both lose charge and accumulate air over a season.

What this question is testing

This tests the pressure profile of a running machine applied to leak direction. The exam keeps returning to the vacuum story, and this question checks that the candidate learned it with the exception attached rather than as a blanket statement.

On the job

This explains a result that confuses technicians on their first chiller: the sniffer alarms at one end of the machine and finds nothing at the other, and both readings are correct. It also explains why a chiller can lose charge steadily while its purge log climbs at the same time. Finding the condenser leaks with the machine running, and the evaporator leaks with the shell pressurized during a shutdown, is how experienced crews split the work.

Memory technique

Cold end sucks, hot end blows. The machine is a vacuum machine only at one end.

Exam tip

Evaporator under a vacuum, condenser slightly above. Leak direction follows the pressure at that spot, not the machine's reputation.

Where to look it up

On a running low-pressure chiller the evaporator is below atmospheric pressure and the condenser is slightly above it.

6. Some high efficiency purge units send their exhaust through an activated carbon canister before the gas leaves the building. What is that canister doing?

  • ADrying the gas so that the water vapor removed with the air does not condense in the vent line

    The purge condenses moisture out into its own separator. The carbon bed is not there to dry the vent gas.

  • Holding back the last traces of refrigerant so less of it leaves with the discharged air
  • CFiltering oil mist so that the exhaust does not stain the roof around the vent termination

    Purge exhaust does not carry meaningful oil mist, and a carbon bed is not an oil filter.

  • DSilencing the discharge so that the purge cycle cannot be heard from the occupied floors

    Noise from a purge discharge is not what a carbon canister addresses, and silencers are not made of activated carbon.

Why B is correct

Activated carbon adsorbs refrigerant vapor onto its surface, so passing the purge exhaust through a bed strips out the traces that survived the condensing stage. That directly improves the purge efficiency figure, which is measured as refrigerant lost per unit of non-condensable gas removed, and it reduces what the owner has to buy back each season.

What this question is testing

This tests knowledge of how purge efficiency is actually achieved rather than how it is defined. The exam ties the efficiency concept to hardware, because the owner's refrigerant bill is the reason the concept exists.

On the job

Owners who have replaced an old thermal purge with a high efficiency unit often see their annual refrigerant purchases fall by a large fraction, and the carbon stage is part of why. For a technician the practical items are checking the replacement date, keeping a spare canister on the shelf and recognizing that a saturated bed quietly stops working without changing anything the operator can see on the panel.

Memory technique

The carbon is a net at the door, catching the refrigerant trying to leave with the air.

Exam tip

Efficiency on a purge means refrigerant kept per pound of air removed. Carbon beds exist to improve that number.

Where to look it up

Purge efficiency is judged by refrigerant lost per unit of non-condensable gas removed; carbon adsorption reduces that loss.

29 more in the bank

Answers and explanations for these are in the app.

  • What is the primary job of the purge unit fitted to a low-pressure centrifugal chiller?
  • How is the efficiency of a purge unit on a low-pressure machine judged?
  • What pressure limit applies when nitrogen is used to raise the pressure inside a low-pressure appliance for leak testing?
  • Where does the air that collects inside a low-pressure chiller during normal operation come from?
  • From what point on a low-pressure chiller does the purge unit normally take its suction?
  • What does a buildup of non-condensable gas do to a running centrifugal chiller?
  • A chiller's purge unit that once ran a few minutes each hour is now running nearly continuously. What does that tell the technician?
  • Why does a purge unit that runs around the clock cost the owner refrigerant as well as electricity?
  • What routine record helps an operator spot a developing leak on a low-pressure chiller before the charge is gone?
  • Why must the pressure used to leak test a low-pressure appliance be kept to a very low ceiling?
  • How can a technician test a low-pressure machine for leaks without raising its internal pressure at all?
  • During a standing vacuum test the pressure climbs a small amount, then holds steady for the rest of the test. What does that pattern point to?
  • What must be installed in the nitrogen line before a low-pressure chiller is pressurized for a leak test?
  • What does a high efficiency purge unit do with the refrigerant it separates from the non-condensable gas it removes?
  • Besides a leak in the shell, what commonly loads a low-pressure machine with non-condensable gas?
  • A purge unit on a chiller discharges to the outdoors several times an hour. How does that discharge stand against the prohibition on releasing refrigerant?
  • A leak has been found on a large low-pressure chiller that falls under the leak repair provisions. Within what period does the rule expect that leak to be repaired?
  • A low-pressure chiller is clearly drawing air in somewhere on its vacuum side and the technician has to find the exact spot. What has to be done to make the leak findable?
  • A chiller cannot be taken out of service yet, and the technician wants to narrow down where air is being drawn in while the machine keeps running. Which instrument can do that?
  • An oil analysis on an older chiller comes back with the moisture figure climbing year over year. Which part of the machine actually takes that moisture out of the refrigerant and the oil?
  • A chiller owner wants to know which of the remaining tubes are likely to fail next rather than waiting for the next flood. Which test gives that information?
  • Water is weeping from the joint where a chiller's waterbox cover bolts to the tube sheet. What kind of leak is that, and what does it require?
  • A chiller's purge unit is discharging normally on every cycle, yet the condenser pressure keeps climbing through the afternoon. What does that combination point to?
  • A chiller with a nine hundred pound charge is losing refrigerant and the repair will take weeks to schedule. Which party carries the legal duty to see that the repair gets made in time?
  • A chiller has been pressurized with dry nitrogen for a leak test and the technician returns hours later to read it. What has to be accounted for before deciding the machine has lost pressure?
  • A single evaporator tube has been identified as the source of a leak on an older chiller. What does plugging that tube accomplish?
  • A chiller holds a standing vacuum test overnight without losing anything, yet its purge runs constantly once the machine is back in operation. Where should the technician look?
  • A chiller has just been retubed and is being prepared to go back into service. What has to be done before the charge is returned to it?
  • Chilled water is finding its way into the refrigerant side of a low-pressure chiller. Which failure allows that to happen?

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