Heat Exchange

Heat exchangers are used in a wide variety of applications. The most common are steam power plants (including nuclear power generation), chemical processing, building heating and cooling, and mobile power plants for automotive, marine, and aerospace vehicles. The main function of heat exchangers is to boil or heat liquid, air, gases, and other matter as efficiently as possible. Classified by their application, the following terms will establish major types:

Boilers: Steam boilers have been used to produce power for over 200 years. Heat is transferred from fuel-fired burners to liquids in tubes surrounding the combustion chamber.

Condensers: Steam is condensed outside of the engine's cylinders by pressure changes. Some nuclear reactors use up to a million feet of tubing in this process.

Shell and Tube Heat Exchangers: This type of heat exchanger is a series of tubes aligned in a parallel direction within a pressurized cylindrical shell. Expansion joints are provided.

Coolers: This system is designed to pass heat from liquid into the atmosphere. Fans may be arranged to pass large volumes of air over tubing with fins attached to provide greater surface area for dissipation of heat form within the tubing.

Radiators: The most commonly known heat exchanger also functions as a cooler but is typically associated with removing heat from the engine coolant in automotive applications.

The use of solder in these systems is mainly for attachment of tubing and pipes used as conduits for the materials being processed. Even in air conditioners, which are sealed systems, the gases circulating throughout the unit are routed through copper or aluminum tubing soldered between the evaporator and the condenser. When a large number of tubes are soldered to a central point, it is possible to use dip methods of soldering. This involves the use of bar solder heated in a large pot to create a sizable quantity of molten solder. Wire solder is used to join individual tubes to header sheet or connectors and to seam low-pressure tanks. This can be done by hand or by automated equipment which feeds the wire into the preheated, fluxed joints while moving the object along on a conveyer. Literally hundreds of thousands of pounds of solder are used annually in heat exchanger applications.

Product Applications
Regardless of the diversity of heat exchanger equipment, there is a common point in their design - each circulates a hot or cold substance through a series of enclosed conduits to an area where its temperature will be changed. This is accomplished by increasing its surface area by spreading the material out (as you would when making noodles from a mass of dough) and then bringing it into close contact with a liquid, gas, or solid matter that will cause cooling or heating. Water and air are important components of the conversion since they are cheap and efficient media for changing the temperature of hot and cold substances. They are primary to our study since the construction techniques for vessels and conduits used to bring about temperature change are natural candidates for solder products.

In the process of increasing a surface area for cooling, we find materials with high thermal conductivity such as copper, brass, stainless steel and aluminum being widely used in tubing, couplings and reservoirs. These materials are also excellent for soldered joints providing high tensile and shear strength with a variety of solder alloys.

Tubing is used in profusion in heat exchangers. The attachment of tubes and piping to distribution points provides the largest single application of solder products in both bar and wire form. The greatest challenge is to bring dozens, hundreds, and even thousands of round tubes to a secure position in a header sheet. This is a junction point for tubing which is normally flat but could be curved and employ compound angles in high-tech equipment found in aerospace vehicles. Each tube must be leak-proof at its attachment point or the whole system is subject to failure. Except for very high-pressure requirements, which dictate welding or brazing, most joints can be soldered effectively. This is especially important when different materials and metal thickness are being joined.

An important prerequisite is to mechanically fix the tubes in place by expanding, countersink and flare, or swaging operations. This allows good alignment of the components when soldering. Called "self-jigging", this also provides stability during cooling to ensure integrity of the soldered joint.

All manufactured products must be cleaned of lubricating and cutting oils prior to the fluxing operation which is absolutely essential to achieving a good soldered joint. Cleaning is accomplished with a variety of solvents, depending on the material being removed. The subject of cleaning and fluxing is specialized. Fluxes will be used in every soldering application to optimize the interaction to the solid combination of solder and joint material. Finding the right one is important.

The listing below shows the most common heat exchanger tubing materials and the types of solders used with them:

The assembly of the tubing and pipe to any structure will require some clearance in the joint to allow capillary action to aid the flow of solder along the surfaces which are being joined. Usually .003 - .05 inches will be adequate.

Fry has designed fluxes specifically for the needs of this market. These needs brought on the development of the Radsol Heat Exchange fluxes. They offer the following benefits:

The Radsol fluxes are designed specifically to be used in the core bake, header, hand solder, tube mill and heater core applications.

Now with the heat exchange industry changing to lead-free solders, Fry Technology has developed and patented the next generation of heat exchange fluxes. They are:

Heat may be applied equally around the area with gas torches, electrical resistance, or induction heaters to make joined metals hot enough to melt the solder. Heating can be separate when the heated tube and header assembly is dipped into the molten solder pot. In addition, components may be pre-soldered, joined, refluxed, and then heated to effect the connection. Soldering of cast fittings to tubing, tubing to connectors, header covers to headers are usually soldered with wire solder placed in the joint until melted.

Future Product and Market Opportunities
In recent years, the use of lead-free solders has become an important new development for prevention of lead poisoning of the user of soldered products as well as the workers involved in their manufacture. These new solders, 97Sn/3Cu and Fry Silver will all be welcomed into the world of heat exchangers. Their high tin alloys will join a variety of base metals to cover the full spectrum of applications from the past and into the future.

Innovations in heat exchanger design will cause some shifting of priorities toward new materials, perhaps an emphasis toward more non-metallic (plastic) radiator housings, etc. The most likely scenario, however, is one in which there is growth in several directions; one emphasizing cost reduction, the other tending to increase performance and reliability. The latter, of course, will emphasize the most efficient thermal conductors … copper, brass, stainless steel, and aluminum to bring about a "more of the same - but better" mindset for development.

With the next market expansion will also come the need for higher levels of quality and certified evidence of being able to do it right the first time. These are promotable qualities and opportunities for enhancement of competitive market position.