Refrigeration / Air Conditioning -...

Refrigeration / Air Conditioning

REFRIGERATION Purpose: The purpose of this package is to provide you with the

knowledge and skills to correctly set up, use, and close down an oxyacetylene plant; and attempt simple silver brazing exercises using an oxyacetylene plant.

THE OXYACETYLENE PLANT You use an oxyacetylene plant to mix oxygen and acetylene gases in the correct proportions and to burn the mixture to produce an extremely hot flame. This flame is deal for: 1. Silver brazing (which is how you join refrigeration pipe work) and; 2. Fusion welding of mild steel (which you will learn more about in a later

assignment). Figure 1 shows a disassembled oxyacetylene plant without the cylinders or trolley. An oxyacetylene plant consists of: 1. An oxygen cylinder 2. An acetylene cylinder 3. A regulator fitted to each cylinder 4. A torch (with various tips) 5. Hoses 6. Accessories such as the cylinder key, lighter and tip cleaning wires,

and 7. A trolley that allows you to move the plant around easily.


Note: The oxygen and acetylene cylinders are available in various sizes. As a refrigeration technician, you would generally use only the smaller sizes that are more easily transported.

Oxygen Cylinder The oxygen cylinder is hollow, painted black, and made of either steel or aluminium.

The oxygen (02) in the cylinder is a colourless, odourless gas that is essential for the combustion (burning) process. The oxygen is charged into the cylinder at a pressure of 17,500 kPag. This pressure falls continually (but slowly) as the gas is used. A valve fitted into the neck of the cylinder receives the oxygen regulator.

Refrigeration / Air Conditioning Safety precautions: 1. Never bring pressurised oxygen into contact with ANY oil or grease.

Oil or grease mixed with pressurised oxygen can explode, without needing a flame or spark to set it off.

2. Never expose oxygen cylinders (or any other pressurised containers) to

heat. Heat can raise the internal pressure to an unsafe level. 3. Never store empty oxygen cylinders with their valves open. Air,

moisture or dirt entering the cylinder affects the purity of the contents when the cylinder is refilled. Also, the moisture corrodes the cylinder internally where such damage is difficult to assess.

4. Never use the oxygen for purposes other than for what it is meant.

Remember that oxygen supports combustion and can instantly create a fire from smouldering material (such as a lighted cigarette).

Acetylene cylinder The acetylene cylinder is painted dull red (maroon). Acetylene gas is a highly combustible gas composed of carbon and hydrogen (C2H2). It is dissolved into a liquid called acetone, which is soaked into a porous material inside the steel acetylene cylinder. The porous material is similar to molecular sieve (which is coarse brick-like material used in refrigeration filter/driers to collect moisture and other contaminants).

Acetylene gas has a strong sweet smell, and is the fuel used to create the flame at the torch. The pressure of a full acetylene cylinder is about 1600 kPag. (However, if an acetylene cylinder holds less than its full charge of acetone, it will not hold its full charge of acetylene even at 1600 kPag). The outlet valve has an unusual left-hand thread to receive the acetylene regulator. (You screw into a left-hand thread in an anti-clockwise direction, instead of the normal clockwise direction used with a standard right-hand thread.) You must be careful when storing or using acetylene cylinders because acetylene can form an explosive mixture with air.

Refrigeration / Air Conditioning Safety precautions: 1. Store acetylene cylinders (that are not being used) in a cool well-

ventilated location, away from combustible materials and away from oxygen cylinders.

2. Never leave apparently empty acetylene cylinders with their outlet

valves open. The acetylene takes a long time to completely leave a cylinder and while it is still escaping it represents a fire and explosion hazard.

3. If an acetylene cylinder develops a leak, move the cylinder into the

open air and notify the supplier. However, if it leaks at the gland around the vale spindle, you can repair this yourself by tightening the gland nut.

4. Never use or store an acetylene cylinder on its side. If you do this,

acetone may collect near the outlet valve and escape with the acetylene when it is released. While this is not dangerous, the acetone gives the flame an orange colour and reduces its heat output. This may be harmful to the welding, brazing or whatever you are doing. Also, the loss of acetone from the cylinder reduces its future storage capacity.

5. Never draw off the acetylene too fast. If you use more than one-fifth of

the cylinder’s capacity per hour the acetone may foam up (like a fizzy drink that has been shaken) and again escape with the acetylene.

Pressure regulators You attach regulators to the oxygen and acetylene cylinders to convert the high and varying cylinder pressures to constant low pressures for the torch. How they work: The outlet pressure from a regulator is controlled by a spring acting on a diaphragm. When the spring is relaxed (pressure-setting knob wound out) the regulator valve is closed and no gas flows from the cylinder. See Fig. 4.

Refrigeration / Air Conditioning When the pressure-setting knob is wound in, the compressed spring moves the valve off its seat, and gas flows into the low-pressure chamber to act against the flexible diaphragm. See Fig. 5. When the pressure in the low-pressure chamber reaches the required level, it marginally overcomes the spring pressure to close the valve. Any decrease in outlet pressure (caused by a gas flow through the torch) will re-open the valve to maintain the outlet pressure. You can vary the outlet pressure by adjusting the pressure-setting knob, and so altering the spring loading. If a regulator creeps, that is, if it passes gas when the pressure-setting knob is released or builds up pressure on the outlet side when the touch valve is shut, it should be repaired. Quite apart from risks, creeping regulators lead to poor work because the flame characteristics continuously change. Pressure gauges: Most regulators have two gauges mounted side by side on top of the regulator body.

One gauge shows the cylinder pressure. The other (closest to the hose connection) shows the outlet pressure to the torch (working pressure). See Fig. 6.

Refrigeration / Air Conditioning Hoses The hoses used with oxyacetylene cylinders are made from canvas-reinforced rubber. The oxygen hose is blue (or sometimes black or green) while the acetylene hose is red. The fittings on each end of the acetylene hose have the unusual left-hand thread so that the hoses cannot be accidentally swapped. Examine hoses regularly for leaks, loose connections or worn patches. If you find a fault either replace the hose concerned or take it to a suitable company for repairs. Do not mend hoses with tape or with short pieces of tubing.

Note: If you mend an acetylene hose with copper tubing, an explosive compound (copper acetylide) may be produced by a chemical reaction between the acetylide) may be produced by a chemical reaction between the acetylene and copper.

Torch Figure 7 shows the torch. The torch has four main parts: 1. The needle valves, which allow fine adjustment of the gas flows to the

tip; 2. The torch body, which provides a comfortable hand grip; 3. The mixer, where the gases are mixed; and 4. The tip, which is available in various sizes to suit different sized flames.

Refrigeration / Air Conditioning Accessories Cylinder key: The cylinder key is a double open-ended spanner with a small cylindrical boss (which has a square hole) welded to one end. One open end of the spanner fits the regulator nuts. The other open end fits the hose connections. The square hole fits the cylinder outlet valve spindles. See Fig. 1. Goggles: Use well-fitting welding goggles with the correct shade of lens when fusion welding, bronze welding, gas cutting, or whenever the flame or molten metal is very bright and dazzling to your naked eyes.

The goggles will not only protect your eyes but also allow you a clear view of the job. You do not need to use welding goggles while silver brazing. Neither the flame nor the molten brazing alloy is overly bright to look at. Also, unlike many other molten metals, molten brazing alloy does not pop or spatter.


Lighter: the safest way to light the torch is to use a flint lighter that is specially designed for the job. You may burn your hands if you use matches to light the torch.

REMEMBER

A mixture of oxygen and acetylene produces an extremely hot flame that is ideal for fusion welding and silver brazing. Oxygen cylinders are black and empty, except for the oxygen they contain. Acetylene cylinders are painted dull red and contain a porous material and acetone, as well as the acetylene gas. They have an unusual left-hand thread on their outlet valves. Never allow oil or grease to meet high pressure oxygen. Acetylene can form an explosive mixture with air. Regulators are precision instruments and you should treat them carefully.

REVIEW QUESTIONS AT THE BACK 1. What problem are you likely to experience when you use an

oxyacetylene plant that has just been dragged into a crawling space beneath a floor or similar confined space?

2. What special precautions do you think you need to take when using an

oxyacetylene set in a confined space?


SETTING UP AND CLOSING DOWN Setting up To set up an oxyacetylene plant 1. Secure both cylinders vertically (preferably in a trolley).

2. Momentarily open and close the outlet valve on each cylinder using

the cylinder key. This is known as cracking the cylinder, and its purpose is to remove any dirt or grit from the outlet that could harm the regulator when it is attached.

3. Screw the correct regulator on to each cylinder. The left-hand thread

on the acetylene equipment prevents you from connecting them wrongly. (Just before each regulator spigot seats hard into the cylinder valve, rotate the regulator body so that it seats evenly. Then, tighten the spigot nut fully by bumping the spanner with the palm of your hand.)

The regulators are colour coded and so it is a simple matter to choose the correct one for each cylinder.


You can recognise the left-hand acetylene threads by a groove machined in the corners of their hexagon nut section. See Fig. 13.

4. Attach one end of each hose to a regulator and the other end of the

torch, observing the colour coding and notches. 5. Wind out the pressure-setting knob on each regulator until the control

spring is quite relaxed. 6. Gently open each cylinder valve to avoid a sudden rush of high-

pressure gas into the regulator, which may damage the mechanism of the pressure gauge. Do not open either valve more than 11/2 turns. Leave the cylinder key on the acetylene cylinder so that you can shut off the fuel gas quickly in case of an emergency.

7. Set the required gas pressure while gas flows through the torch. That

is, open the acetylene torch valve, and wind in the acetylene pressure-setting knob until the outlet gauge registers the working pressure needed for the job in hand. Close the acetylene torch valve and then repeat this procedure for the oxygen.

8. Check all connections for leaks. The oxyacetylene plant is now ready for use. Closing down To close down an oxyacetylene plant 1. Close the acetylene valve on the torch to extinguish the flame. 2. Close the oxygen valve on the torch. 3. Shut off the oxygen and acetylene cylinder valves.

Refrigeration / Air Conditioning 4. Open the oxygen and acetylene valves on the torch, and drain the

gases from the torch, hoses and regulators. 5. Release the pressure-setting knobs on the regulators. 6. Close the torch valves.

REMEMBER

Before you fit a regulator to a cylinder, crack open the cylinder valve. Never use too much force when tightening connections. Always follows the correct sequence when setting up or closing down an oxyacetylene plant.

Review Questions 1. What likely harm would be caused if an unsecured cylinder on an

oxyacetylene plant fell from its standing position on to its side? 2. Why should you not extinguish an oxyacetylene flame by closing the

oxygen torch valve before the acetylene?


TIP SIZES AND FLAME CONTROL Choosing the right tip size As you gain more experience in the use of an oxyacetylene plant, you will learn how much heat you need for various welding, brazing, or heating jobs. The amount of heat in a flame is related to the flame’s size. You can vary this to some extent by adjusting the torch valves. However, you must choose a tip size that allows the flame to be both stable and quiet. If your tip size is too small for the flame you are using, the flame will be harsh, noisy and likely to push molten metal away. If your tip size is too large for the flame you are using, the flam will be soft, unstable and likely to backfire.

Note: A backfire occurs when the flame blows out and relights almost straight away making a loud bang.

Tips are made of pure copper and numbered in tenths of a millimetre according to the size of the outlet hole. The larger the number, the larger the tip size.

Choosing the right regulator settings Tip sizes and regulator settings for various gas welding jobs will be covered in a later assignment on welding. For brazing work, however you will find that a number 12 tip will do most jobs with both regulators set at about 50 kPag.

Note: Under no circumstances allow the pressure in an acetylene line to rise above 100 kPa. When separated from acetone and subjected to such high pressure, acetylene becomes unstable and can explode.

Refrigeration / Air Conditioning Tip cleaning Always keep the holes in your tips clear of dirt using the proper tip cleaning wires. See Fig. 1 again. Carefully insert a tip cleaning wire straight into a clogged tip hole. Never insert it at an angle. Never use a tip cleaning wire as a file to enlarge the hole in a tip. Flashbacks Never try to use the very last bit of gas from an oxygen or acetylene cylinder. Instead, renew the cylinder as soon as it pressure drops to the point where the regulator can no longer maintain the required outlet pressure. (Do not try to readjust the regulator.) If you try to use the last bit of gas from a cylinder, you could cause a flashback. A flashback is where the flame burns back inside the equipment instead of at the end of the tip. You will normally see a flashback as a sudden loss of flame at the tip, followed by a shrill hissing sound. If this occurs, immediately close the oxygen and acetylene valves on the torch – in that order. Then, if you do not know the cause of the flashback, have the equipment checked to make sure it is safe.

Most suppliers of industrial gases advise fitting a flashback arrestor between each regulator outlet and its connecting hose. Flashback arrestors are designed to reduce the risk of flashbacks and, if a flashback does still occur, to prevent it from causing serious damage or injury.

Refrigeration / Air Conditioning The flashback arrestor shown in Fig. 15 has a 1. Flow cut-off valve to stop the gas flow 2. Flame arrestor to extinguish the flame 3. Non-return valve to prevent reverse gas flow 4. Relief valve to vent excess pressure, and 5. Warning signal level to show that a flashback has occurred. Replace any hose that has been burnt by a flashback. Lighting the torch Direct the torch well clear of the cylinders and of any combustible material. Open the acetylene valve on the torch and immediately light the issuing gas, using a flint lighter. If the flame blows out or burns away from the tip, turn down the acetylene torch valve slightly. The flame should be bright with a minimum of smoke. See Fig. 16. Carburising flame To get a carburising flame, you next gently open the oxygen valve on the torch. Note that, as the oxygen increases, an inner white cone appears at the tip. At this stage, you will see three distinct flame zones, that is 1. A white inner cone, surrounded by 2. A whitish feather, in turn, surrounded by 3. A bluish outer flame Such a flame (Fig. 17) is called a carburising or reducing flame.

Refrigeration / Air Conditioning You can use a carburising flame to hardface steel. In this process, some of the unburned carbon in the flame is dissolved into molten steel, which has a surface-hardening effect on it. Neutral flame If you gradually open the oxygen valve further, the whitish feather will retract and disappear to give a neutral flame. See Fig. 18. A neutral flame has a brilliant white inner cone, surrounded by a blue envelope. The neutral flame is the hottest kind of flame (about 31000C). It is used for fusion welding mild steel, for silver brazing, and for most heating applications. Oxidising flame If you further increase the oxygen supply to the flame (or alternatively reduce the acetylene supply), the inner cone will shorten, and the flame will have a harsher sound. This flame is called an oxidising flame. See Fig. 19. An oxidising flame is used for bronze welding, and for fusion welding brasses or bronzes. The excess oxygen in the flame reduces the amount of dangerous zinc fumes given off.


REMEMBER

Use the right tip size and regulator settings. Backfires and flashbacks can be avoided by having the equipment regularly maintained, and by using it correctly. You can use flashback arrestors to protect yourself and the oxyacetylene plant from dangerous flashbacks. The three basic kinds of oxyacetylene flame are carburising, neutral and oxidising.

Review Questions 1. What size outlet hole has a number 8 tip? 2. Assume you have set up an oxyacetylene plant correctly and chosen

the right tip size and regulator settings for the work you are about to do. If you open the acetylene torch valve, light the issuing gas, and find that the flame is very yellow and leaves thick soot particles floating in the air, what have you done wrong?


SILVER BRAZING The metals you intend to join (whether by welding or silver brazing) are called the parent metals or base metals. When you silver braze, you do not melt the parent metals. Instead, you bond them together with a thin layer of silver brazing alloy that has a melting point over 500oC but less than those of the parent metals. Silver brazing is very useful for making joints 1. That withstand vibration; 2. That resist corrosion; 3. Where a large heat input would cause damage; and 4. Between dissimilar (different) metals such as between any combination

of copper, brass, mild steel or stainless steel. These properties make silver brazing ideal for refrigeration work. As a refrigeration engineer, you will use silver brazing to join copper tubing, connect copper tubing to brass shutoff valves, and connect copper tubing to steel spigots on suction accumulators, and so on. Rules for good silver brazing are as follows: 1. Choose the right brazing alloy. 2. Obtain a suitable mating joint. 3. Thoroughly clean the mating surfaces. 4. Apply the flux (only if one is needed). 5. Use dry nitrogen gas to prevent oxidation of the inside surfaces. 6. Use the correct flame and heating technique. Let us look at these things in more detail. Choosing the right brazing alloy Table 1 shows a typical manufacturer’s chart of brazing alloys. The upper part of Table 1 shows the Easyflo alloys. The bottom shows the Silfos alloys together with Tecfos.

Refrigeration / Air Conditioning Both the Easyflo and Silfos alloys are classified by their silver content. For example, Easyflo 42 contains 42% silver, and Silfos 15 contains 15% silver. Silver is a very expensive metal and so the higher the silver content, the more expensive the alloy becomes. Table 1 also shows this manufacturer’s colour coding used on the end of each rod for identification purposes. Other manufacturers use different names and colour codes for their brazing alloys. Generally, the company that employs you will stock only two brazing alloys. One (often Silfos 15) is selected for copper-to-copper joints in refrigeration tubing. The other (often Easyflo 42) is selected for joining copper tubing to either brass, mild steel or stainless steel. In the trade, these two alloys are commonly referred to simply by their trade names, Silfos and Easyflo.

WARNING

In geothermal areas of New Zealand, where sulphurised gases are in the air, Silfos and Tecfos suffer from corrosion and so cannot be used. In those areas, you must use Easyflo on all brazed joints.

Obtaining a suitable mating joint Silver brazing relies on capillary action to draw the molten brazing alloy into a gap between close-fitting components. If this capillary action does not occur, the joint will be weak. See Fig. 20.

Fig. 20 Figure 21 shows capillary action. If the top sheet of glass is moved until it touches the drop of water, capillary action will draw the water between the glass sheets. Note: Capillary action will still draw the water between the sheets if they are vertical when the flow must occur against gravity.


The right gap: To produce capillary action you need the right gap. The recommended gap for silver brazing alloys is about 0.1 mm to 0.2 mm.

If you use a gap that is too small (press fit) or too large (sloppy fit), you hinder the capillary action and the molten brazing alloy is unlikely to penetrate properly. If you use a gap that is too large (a sloppy fit) you also waste expensive brazing alloy and some molten alloy may run right through the joint, particularly in a vertical one.

The right lap: To give the joint strength, you need the right lap. The length of the lap should be at least four times the thickness of the thinner metal being

joined.

Figure 24 shows factory-swaged ends (socket joints). The gaps and laps will automatically be correct in these, provided the tubing and fittings are of the correct refrigeration grade and provided the tubing is inserted completely into the swaged ends.

Refrigeration / Air Conditioning The gaps and laps will also be correct if you obey the manufacturers’ instructions when using tube expanders or other manual swaging tools.

Cleaning the mating surfaces Just before you braze, remove all scale, dirt, oil or grease from the joint area. If the scale is heavy, you may need to start cleaning the mating surfaces with a file. However, always finish off with a fine grade of emery tape or similar, to leave a smooth shiny finish. Make sure that you do not leave filings or dirt inside the components. Applying flux To get the best joint between dissimilar metals, you must use Easyflo, which requires that you also use flux. You will generally need only one flux (general purpose flux) for all your Easyflo joints. You usually buy this flux as a white powder. Mix it into a paste with water so that it will stick to the work and will not be blown away by the flame. When you heat the flux to a clear molten state, it 1. Dissolves the light oxide films that still exist on otherwise clean mating

surfaces, 2. Excludes air to prevent new oxides forming during brazing, and 3. Promotes wetting of the mating surfaces by the brazing alloy.

Note: The brazing flux melts just before the ideal brazing temperature is reached.

Refrigeration / Air Conditioning Try to minimise the amount of flux and moisture getting inside the system. Once inside, these contaminants get carried around by the refrigerant and oil. The filters and strainers will remove some flux, but some may reach the high temperature area at the compressor discharge. Once heated, flux takes an active part in the decomposition of both oil and refrigerant. Moisture is just as harmful. It readily combines with most refrigerants to form corrosive acids. Apply the flux as follows: 1. Insert the end of the tubing part way into the fitting and then spread the

flux paste around the mouth of the joint. 2. Push the piping all the way in and rotate the fitting or tubing a few

times to spread the flux evenly over the mating surfaces. 3. Add further flux while you braze by

(a) Dipping the heated end of the alloy rod into the flux powder (or paste) and

(b) Applying the tuft of flux (which sticks to the rod end) directly to

the joint mouth as required.

To prevent external corrosion, you must remove the glass-like flux residue that remains on the outside of the joint after you have finished brazing. Remove it by chipping, wire brushing and then wiping with a warm wet rag. (The wet rag will also cool the joint enough to prevent you burning yourself on it as you concentrate on brazing nearby joints.)

WARNING

Use eye protection when chipping the flux away.

Avoid using flux if you can. Always braze copper-to-copper joints with either Silfos or Tecfos unless you are in a geothermal area of New Zealand). The phosphorus content in these rods as a flux by itself. Using dry nitrogen If air is present inside components, copper oxide scale will form inside as you braze them. When the refrigerant later passes through, it will pick up this scale and cause it to block filter/driers and mix with the compressor oil to form sludge.

Refrigeration / Air Conditioning If refrigerant or refrigeration oil are inside components that are at brazing temperatures, they will break down into acids and other damaging contaminants. To avoid such internal contamination, replace the air, refrigerant or oil inside the components you are heating with an inert gas such as dry nitrogen. Applying the nitrogen: Connect the components you are about to braze to a nitrogen cylinder fitted with a suitable pressure regular. Allow just enough nitrogen flow through them so that you can feel a slight breeze at the outlet end of the line. Often, you can apply the nitrogen through a gauge port on a compressor service valve. See Fig. 26.

Fig. 26 Drifting nitrogen through pipework Never force nitrogen into pipework too rapidly while brazing because any significant positive pressure within the pipework will force pin-hole leaks in the brazing alloy. Restrict the flow of nitrogen leaving larger piping (say above 16 mm outside diameter) to make sure the piping gets completely filled with nitrogen. (An end cap with a small hole of say 6 mm diameter cut in it is ideal for this.) Using the correct flame and heating technique When brazing use a slightly-soft neutral flame or a slightly carburising flame. Never use an oxidising flame because any excess of oxygen will harm the joint by causing it to oxidise.

Note: Instead of using an oxyacetylene flame, some people use an air/acetylene, air/propane or similar flame. While they need only one gas cylinder, these flames are much larger and cooler than oxyacetylene.

Refrigeration / Air Conditioning Support the assembly in such a way that there is no strain on the joint while you are heating it. Quickly and evenly, bring the whole joint to a temperature slightly above the melting point of the filler alloy being used. Keep the flame moving in a circular motion.

For a good spread of heat, the inner cone should not be too close to the metal.

Fig. 27 Play the flame more on the part of the joint that needs more heat. For instance, play it on the thicker metal, or on the metal that is closer to a bulky fitting that will be drawing heat away. Avoid producing local hot spots as you bring the separate mating surfaces to the brazing temperature at the same time. Flux: If flux is being used (an Easyflo joint) it will bubble and melt to a clear liquid. At this stage, start prodding with the alloy rod to see if it melts when touched against the parent metals. The molten flux will penetrate the joint ahead of the molten alloy. Any black patches that form on the flux during heating indicate that the flux cannot dissolve the oxides, or that dirt or grease are present. The cause may be too much contamination of the joint, insufficient flux or excessive oxidation of the joint due to overheating. Let the joint cool, reclean it, add more flux and try again. However, patches of deposited brazing alloy and flux will make subsequent cleaning harder. Concentrate on getting it right the first time.

Always melt the filler alloy with the job temperature by touching the rod directly to the joint mouth. Never melt it directly by the flame. The capillary flow into the joint occurs by a gentle force that you maintain by heating the joint in the right areas and in the correct

sequence.

Fig. 28 Apply the filler alloy to the joint mouth

As the alloy melts into the joint, direct the flame towards the bottom to promote the capillary action and encourage the alloy to flow deeper into the joint.


When using Easyflo, make sure the brazing alloy develops a smooth concave (curved inwards) surface at the joint (see Fig. 29). You do this by controlling the flame and making sure all the alloy’s surface is continually covered with flux. Silfos or Tecfos does not solidify into such a smooth surface. Instead, if correctly applied, the finished joint should be similar to that shown in Fig. 30. A visible slight mound of Silfos at the joint mouth confirms that you have completely sealed the joint. The capillary action is strong enough for you to make a brazed joint in any position. However, an upside down joint is difficult to achieve with Easyflo because the flux tends to dribble down and encourages the filler alloy to follow.

Work around large pipes: When you braze a large diameter pipe, you cannot bring the whole joint to the correct temperature at once. Instead, move the torch, followed by the filler rod, at a suitable speed around the circumference of the joint. Avoid blocking capillary tubes with brazing alloy: Take particular care when you braze a small-bore capillary tube in place. Heat will conduct along the tube because of its small size. You should use therefore, a very small flame, and insert the tube far enough into the larger tubing or fitting to prevent the alloy flowing along the capillary tube and blocking its open end. However, be careful not to insert the capillary tube so far that it contacts an inner component that will restrict the flow through it. Avoid overheating components: When brazing tubing on to sight glasses, expansion valves, vibration eliminators, and similar components that can be damaged by excessive heating, protect them by 1. Directing the brazing flame away from the component, and 2. Wrapping them in a wet rag.


Fig. 31 Protecting a component from overheating

In some instances, for example, when brazing a diaphragm valve in place [Fig. 31(b)] you must remove the internal parts first and replace them only once the body has cooled. Avoid pressure build-up in sealed sections: When brazing a container object, or a closing joint in piping, open a valve to relieve the pressure that builds up as the heat is applied. If you cannot do this, heat an area way from the joint first. Then, allow it to cool as you braze the joint. The contraction of the gases in the cooling area counters the expansion of the gases in the warming area. This prevents any overall rise in internal pressure, and allows you to successfully seal the joint.

REMEMBER

1. Never use an oxidising flame; 2. Try to prevent flux or brazing alloy from entering and

contaminating the system; 3. Use the temperature of the parent metal and not of the direct

flame to melt the brazing alloy, and 4. Use dry nitrogen to prevent internal oxidation.

REVIEW QUESTIONS 1. Why do you need to do more work when you braze copper tubing used

to carry refrigerant, than when you braze copper tubing used to carry water?

2. Assume you have a very sloppy joint between two different size pipes

that you need to braze together. Suggest a good practical way of getting the correct clearance fit for proper capillary action of the molten alloy and for maximum strength of the finished joint.


FURTHER ADVICE ON RUNNING PIPEWORK Use only refrigeration grade copper tubing Use only refrigeration grade copper tubing that has been dehydrated (dried) and capped. See Fig. 32. This piping is free of internal corrosion, dust and grease. The end caps keep it clean internally.

Fig. 32 Dehydrated and sealed copper tubing

Other tubing such as plumbing tubing has different diameters and wall thicknesses and often contains dirt and grease. Grease can contaminate a refrigeration system with sulphur compounds and waxes. Sulphur compounds take part in chemical reactions with refrigerants and refrigeration oils. Wax tends to collect in low temperature areas and block metering devices. Keep tubing sealed When running pipelines, never leave the ends open for air to enter. Use the plugs or caps supplied with the tubing to keep the ends sealed (whenever you are not passing dry nitrogen through).


Fig. 33 Using a tube cutter you must use a hacksaw, slope the tubing downwards so that the metal

ake similar precautions to prevent

Fig. 34 The burr

pocket knife is a handy tool for removing this burr.

burrs are left inside copper tubing

. Metal fragments may dislodge from them later, and

. The burrs will restrict refrigerant flow and oil return.

you must cut a fixed horizontal piece of tubing, scoop the metal particles out

void having to cut fixed vertical tubing because it is difficult to remove the

o not force a tube cutter to take overly deep cuts because this will produce a

eplace the cutting wheel on your tube cutter if it cannot cut tubing without

Cut tubing correctly Whenever you have to cut tubing, you should generally use a tube cutter rather than a hacksaw.

Iffragments fall out as you cut.

Tmetal fragments falling into tubing when removing the burr left by a tube cutter. See Fig. 34.

A If 1 2 Ifas that come away when removing the burr. Aburr from the lower section without some metal shavings falling inside. If you have no choice but to do this, take great care to remove all the shavings. Dlarger burr. Rleaving a heavy burr.

Refrigeration / Air Conditioning A tube cutter is likely to leave a heavier burr on soft drawn copper tubing than on hard draw tubing. The softer tubing is more likely to curve inwards under the pressure of the cutting wheel. To reduce this problem, take more time to cut soft drawn tubing by applying less cutting pressure. Cutting capillary tubing: You must use a special technique to cut capillary tubing so as to avoid restricting the size of its tiny bore by the cutting process. Most engineers find the best method is to 1. Notch either side of the tube (where you want to cut it) with the corner

of a file; and then 2. Break the tubing by gently bending it backwards and forwards a

number of times at that point. Avoid using unnecessary brazed joints If you use brazed joints unnecessarily you increase the cost of an installation and increase the number of joints that you must later check for leaks. Use tube expanders: Where you can, use tube expanders instead of sweat unions to make joints between straight pieces of tubing.

To expand (swage) the end of a length of hard drawn copper tubing, you first anneal (soften) the end by heating it to a dull red colour. You can then either allow it to cool slowly or quench it. The best way, usually, is to quench it with a wet rag to avoid burning yourself on it later, and to speed up your work progress. You should always use dry nitrogen gas to keep the inside of the tubing clean while it is at oxidising temperatures. Once the end of the tubing is annealed, insert the head of the tool inside, and simply squeeze the two handles together. This neatly expands the end of the tube into the form of a female socket.


Note: You can expand the ends of 16 mm OD (outside diameter) tubing, or smaller with a flaring tool shown in Fig. 37.

Replace the spinner with the correct size swaging head. Clamp the tubing so that it extends further above the block than that shown in Fig. 38(a). Then, tighten the swaging head into the tubing to expand it.

Use tube benders: Use tube benders instead of sweat bends on tubing of up to 19 mm OD. Figure 36 shows how to use lever tube benders to make a 90o bend.

Fig. 36 Lever tube benders

To use lever tube benders: 1. Mark the tube where you want the 90o bend. 2. Raise the form handle. 3. Position the tubing in the groove, as in Fig. 36(b) and check that it is

engaged in the tube clamp. 4. Lower the form handle to the position shown in Fig. 36(b) making sure

that your mark lines up with the letter L on the form lever. 5. Pull the lever handle down in the direction of the arrow until the 0 mark

on the form lever lines up with the 90o mark on the form wheel.

Refrigeration / Air Conditioning Use a flaring tool: Use a flaring tool (Fig. 37) to make joints in small diameter copper tubing where the joints are likely to be disconnected and reconnected in the future. For example, use flared joints when installing a TEV and filter / drier in a small commercial coolroom system.

Fig. 37 A flaring tool To make a flare on the end of a piece of soft drawn (or annealed hard drawn) copper tubing: 1. Ensure that the end of the tube is cut squarely, and then carefully

scrape away the burr left by the tube cutter. 2. Place the flare nut on the tubing with the open end toward the end of

the tubing. 3. Clamp the tube into the flaring tool.

The tube should extend one third of the height of the flare above the surface of the flare block. See Fig. 38(a). 4. Place a drop of refrigeration oil on the flaring tool spinner where it will contact the tubing. Tighten the spinner against the tube to form the flare. See Fig. 38(b).

Fig. 38

Refrigeration / Air Conditioning Keep components sealed before fitting If a component that you wish to install arrives properly dehydrated and sealed, keep it sealed until the moment you install it.

Leave the protective seals on components such as moisture indicators and filter / driers until immediately before you fit them to prevent air and dust entering.

Fig. 39 Seal removal Blast dry nitrogen through Before making the final connection in a piping run, use the dry nitrogen you have on hand (for preventing internal contamination) to remove any loose debris that you may have accidentally left inside. How to do this: 1. Increase the pressure setting on your regulator. 2. Hold your hand over the free end of the piping while nitrogen is flowing

into the other end of the regulator. 3. Allow the pressure to build up as high as you can with your hand

blocking the outlet. (You are unlikely to manage better than about 200 kPag).

4. Remove your hand suddenly to blast the nitrogen and any grit, scale,

flux or other solid debris out of the pipework.

BEWARE]

High velocity grit could cause eye injuries. Take suitable precautions. Do NOT do this in a confined space because the nitrogen could exclude the oxygen from the air you breathe and kill you by asphyxiation.


REMEMBER

Leave protective caps on components until immediately before fitting them. Use only refrigeration grade copper tubing that has been properly dehydrated and capped. Reseal tubing immediately after you have cut a short length from it. Never leave the end of pipework open for air to enter. Try to use a tube cutter rather than a hacksaw to cut tubing. A good idea is to blast nitrogen through pipework to remove any debris before making the final connections. However, be careful how you do this.

REVIEW QUESTIONS: ANSWERS TO REVIEW QUESTIONS 1. Dragging an oxyacetylene plant into a confined space is likely to

involve lying the acetylene cylinder down instead of keeping it upright. The acetone will soak into the porous material near the outlet valve. This means the acetone will probably leave the cylinder when the acetylene is later drawn off. This acetone would lower the heat of the flame and reduce the storage capacity of the cylinder when it is next refilled with acetylene.

Note: If you equip your oxyacetylene plant with long hoses, you may

avoid having to drag the cylinders into confined spaces. 2. When using an oxyacetylene set in a confined space, you need to (a) Have adequate fresh air ventilation;

(b) Confine the flame to where you want it with sheetmetal shields or similar, and ensure that you cannot possibly ignite hidden combustible material for example, material inside a wall cavity;

(c) Have someone stand by with a fire extinguisher; and (d) Make sure other workers in the vicinity know what you are doing

and that everyone knows the nearest exist should something go wrong with your fire-prevention efforts.

Refrigeration / Air Conditioning 1. If a cylinder on an oxyacetylene plant falls over on to its side (a) Its regulator is likely to be severely damaged;

(b) At least one pressure gauge on the regulator is likely to get smashed, and

(c) The hose is likely to be broken where it connects on to the

regulator as this is a common point of impact.

(If the cylinder valve breaks off in the fall, the cylinder and valve are likely to become dangerous projectiles, shooting in different directions.)

2. You should not extinguish an oxyacetylene flame by closing the oxygen

torch valve before the acetylene because, instead of going out with a pop, the flame would become very sooty and go out only gradually as you close the acetylene torch valve.

1. A number 8 tip has a 0.8 mm outlet hole (eight tenths of a millimetre). 2. If, when you light the flame it is very yellow and sooty, you have not

opened the acetylene torch valve wide enough. 1. You need to do more work when you braze refrigerant tubing because

you must take steps (use dry nitrogen gas) to keep the inside of the tubing perfectly clean.

2. Three practical ways of converting a sloppy joint into the correct fit for

proper capillary action and maximum joint strength are as follows:

(a) Use tube expanders to expand the inner tube to a suitable outside diameter to eliminate the excessive gap.

(b) Slip a short off-cut (collar) of an intermediate sized tube into the

space between the two tubes to take up the excessive gap. (c) Crimp the outer tube on to the thinner tube (after annealing it) by

pinching it with a pair of pliers or similar. See Fig. 40. (You should have explained one of the above methods)

Fig. 40


ASSESSMENT 1. Why is an air / acetylene flame generally unsuitable, compared to

oxyacetylene for making a brazed joint in a very confined space? 2. Which brazing alloy in Table 1 is the cheapest? Say why. 3. When do you need to use flux for silver brazing? 4. During brazing, you heat a correctly chosen flux. The melting of the

flux to a clear liquid indicates two important facts. What are they? 5. What two basic kinds of flame can you use for silver brazing? 6. How do you control the depth of penetration when brazing? 7. How does the amount of gap affect the strength of a brazed joint? 8. Assume you have two rods of silver brazing alloy, one Silfos and one

Easyflo, but the end colour codes on both are missing. How can you tell which rod is which?

9. What is the diameter of the outlet hole in the following? (a) A No. 6 tip (b) A No. 12 tip 10.

What danger is there in using dry nitrogen while in an underfloor pipe duct, such as the one shown in Fig. 41?

Fig. 41 Supermarket pip duct

Refrigeration / Air Conditioning Service Gauge Manifold Set, Connect To & Disconnect From a Basic Vapour Compression System: Purpose: The purpose of this section is to provide you with the underpinning knowledge and skills required to safely connect and disconnect a service gauge manifold set to a Basic Vapour Compression System and to minimise the loss of any refrigerant in the process. Ensuring compliance with HB20.1, HB20.2 & HB20.3 Codes of Good Practice (Parts 1, 2 and 3). Fitting Service Gauges to a “Pumped Down” System

1. Check that the system is electrically isolated. 2. Remove the valve stem caps from the discharge and suction service valves

and check the valves are fully back-seated.

3. Check that the manifold hand valves are closed and the centre hose (generally the yellow hose) is sealed / capped.

4. Remove the service port caps and connect the manifold gauge lines,

(generally the blue hose) from the compound gauge to suction service valve and (generally the red hose) from the pressure gauge to discharge service valve.

5. Where fitted, slightly loosen the service valve “gland”.

6. “Crack” or slightly front-seat the service valves and slightly purge the gauge

lines.

7. Switch the system electrics ON.

8. Open the liquid line hand valve. When the low side pressure increases the LP control closes energising the compressor.

9. Monitor the system operation and pressures.

“Pumping Down” the System and Removing the Service Gauges.

1. Fully backseat the discharge service valve. 2. Where fitted, tighten the discharge service valve “gland”. 3. Open the manifold hand valves. This will enable any refrigerant in the gauge

lines to flow back to the system via the “low side”.


4. Close the liquid line hand valve. The system will “pump down” i.e. the refrigerant flow to the evaporator has been isolated and as such the system will draw the refrigerant from the low side and pump it into the high side where it will condense and collect in the liquid receiver. When the low side pressure reaches the cut-out setting of the LP control the compressor will switch OFF.

5. In the event of an increase in pressure, there is a pump down over ride switch

fitted to the system to enable further pressure reduction.

6. Electrically isolate the system.

7. Fully backseat the suction service valve.

8. Where fitted, tighten the suction service valve “gland”.

9. Remove the manifold gauge lines and seal / cap them.

10. Refit all valve stem caps and service port caps.

11. Leak test the service valves and ports.

Note: The centre port of the manifold gauge set should be capped or if a gauge line is fitted, the line will be sealed to prevent the loss of refrigerant.

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