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FEATURECOMMERCIAL REFRIGERATION

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May 2015 | HVAC&R Nation | www.hvacrnation.com.au

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REFRIGERATION SUPERHEATTo understand superheat in connection with an evaporator and system performance, we must first understand how it occurs, the reasons for setting the correct superheat in evaporator conditions, and the effect it has on the system.

What’s superheat?Refrigerant undergoes several changes in a vapour-compression system. The temperature at which refrigerant fluid changes state from liquid to vapour and, conversely, from vapour to liquid, is known as the “saturation temperature” of that refrigerant.

It is important to note that the saturation temperature of the liquid and the saturation temperature of the vapour are the same for any given pressure, i.e. the temperature of vaporisation of the liquid and the temperature that the vapour will condense.

So, it follows that any temperature rise above saturation of the liquid will result in a vaporisation of some of the liquid and, conversely, that reduction in temperature of vapour will cause a condensation.

A refrigerant vapour that is above saturation temperature (corresponding to its pressure) is a superheated vapour. When energy is applied to a vapour, causing the vapour’s temperature to rise and convert from a liquid to vapour state, then it is referred to as superheated vapour, or superheat.

Why is it important?Most evaporator systems in commercial refrigeration use a thermostatic expansion valve to meter refrigerant into an evaporator. This is one of the two points in a vapour compression system that are the “dividing points” between the high and low sides of the system.

The other dividing point is the refrigeration compressor between suction and discharge.

The expansion valve, or TX-valve, feeds high-side liquid refrigerant though an orifice into the evaporator. Here it boils off and expands into a vapour, collecting heat from the medium being passed over the evaporator coils and fins.

In this example, we’ll use air in a cool room or a refrigerated case, for clarity.

As the refrigerant temperature rises in the evaporator and collects the heat from the air, it reaches a point where it can perform no further useful work. To find this optimum point and use the evaporator surface, we set the “superheat” point on the evaporator.

What happens if you get it wrong?Too low a superheat setting may result in some liquid not being vapourised in the evaporator, which leads to some liquid being returned to the suction side of the compressor. This can cause liquid knock and damage to the compressor.

If superheat is set too high, not all of the effective surface area of the evaporator will have been used, resulting in poor refrigerated case or cool room performance.

Many refrigeration technicians assume that by checking the coil-in to coil-out temperature they are able to determine the evaporator superheat. However, they are wrong.

SHARPEN YOUR TOOLSThere are a few areas of commercial refrigeration where most fridgies could, in all honesty, lift their game a little bit. HVAC&R Nation spoke to Refrigeration Innovations principal Dave Redden, M.AIRAH, about four of the most commonly misunderstood and mismanaged areas of commercial refrigeration.

May 2015 | HVAC&R Nation | www.hvacrnation.com.au

Using an accurately calibrated suction gauge, measure the evaporator pressure at its outlet; most manufacturers provide a point to do this. Then, with a fast-response temperature probe, securely fastened as close as possible to the TX sensing bulb and insulated against air movement, check the temperature of the coil outlet.

By converting the pressure reading on the gauge to temperature and subtracting from the temperature reading of the probe, you will have determined the evaporator superheat.

Of course, this will be determined over the cycle of the TX and the evaporator. There is no point setting the superheat upon start-up, nor immediately after a defrost cycle when the TX will be wide open and the evaporator working at capacity. Observe several cycles to ensure accuracy.

COMMISSIONING AND CONTROLSThe importance of correctly commissioning a refrigeration system cannot be overemphasised.

The system design engineer carefully calculates loads on the compressor rack or condensing unit to obtain maximum efficiency, and the technician runs pipework according to best practice. There is no point in doing these things if the system is simply going to be charged with refrigerant, switched on, and left.

Following a pre-determined, methodical and recorded process is the only way to ensure the plant performs to its design criteria and the client’s expectation.

Additionally, the defects liability period cost is borne by the installing contractor. On top of which, poor commissioning results in poor performance, high and unnecessary power use and adverse recommendations from the client.

Pre-start-up check and tests are vital. Ensure that the electricians have checked that all connections are tightened and connected according to the wiring diagrams – don’t just assume that they are. Most time spent on commissioning is tracing control and erroneous alarm issues.

Where does commissioning start and end?Some companies believe it starts at the pressure testing, some when refrigerant is added. Both these times involve the installing company. But, certainly, whomever is signing the commissioning sheets needs to be present at these stages.

Safety is absolutely paramount. The electrical side, once checked and ready to run, could be considered the first stage. The setting of safety pressure controls for high and low cut-outs is next. You should never rely on the scales on the sides of pressure controls as the only method of setting; use a gauge and dry nitrogen through a pressure regulator to determine that the correct set-points are obtained.

Think of it in the same light as pilots conducting pre-flight checks: it’s too late on take-off to realise that the correct settings for flight are not enforced. Similarly, once plant is running, it’s too late to discover the oil pressure settings cannot protect the compressor from damage.

As the industry transitions to natural refrigerants, and R744 (CO2) becomes the norm in larger installations, the setting up and testing of gas detectors becomes even more paramount.

You can’t smell or see CO2, so you must ensure all detectors, alarms and exhaust fans in the plant room are fully functional before charging the system.

Condenser set-points, compressor high and low set-points, evaporator set-points, and defrost periods and durations – are they spaced sufficiently? – are all valid reasons for being methodical.

Doing it right at the start saves a great deal of work – not to mention embarrassment – down the line.

PIPE SIZINGPipe sizing often comes up in discussions about continuing installation problems. If it’s done right, the installation will perform as designed; get it wrong, and the consequences are there for the life of the installation.

Even in a somewhat simplistic view, there are many things to consider during the design of a refrigeration piping system: types of refrigerants used, pressure of the refrigerant (CO2 is a good example), distance to be run and the route taken, temperature of the refrigerating system, design ambient, and many others.

Incorrect pipe selection generally falls into three categories: undersizing, oversizing, and incorrect wall thickness and grading.

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www.hvacrnation.com.au | HVAC&R Nation | May 2015

UndersizingRefrigerant piping is often undersized through ignorance, convenience or through trying to save on costs. Undersized piping results in high internal refrigerant velocities and good oil return but high line-per-metre pressure drops and resultant loss in system capacity; as well as compressor overheating due to compressor cooling losses and compressor failures.

Undersizing can also be caused by using too many bends and fittings in the pipe run. As with any fluid flow, friction creates resistance to flow, so the longer the pipe run and the more bends and fittings included creates friction penalties and a pressure drop in the run.

A pressure drop will then create capacity reduction at the compressor in the case of the suction line, and capacity reduction at the expansion device and evaporator in the case of the liquid line.

Bad pipe design can cause liquid “hang up” in the condenser, which can reduce capacity and cause erratic operation of the expansion device.

OversizingOversized refrigeration piping commonly results in low refrigerant velocities, poor oil return but also low line-per-metre pressure drops, thus no resultant system capacity losses are experienced. But it’s false economics. The selection and installation of refrigeration piping continues to be the source of concern and frustration to many members of the industry. It can lead to

inefficient system operation and performance. Also, it can lead to the voiding of manufacturer warranties in some cases, which in turn damages the reputation of the industry in the eyes of consumers.

Oversizing can also result in compressor oil losses and compressor failures due to a lack of lubricant.

Incorrect wall thickness and gradingPopular refrigerants such as R410A and R744 (CO2) operate at much higher system pressures, making it essential to ensure the pipe wall thickness and therefore the maximum safe working pressure of the pipe system and all line components used are correctly rated. Incorrect wall thickness can result in pipe-work failures and splitting pipes, leading to refrigerant leaks.

Note: It is the velocity of the vapour at the pipe surface rather than the overall gas velocity that is important. This velocity is directly related to the inside of the pipe diameter and the density of the refrigerant. It could be considered as follows: the lower density and pressure of the vapour and the larger the inside diameter, the higher average velocity needed to produce a given surface velocity.

BASIC MAINTENANCEMany clients do not see the necessity of carrying out this vital task … until late one Friday evening on a

very hot day, when the system gives out. This has a great deal to do with clogged condensers, and slight shortage of refrigerant charge. System that tick along reasonably well during the cooler months, suddenly stop coping.

And aside from instances of catastrophic failure on hot days, poorly maintained plant costs more to run – if you explain that to your client, their ears are sure to prick up. The following sections cover some key maintenance points for different equipment and systems.

CondensersCondensers are the most exposed part of the system: stuck on the roof or down an alley, busily extracting as much passing detritus as possible. It’s important to regularly clean the fins, using dry nitrogen after an initial brush-down to remove larger items.

Hosing out with water is a good start; then inspect the fins for damage and corrosion. Tell-tale signs of oil trace around joints and return bends are a good indication of wear and tear.

Check the operation and rotation of fan motors – the amount of condensers with one or more motors operating in the wrong direction is astounding, and a sign of poor commissioning. Replace any that do not run at speed.

If the system is fitted with heat reclaim, check the three-way valves for operation and any leaks. Also, run the leak detectors over all surfaces.

COMMERCIAL REFRIGERATION FEATURE / 23

May 2015 | HVAC&R Nation | www.hvacrnation.com.au

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CompressorsIn the compressor, check oil levels and colour – an initial sight-check for contamination is a good start. It’s also preferable to carry out a proper acidity check with one of the many kits available.

Also check for oil traces and leaks, and listen to the compressor operating – any fridgie worth his salt should be able to hear if anything is wrong: bearing wear, knocking from the heads, etc.

If in any doubt, check the amps draw – does it match the rating plate?

A good, quick test on compressors fitted with suction and discharge shut-off valves is to fit your gauges and carry out a compression check. This should reveal a lot. After ensuring the compressor is safe, check all electrical connections for tightness or signs of burning.

Check the head fans on low-temperature compressors are free-running and up to speed. Check and record

HP/LP settings. When inspecting bolts and pipework, check for tightness, ensure brackets are tight, and ensure there is no rubbing of pipes or control lines.

Cool roomsWhen it comes to cool rooms, it’s important to remember that evaporators, after condensers, are the most affected by dirt. Ensure they are clean and the coils are not blocked, and treat with a proprietary coil cleaner and pressure wash – remember to clear the area of stock for this procedure.

Check fan motors and blades, again for speed and rotation.

TX valves should be checked – is the bulb secure? Check the superheat. Also carry out a thorough leak test, check electrical connections, fans and defrost heaters, and drain heaters, check for damage, as well as condensate drains. Door hardware should be checked – check whether they close and seal, hinges are in good order, and the running gear on sliding doors.

Cool rooms suffer enormous damage where pallets are stored; note and report any panel damage.

Leak detectors should be checked, if fitted. Lighting should conform to standards, report any faulty lighting, or in the case of tubes or globes, ensure they are replaced.

Personal trapped alarms, whether the twist-bell type or electronic type, need to be checked and in good order – there’s good money on at least 50 per cent missing the bell housings.

Low-temperature rooms are fitted with pressure-relief ports. Ensure they are fully functional and free of ice.

Refrigerated casesAs for maintenance on refrigerated cases, things were a lot easier in the “good old days” when stores closed on Saturday afternoons and Sundays. There was always a good opportunity to carry out case maintenance. Now, in the days of 24/7 trading . . . not so much.

However, if you are fortunate enough to schedule case maintenance with the highly prized empty case, start with a good pressure clean of the air ducts, honeycombs and evaporators. Remove any debris, and ensure the drains are free running – the greatest sale of fan motors would be the result of blocked drains. Check that fan motors are operating correctly – remove the blades; more often than not, there will be plastic wrapped around the shaft.

Leak-test all components, and check the TX-valve bulb for secure fitting. Also check the superheat setting: often someone has given it a tweak since commissioning, usually because of some other system fault or poor airflow, reducing performance. Check electrical connections, especially defrost heaters, and make sure they have not crept out of their brackets.

Lighting should be inspected for safety. The case should be checked to ensure it is using the correct tubing for the product.

Finally, ensure all evaporator covers and fan panels are correctly fitted and affixed. The case manufacturer did not fit those screws for fun, and they are there for a specific reason, air pressure being the main one.

One of the great advantages of control systems is the ability to look at system operation graphs – take time to sit down and go through them, because they can tell you much. ▲

Fridgie festNext month, AIRAH’s Refrigeration 2015 Conference will touch down in Sydney.

AIRAH’s Refrigeration 2015 Conference will be held in Sydney, June 2–3, commencing with a site visit and dinner on day one, followed by a full-day of conference technical presentations on day two.

AIRAH CEO Phil Wilkinson, F.AIRAH, says that much has happened since AIRAH last held a refrigeration conference in 2013.

“It’s been two years since we held an AIRAH refrigeration conference, with considerable changes taking place in the legislative and regulatory space during that time,” Wilkinson says. “At the same time, applications of new technology, emerging trends and exciting work taking place in research mean that there is plenty to discuss at a gathering of Australia’s foremost refrigeration professionals.”

Conference committee chair Julian Hudson, M.AIRAH, says presentation topics will cover everything from replacing R22 with natural refrigerants, to the use of hydrofluoroolefins in stationary systems.

Aimed at consultants, students, researchers, government and other industry professionals who have an interest in the design of refrigeration systems, this conference is the fourth in a series of highly successful AIRAH refrigeration conferences.

The conference will be held in conjunction with AIRAH’s Energy Efficient Industrial Refrigeration training course, held June 1–2. This course covers topics such as energy efficiency in temperature-controlled warehouses, and cooling, chilling and freezing in food processing plants.

Register nowFor more information on AIRAH’s Refrigeration 2015 Conference, or to register, visit www.airah.org.au/Refrigeration2015

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