What are the top 10 questions to ask an air conditioning company?

1. What type of air conditioning is best for my home?
   
2. What are the benefits of each type of air conditioning?
   
3. How will the air conditioning be installed?
   
4. What is the warranty?
   
5. How often does the air conditioning need to be serviced?
   
6. What are the costs of installation and annual upkeep?
   
7. How long will the air conditioner last?
   
8. What are the emissions of the air conditioning unit?
   
9. How noisy is the air conditioning unit?
   
10. Which room will the air conditioner be installed in?

The first thing to look for in an HVAC company is the knowledge of the technicians and part of this can be determined from air conditioning companies reviews. You can ask about their experiences with challenging situations, their past training and their dedication to the field. If you have just graduated from HVAC school, you should also highlight your enthusiasm and interest for the field. The employer will notice how eager you are to learn, and they will be more inclined to hire you as a new employee.

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Another you want to ask is the level of expertise and knowledge of the HVAC technicians. They should be able to tell you how much experience they have in their field. If they don’t, they can’t get the job done safely and may even have to leave you fuming because they didn’t have the parts they need. It’s always better to get an experienced technician rather than a cheap one, because the latter may not be able to reach some areas in the house.

Make sure the HVAC contractor is insured and carries a satisfaction guarantee. You should also ask about the paperwork they’ll need to complete the project. If you’re hiring an HVAC company, you need to ensure the paperwork is done properly. If you’re getting an HVAC repair in your area, make sure you go with an air conditioning company that has been in business for several years. A long-term relationship is best for you and your home.

You can also ask about their energy rating. These ratings are important for deciding on which AC system is best for your home. The company should include a satisfaction guarantee and make sure you are satisfied with the work. You should also ask about the company’s maintenance plan. Most HVAC companies will offer a maintenance program after the installation. If necessary, the company should replace or repair components in-house.

The HVAC company should explain the energy efficiency of their system. The technician should be able to explain the energy consumption of its equipment. A good contractor should be able to explain how many BTUs are required to cool or heat one degree of water. You should ask about the manufacturer warranty of the equipment. If you’re not sure about the energy rating of your system, ask the HVAC company to tell you their energy ratings.

The HVAC company should be able to tell you what the BTU is. British thermal units are a unit of energy, which means it measures the energy required to heat or cool a liter of water. For this reason, it is important to ask your potential HVAC technician the BTU in your area. It will provide you with the best estimate for your project. This information will be vital for you to choose the best air conditioning company for your home.

Do you know the BTU? What is the BTU? The BTU is a measurement of the amount of energy needed to heat or cool a single degree of water. An HVAC company should be able to explain the BTU to you in plain language. Moreover, you should ask about the quality of the HVAC technician. Do you trust the technician? If so, you can also trust the results.

The BTU is an indicator of how much energy the air conditioner uses. A high BTU is a sign of a high-efficiency system. If you have a high-efficiency model, it can be beneficial to consider a higher-efficiency unit. An efficient HVAC company should be able to tell you if there are any problems with the unit, or if they will only take a few days to repair the problem.

What are the BTU? This is the number of BTUs needed to heat or cool a degree of water. The BTU is the energy required to heat or cool a unit of water. The BTU also refers to the amount of heat that is pulled out of the air in an hour. The technician should be able to explain these factors to you so you can make an informed decision.

How to braze?

There are six fundamentals of brazing that every brazer should follow to ensure consistent and repeatable joint quality, strength, hermeticity, and reliability. They are generally simple to perform (some may take only a few seconds), but none of them should be omitted from your brazing operation if you want to end up with sound, strong, neat-appearing joints.

For the sake of simplicity, we'll discuss these six brazing process steps mainly in terms of "manual brazing," that is, brazing with hand-held torch and hand-fed filler metal. But everything said about manual brazing applies as well to mass production brazing. The same brazing process steps must be taken, although they may be performed in a different manner.

For more on the uses of brazing, watch this video.

Conversely, if the gap is wider than necessary, the strength of the joint will be reduced almost to that of the filler metal itself. Also, capillary action is reduced, so the filler metal may fail to fill the joint completely again lowering joint strength.

So the ideal clearance for a brazed joint, in the example above, is in the neighborhood of .” (.038mm.) But in ordinary day-to-day brazing, you don’t have to be this precise to get a sufficiently strong joint.

Capillary action operates over a range of clearances, so you get a certain amount of leeway. Look at the chart again, and see that clearances ranging from .001” to .005” (.025 mm to .127 mm) still produce joints of 100,000 psi (689.5 MPa) tensile strength.

If you’re joining two flat parts, you can simply rest one on top of the other.

The metal-to-metal contact is all the clearance you’ll usually need, since the average “mill finish” of metals provides enough surface roughness to create capillary “paths” for the flow of molten filler metal. (Highly polished surfaces, on the other hand, tend to restrict filler metal flow.)

However, there’s a special factor you should consider carefully in planning your joint clearances. Brazed joints are made at brazing temperatures, not at room temperature. So you must take into account the “coefficient of thermal expansion” of the metals being joined. This is particularly true of tubular assemblies in which dissimilar metals are joined.

As an example, let’s say you’re brazing a brass bushing into a steel sleeve. Brass expands, when heated, more than steel. So if you machine the parts to have a room temperature clearance of .002”-.003” (.051 mm-.076 mm), by the time you’ve heated the parts to brazing temperatures the gap may have closed completely! The answer? Allow a greater initial clearance, so that the gap at brazing temperature will be about .002”-.003” (.051 mm-.076 mm.).

Of course, the same principle holds in reverse. If the outer part is brass and the inner part steel, you can start with virtually a light force fit at room temperature. By the time you reach brazing temperature, the more rapid expansion of the brass creates a suitable clearance.

For more information on good fit and proper clearance, watch our video here!

Designing Lap Joints

The first thing you'll notice is that, for a given thickness of base metals, the bonding area of the lap joint can be larger than that of the butt joint and usually is. With larger bonding areas, lap joints can usually carry larger loads.

The lap joint doubles the thickness at the joint, but in many applications (plumbing connections, for example) this reinforcement of the joint is preferable. Resting one flat member on the other is usually enough to maintain a uniform joint clearance. And, in tubular joints, nesting one tube inside the other holds them in proper alignment for brazing. However, suppose you want a joint that has the advantages of both types; single thickness at the joint combined with maximum tensile strength. You can get this combination by designing the joint as a butt-lap joint.

Designing to distribute stress

When you design a brazed joint, obviously you aim to provide at least minimum adequate strength for the given application. But in some joints, maximum mechanical strength may be your overriding concern. You can help ensure this degree of strength by designing the joint to prevent concentration of stress from weakening the joint.

Spread the stress

Figure out where the greatest stress falls, then impart flexibility to the heavier member at this point, or add strength to the weaker member. When you're designing a joint for maximum strength, use a lap to increase joint area rather than a butt, and design the parts to prevent stress from being concentrated at a single point. There is one other technique for increasing the strength of a brazed joint, frequently effective in brazing small-part assemblies. You can create a stress-distribution fillet, simply by using a little more brazing filler metal than you normally would, or by using a more "sluggish" alloy. Usually you don't want or need a fillet in a brazed joint, as it doesn't add materially to joint strength. It pays to create the fillet when contributing to spreading joint stresses.

Designing for service conditions

In many brazed joints, the chief requirement is strength. And we've discussed various ways of achieving joint strength. But there are frequently other service requirements which may influence the joint design or filler metal selection. For example, you may be designing a brazed assembly that needs to be electrically conductive. A silver brazing filler metal, by virtue of its silver content, has very little tendency to increase electrical resistance across a properly-brazed joint. But you can further insure minimum resistance by using a close joint clearance, to keep the layer of filler metal as thin as possible. In addition, if strength is not a prime consideration, you can reduce length of lap. Instead of the customary "rule of three," you can reduce lap length to about 1-1/2 times the cross-section of the thinner member.

If the brazed assembly has to be pressure-tight against gas or liquid, a lap joint is almost a must since it withstands greater pressure than a butt joint. And its broader bonding area reduces any chance of leakage. Another consideration in designing a joint to be leak proof is to vent the assembly. Providing a vent during the brazing process allows expanding air or gases to escape as the molten filler metal flows into the joint. Venting the assembly also prevents entrapment of flux in the joint. Avoiding entrapped gases or flux reduce the potential for leak paths. If possible, the assembly should be self-venting. Since flux is designed to be displaced by molten filler metal entering a joint, there should be no sharp corners or blind holes to cause flux entrapment. The joint should be designed so that the flux is pushed completely out of the joint by the filler metal. Where this is not possible, small holes may be drilled into the blind spots to allow flux escape. The joint is completed when molten filler metal appears at the outside surface of these drilled holes.

To maximize corrosion-resistance of a joint, select a brazing filler metal containing such elements as silver, gold or palladium, which are inherently corrosion-resistant. Keep joint clearances close and use a minimum amount of filler metal, so that the finished joint will expose only a fine line of brazing filler metal to the atmosphere. These are but a few examples of service requirements that may be demanded of your brazed assembly. As you can see both the joint design and filler metal selection must be considered.

If you'd like to watch a hands on demonstration of good fit and proper clearance, check out this video.

Start by getting rid of oil and grease.

In most cases you can do it very easily either by dipping the parts into a suitable degreasing solvent, by vapor degreasing, or by alkaline or aqueous cleaning. If the metal surfaces are coated with oxide or scale, you can remove those contaminants chemically or mechanically. For chemical removal, use an acid pickle treatment, making sure that the chemicals are compatible with the base metals being cleaned, and that no acid traces remain in crevices or blind holes. Mechanical removal calls for abrasive cleaning. Particularly in repair brazing, where parts may be very dirty or heavily rusted, you can speed the cleaning process by using a grinding wheel, or file or metallic grit blast, followed by a rinsing operation.

Once the parts are thoroughly clean, it’s a good idea to flux and braze as soon as possible. That way, there’s the least chance for recontamination of surfaces by factory dust or body oils deposited through handling.

For more on properly cleaning the metals, check out this blog post or this video.

How much flux do you use?

Enough to last throughout the entire heating cycle. Keep in mind that the larger and heavier the pieces brazed, the longer the heating cycle will take - so use more flux. (Lighter pieces, of course, heat up faster and require less flux.) As a general rule, don't skimp on the flux. It's your insurance against oxidation. Think of the flux as a sort of blotter. It absorbs oxides like a sponge absorbs water. An insufficient amount of flux will quickly become saturated and lose its effectiveness. A flux that absorbs less oxides not only insures a better joint than a totally saturated flux, but it is a lot easier to wash off after the brazed joint is completed.

Flux can also act as a temperature indicator, minimizing the chance of overheating the parts. Handy & Harman's Handy Flux, for example, becomes completely clear and active at °F/593°C. At this temperature, it looks like water and reveals the bright metal surface underneath - telling you that the base metal is just about hot enough to melt the brazing filler metal. In this video, we'll show you how Handy® Flux appears during the brazing process.

Handy Flux temperature indicator chart

Temp Appearance Of flux 212°F (100°C) Water boils off 600°F (315°C) Flux becomes white and slightly puffy, starts to "work" 800°F (425°C) Flux lies against surface and has a milky appearance °F (593°C) Flux is completely clear and active, looks like water. Bright metal surface is visible underneath. Test the temperature by touching brazing filler metal to base metal. If brazing filler metal melts, assembly is at proper temperature for brazing.

Fluxing is an essential step in the brazing operation, aside from a few exceptions. You can join copper to copper without flux, by using a brazing filler metal specially formulated for the job, such as Handy & Harman's Sil-Fos or Fos-Flo 7. (The phosphorus in these alloys acts as a fluxing agent on copper.) You can also omit fluxing if brazing occurs in a controlled atmosphere (i.e. a gaseous mixture contained in an enclosed space, usually a brazing furnace). The atmosphere (such as hydrogen, nitrogen or dissociated ammonia) completely envelops the assemblies and, by excluding oxygen, prevents oxidation. Even in controlled atmosphere brazing you may find that a small amount of flux improves the wetting action of the brazing filler metal.

For more tips on proper fluxing, watch this video.

Better Joint Design

An axiom of metal joining is that proper joint design is the path to efficient fixturing. Brazing experts at Lucas Milhaupt offer these tips for improving your joint design:

• Make component pieces of the assembly self locating, so the fixture only supports and cradles the components.
• Allow space for the filler metal to flow and for flux to be forced out of the joint.

Example Problem: Where a tube enters a fitting or casting, some manufacturers use a press fit to keep externally applied alloy from reaching the bottom of the joint, where it might plug a hole in the fitting. Unfortunately, molten flux reaches the bottom of the blind hole and is trapped there, as alloy melts and tries to enter the joint. The alloy cannot displace the flux, so heavy flux inclusions and poor joint quality result.

Example Solution: Use a slip fit and a buried preform in the hole. The alloy is induced by heat to flow to the top of the joint, pushing the flux out. This leaves the hole open and results in a sound joint. Take into account the expansion and contraction characteristics of the metals being joined. If possible, design the joint so the higher-expansion material is the outer member of the joint. (It will expand more than the inner member, providing clearance where the filler metal will flow.)

Silver brazing is a cost-effective method for joining metals, especially when joints are designed for maximum brazing efficiency and fixtures are designed as described. Many products manufactured today could be redesigned for brazing to reduce manufacturing costs. Even though silver is expensive, it represents a small percentage of total assembly costs.

First, apply heat broadly to the base metals. If you’re brazing a small assembly, you may heat the entire assembly to the flow point of the brazing filler metal. If you’re brazing a large assembly, you heat a broad area around the joint.

All you have to keep in mind is that both metals in the assembly should be heated as uniformly as possible so they reach brazing temperature at the same time. When joining a heavy section to a thin section, the “splash-off” of the flame may be sufficient to heat the thin part. Keep the torch moving at all times. When joining heavy sections, the flux may become transparent—which is at °F (593°C)—before the full assembly is hot enough to receive the filler metal.

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In torch brazing, a variety of fuels are available—natural gas, acetylene, propane, propylene, etc., combusted with either oxygen or air. The most popular is still the oxy/acetylene mixture. When it comes to safely brazing with oxy-acetylene torches, let's look at two important aspects: safety equipment, plus procedures for safe operation. This is serious business: arc rays and sparks can result in loss of sight, fume inhalation can lead to lung damage, and other accidents can cause burns, fires, or explosions.

Oxy-acetylene Torch Safe Operation

In addition to using safety equipment, workers should practice safe operation to prevent flashbacks.  Keep acetylene and oxygen separate until the torch is ignited. When starting a torch, the acetylene valve should be opened first.  Next, the torch should be ignited, and then oxygen can be introduced. Please note that opening both gas valves prior to ignition can cause gas backflow into either gas hose, leaving the system vulnerable to flashback.

After use, it is critical that both gas lines be emptied separately-one at a time-through the torch.  If this "bleeding" of gas lines is done simultaneously for oxygen and acetylene, any pressure difference between the gas lines will cause backflow of one gas into the other line, so this should be avoided.

Flashbacks can also be caused by brazing with multiple torches, simultaneously, on one part. If using dual torches to heat both sides of a part, do not aim the torches at each other, but rather, angle each torch toward the part.  If one torch should cause flashback in the other, operators will hear a loud hissing sound and should immediately turn off the gas by closing first the acetylene valve and then the oxygen valve.

Flux Changes During Brazing Process

Some metals are good conductors—and consequently carry off heat faster into cooler areas. Others are poor conductors and tend to retain heat and overheat readily. The good conductors will need more heat than the poor conductors, simply because they dissipate the heat more rapidly.

In all cases, your best insurance against uneven heating is to keep a watchful eye on the flux. If the flux changes in appearance uniformly, the parts are being heated evenly, regardless of the difference in their mass or conductivity.

Depositing Filler Metals

You’ve heated the assembly to brazing temperature. Now you are ready to deposit the filler metal.

In manual brazing, all this involves is carefully holding the rod or wire against the joint area. The heated assembly will melt off a portion of the filler metal, which will instantly be drawn by capillary action throughout the entire joint area.

You may want to add some flux to the end of the filler metal rod— about 2” to 3” (51 mm to 76 mm)— to improve the flow. This can be accomplished by either brushing on or dipping the rod in flux. On larger parts requiring longer heating time, or where the flux has become saturated with much oxide, the addition of fresh flux on the filler metal will improve the flow and penetration of the filler metal into the joint area.

However, there is one small precaution to observe. Molten brazing filler metal tends to flow toward areas of higher temperature. In the heated assembly, the outer base metal surfaces may be slightly hotter than the interior joint surfaces. So take care to deposit the filler metal immediately adjacent to the joint. If you deposit it away from the joint, it tends to plate over the hot surfaces rather than flow into the joint. In addition, it’s best to heat the side of the assembly opposite the point where you’re going to feed the filler metal. In the example above, you heat the underside of the larger plate, so that the heat draws the filler metal down fully into the joint. (Always remember—the filler metal tends to flow toward the source of heat.)

Methods for Flux Removal

After brazing, flux forms a hard, glass-like surface and can be difficult to remove. What is the best cleaning method? You can remove excess flux by various means; the most cost-effective approaches involve water.

Industry flux standards focus on water-based fluxes. AMS and AMS stipulate that all fluxes conforming to these specifications should be soluble in water at 175°F/79°C or less after brazing. Therefore, brazing fluxes are typically designed to dissolve in water.

The most common methods for post-braze flux removal are:

Soaking/wetting

Use hot water with agitation in a soak tank to remove excess flux immediately following the braze operation, and then dry the assembly. When soaking is not possible, use a wire brush along with a spray bottle or wet towel. When using a soak bath of any kind, change the solution periodically to avoid saturating the cleaning solution.

Quenching

This process induces a thermal shock that cracks off residual flux. When quenching a brazed part in hot water, take care to avoid compromising the braze joint. Quench only after the braze filler metal has solidified to avoid cracks or rough braze joints. Note that quenching can affect base material mechanical properties. Do not quench materials with large differences in coefficients of thermal expansion to avoid cracks in the base materials and tears within the braze alloy.

You can use more elaborate methods of removing flux as well—an ultrasonic cleaning tank to speed the action of the hot water, or live steam. Additional cleaning methods include:

• Steam lance cleaning - This process employs superheated steam under pressure to dissolve and blast away flux residue.
• Chemical cleaning - You can use an acidic or basic solution, generally with short soak times to avoid deteriorating the base materials. For chemical soaks, monitor the pH level to determine when to change the solution.
• Mechanical cleaning - Clean residue from brazed joints with a wire brush or by sandblasting. Be advised that soft metals-including aluminum-require extra care, as they are vulnerable to the embedding of particles.

Always ensure that your cleaning method is compatible with base metal properties. Some metal groups achieve a desired effect from a special treatment after cleaning. Stainless-steel and aluminum parts, for example, may benefit from chemical immersion to improve surface corrosion resistance.

The only time you run into trouble removing flux is when you haven’t used enough of it to begin with, or you’ve overheated the parts during the brazing process. Then the flux becomes totally saturated with oxides, usually turning green or black. In this case, the flux has to be removed by a mild acid solution. A 25% hydrochloric acid bath (heated to 140-160°F/60-70°C) will usually dissolve the most stubborn flux residues. Simply agitate the brazed assembly in this solution for 30 seconds to 2 minutes. No need to brush. A word of caution, however—acid solutions are potent, so when quenching hot brazed assemblies in an acid bath, be sure to wear a face shield and gloves.

After you’ve gotten rid of the flux, use a pickling solution to remove any oxides that remain on areas that were unprotected by flux during the brazing process. The best pickle to use is generally the one recommended by the manufacturer of the brazing materials you’re using. Highly oxidizing pickling solutions, such as bright dips containing nitric acid, should be avoided if possible, as they attack the silver filler metal. If you do find it necessary to use them, keep the pickling time very short.

Brazed Joint Examination Methods: Nondestructive testing methods

Nondestructive testing methods of checking quality and specification conformance include:

Visual examination - with or without magnification-for evaluating voids, porosity, surface cracks, fillet size and shape, discontinuous fillets plus base metal erosion (not internal issues such as porosity and lack of fill)

Leak testing - for determining gas- or liquid-tightness of a brazement. Pressure (or bubble leak) testing involves the application of air at greater-than-service pressures. Vacuum testing is useful for refrigeration equipment and detection of minute leaks, employing a mass spectrometer and a helium atmosphere.

Radiographic examination - useful in detecting internal flaws, large cracks and braze voids, if thickness and X-ray absorption ratios permit delineation of the brazing filler metal-cannot verify a proper metallurgical bond (pictured right)

Proof testing - subjecting a brazed joint to a one-time load greater than the service level-applied by hydrostatic methods, tensile loading or spin testing

Ultrasonic examination - a comparative method for evaluating joint quality, in immersion mode or contract mode-involves reflection of sound waves by surfaces, using a transducer to emit a pulse and receive echoes (pictured to the right)

Liquid penetrant examination - dye and fluorescent penetrants may detect cracks open to the surface of joints-not suitable for inspection of fillets, where some porosity is always present

Acoustic emission testing - evaluating the extent of discontinuity-using the premise that acoustic signals undergo a frequency or amplitude change when traveling across discontinuities

Thermal transfer examination - detects changes in thermal transfer rates due to discontinuities or unbrazed areas-images show brazed areas as light spots and void areas as dark spots

Brazed Joint Examination Methods: Destructive testing methods

There are also several destructive and mechanical testing methods, often used in random or lot testing:

Peel testing - useful for evaluating lap joints and production quality control for general quality of the bond plus presence of voids and flux inclusions-where one member is held rigid while the other is peeled away from the joint

Metallographic examination - testing the general quality of joints-detecting porosity, poor filler metal flow, base metal erosion and improper fit

Tension and shear testing - determines strength of a joint in tension or in shear-used during qualification or development rather than production

Fatigue testing - testing the base metal plus the brazed joint-a time-consuming and costly method

Impact testing - determines the basic properties of brazed joints-generally used in a lab setting

Torsion testing - used on brazed joints in production quality control-for example, studs or screws brazed to thick sections

Failed Brazing Inspection

The size, complexity and severity of the application determine the best inspection method, and several methods may be required. If you are unable to develop an accurate and dependable method of inspecting a critical brazed joint, consider revisiting your joint design to allow adequate inspection.

Examining finished joints may be the final step in the brazing process, but inspection procedures should be incorporated into the design stage. Both nondestructive and destructive methods may be employed, depending on the application, service and end-user requirements plus regulatory codes and standards.

Once the flux and oxides are removed from the brazed assembly, further finishing operations are seldom needed. The assembly is ready for use, or for the application of an electroplated finish. In the few instances where you need an ultra-clean finish, you can get it by polishing the assembly with a fine emery cloth. If the assemblies are going to be stored for use at a later time, give them a light rust-resistant protective coating by adding a water soluble oil to the final rinse water.

Watch this video for more on how to properly clean joints.

Pulling It All Together: Brazing Fundamentals

We’ve discussed the six basic steps required in correct brazing procedures. And we’ve gone into a fair amount of detail in order to be as informative as possible. To get a more balanced picture of the overall brazing process, it’s important to note that in most day-to-day brazing work, these steps are accomplished very rapidly.

Take the cleaning process, for example. Newly-fabricated metal parts may need no cleaning at all. When they do, a quick dip, dozens at a time, in a degreasing solution does the job. Fluxing is usually no more than a fast dab of a brush or dipping ends of the parts in flux. Heating can often be accomplished in seconds with an oxy-acetylene torch. And flowing the filler metal is virtually instantaneous, thanks to capillary action. Finally, flux removal is generally no more than a hot water rinse, and oxide removal needs only a dip into an acid bath.

There are exceptions to the rule, of course, but in most cases a brazed joint is made fast—considerably faster than a linear welded joint. And, as we’ll see later on, these economies in time and labor are multiplied many times over in high production automated brazing. The pure speed of brazing represents one of its most significant advantages as a metal joining process.

Reach out to our brazing experts at Lucas Milhaupt if you have further questions regarding brazing, or attend our next Advanced Fundamentals & Brazing by Design 2 day course.

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