Coil Performance
Airflow around the coil’s pipe or tubing is one of the keys to coil performance. The coil should be designed to maximize the amount of time heated air is in contact with the coil. There are two basic approaches to this problem, the American approach using the pancake coil and the European approach. In a vertical pancake coil this means heated air is pulled around the diffusion plate toward the center of the coil pancakes. In a horizontally oriented pancake coil machine the hottest air is pulled toward the top of the coil, creating hot spots on the upper portions of each pancake.

The gun type burner in vertical style pancake equipment is mounted in the bottom of the coil. This allows the heated air to rise as quickly as possible out of the combustion chamber, only slightly slowed and deflected by the diffusion plate.

Coil Materials

Coils may be constructed of either pipe or tubing. From 80 to 130 feet of pipe or tubing is generally used in a coil. SAE schedule 40 or 80 pipe foot lengths is the standard pipe used in coil construction. Coils wound from pipe will have welded joints where the lengths of pipe are connected. Interior diameters range from 3/8 inch to ¾ inch or more, generally the greater the flow, the larger the coil ID. The smaller the ID, the faster the water travels through the coil and the shorter the time it is exposed to the heat exchange process. The larger the ID, the slower the water flow through the coil.

A single, bucket-type coil is rarely seen by itself but is more common as a cold-water leg for a pancake type coil or as part of a multiple-pass coil arrangement. In a multiple pass design, two or more buckets are set inside each other and heated air passes both inside and outside each bucket before escaping from the machine.

Pancake Coils

The pancake or helical coil is composed of spirals of pipe or tubing set directly in front of the burner flame which is deflected by a steel baffle plate to spread the flame and pass heated air between gaps in the spiral. Generally two or more spirals are stacked together to provide more surface areas for heat exchange.

Pancake type coils have a diffusion plate, which deflects the burner flame away from the center of the combustion chamber and makes hot air pass around the coil pancakes. This diffusion plate or baffle acts as a shield to help protect the coil from the blowtorch effect of the forced-draft oil burner firing directly at the coil’s pancakes.

Multiple-Pass Coils

The multiple-pass coil is designed to make it as hard as possible for the heated air to escape from the combustion chamber. This design is also called a labyrinth combustion chamber. The design increases the amount of time hot air is in contact with the coil. Most multiple-pass or European-style coil designs are vertically oriented. The fuel nozzle and electrodes are mounted at the top of the boiler assembly, firing down into the combustion chamber.

The hot air generated by the burner passes through the inside leg of the coil, then rises, (since hot air is lighter than cooler air). As the heated air rises, it heats the outside of the inside leg of the coil and the inside of the outside leg. However, the hot air is trapped at this point because there is no stack in this section of the combustion chamber. The airflow then is forced downward by blower driven incoming air and heats the outside of the outer leg before exiting through the stack. Direction of airflow in this manner increases the direction and efficiency of the heat exchange process.

Three Main Coil Problems
There are three main coil problems: freeze-up, sooting and scaling. Freeze-up results from water being left in a coil that is exposed to below freezing temperatures. When water freezes it expands and this expansion can cause a coil to burst.

1. Freeze-Up Problem - The coil bursts when low temperatures cause water retained in the coil to freeze. When water turns to ice it expands or takes up more space and the coil may burst.
2. Scaling – Heat can cause impurities, including lime, calcium, minerals and cleaning chemicals, to separate from the water and bake onto the inside of the coil. This insulates the coil, making water harder to heat, and eventually restricts or stops water flow through the coil. This tighter restriction will also cause more strain on the pump and a higher amp draw by the electric motor.
3. Sooting- Carbon deposits from fuel that does not completely burn in the combustion chamber can form on the outside of the coil. These deposits insulate the coil and can greatly reduce heat exchange efficiency. This sooting may be caused by a number of factors. Essentially, soot is carbon molecules left over from incomplete combustion of the fuel. Since perfect combustion does not happen, there will always be some soot. Soot will also form from natural gas combustion but will not be as heavy or form as rapidly since natural gas has fewer carbon molecules to begin with. Excess air lightens carbon buildup but also lowers ignition temperature, which may result in more unburned fuel and consequently leave more carbon or soot deposits. A qualified service technician should perform any air or burner adjustments.

The coil should never be heated while "dry". Always make certain water is running all the way through the system before turning on the burner. A hot water machine should have a safety device to prevent burner operation when there is a no flow.

Preventing Freeze-Up
Freeze-up is generally more of a problem in warmer parts of the United States where freezing temperatures are not encountered so often. In these areas less care in freeze-up prevention is taken than in areas where winter temperatures regularly fall below the freezing mark.

Freezing may plug the boiler inlet and discharge hose without actually bursting the coil or hose. When heat is applied to the coil, ice in the coil will melt long before the ice blocking either the inlet or outlet. The melted ice retained in the coil can be superheated and with both ends of the coil blocked by ice, a steam explosion will result. Check for full continuous water flow out of the nozzle before turning on the burner if you suspect the cleaner might have been subjected to freezing conditions.

Basically there are three ways to prevent coil freeze-up:
1. Keep the machine warmer than 32 degrees Fahrenheit – that is, keep it in a heated shed or garage.
2. Drain or blow (with compressed air) all the water from the coil.
3. Fill the coil with antifreeze.

The most effective method of preventing freeze-up is to keep the equipment warm. A cleaner that is stored in a heated garage, warehouse or other heated area will not freeze unless the heating system fails. Some equipment is equipped with an air valve or air valve stem similar to that mounted on an automobile tire to allow connection of a compressed air line to force water out of the system. This method is not always effective and does not remove water form the pump or other components.

Equipment with vertical coils often have the water outlet at the bottom of the coil. This allows for gravity draining of the coil but makes no provision for draining the pump or other components. However, gravity is more effective than compressed air in removing water from the coil.

Filling the entire fluid system with antifreeze protects all of the components, not just the coil, and can be very effective if done properly. There are three acceptable methods, pouring the antifreeze into the float tank (for equipment with float tanks), introducing the antifreeze through the water inlet (for cleaners without float tanks), and introducing the antifreeze through the chemical line (for equipment with upstream chemical injection only).

System With Float Tank

If the cleaner has a float tank, antifreeze or antifreeze solution can be poured directly into the float tank. Use the flowing procedure:
· Disconnect the inlet water line.
· Open the float tank lid.
· Run the cleaner until the float tank is about one-half full of water.
· Fill the float tank with antifreeze or antifreeze solution. If sub-zero weather is expected use undiluted antifreeze.
· Remove the pressure nozzle from the lance.
· Place the lance in the float tank so that a closed loop is formed.
· Run the cleaner until antifreeze has circulated completely through the equipment.
· When returning the
equipment to service, the antifreeze can be drained into a five-gallon bucket and saved for reuse.
· Once the antifreeze has been drained, flush the equipment thoroughly by operating it with plain water.
· Reintroduce the antifreeze into the system at the end of the day.

System Without Float Tank

If the cleaner does not have a float tank, fill the system with antifreeze, using the following procedure:
· Keep a five-gallon bucket on hand to hold your antifreeze solution.
· When the day’s cleaning work is completed, remove the pressure nozzle and run the cleaner until all remaining chemical is flushed from the system.
· Disconnect the inlet line from the water source and place it in the antifreeze container.
· Run the machine until it is filled the antifreeze. The antifreeze will be detectable by color and foaming.
· To remove the antifreeze simply place the discharge hose in the bucket and run the cleaner until the original antifreeze level is reached.
· Reconnect the cleaner to the water supply and flush it for about 30 seconds.
· Once the cleaner has been flushed, replace the water line and operate the equipment normally.
· Reintroduce the antifreeze into the system at the end of the day.

Using Upstream Injection
In the field or in an emergency situation, antifreeze can be introduced into a cleaner through the chemical line. Antifreeze will be diluted as it enters the system. This works only with cleaners with upstream chemical injection, that is, cleaners with float tanks. And, no antifreeze enters the float tank or low-pressure lines leading to the pump and chemical valve.

To use the chemical valve to introduce antifreeze into the system, follow this procedure:
· Place the chemical line in a container of undiluted antifreeze.
· Remove the pressure nozzle from the lance.
· Open the chemical valve to its maximum setting. This will allow the antifreeze to mix in a solution with the water entering the system. This dilution will be adequate for all but the coldest conditions.
· Operate the cleaner in a normal manner until antifreeze begins to come out of the end of the lance. Antifreeze flow can be determined by change in color of the fluid and foaming.
· To return the equipment to service, drain the antifreeze and flush the cleaner wit pain water.

Note: The antifreeze solution can be drained into a five-gallon container for reuse. If this solution is reused it must be poured into the float tank rather than introduced through the chemical line of it will be diluted to an unacceptable level.

Important Safety Warning!
Freezing may plug the boiler inlet and discharge hose without actually bursting the coil or hose. When heat is applied to the coil, ice in the coil will melt long before the ice blocking either the inlet or outlet. The melted ice retained in the coil can be superheated and with both ends of the coil blocked by ice, a steam explosion will result. Check for full continuous water flow out of the nozzle before turning on the burner if you suspect the cleaner might have been subjected
to freezing conditions.

Scaling

In most cases, and particularly where water is more than moderately hard, scale will build up inside a hot water machine’s heating coil. As the water is heated, impurities – including calcium, manganese chloride and silicates – precipitate out of the water and form deposits on the inside of the coil. Not only does scale restrict the flow of water through the coil, it is an excellent insulator and its buildup will significantly reduce the coil’s efficiency as a heat exchanger. Since the coil is not passing heat on to the water, the outside of the coil can get very hot. The cleaner
may seem to operate in a normal manner but the water will not reach previous temperatures. A loss in output temperature may result from scaling.

Fighting Scale
There are a number of weapons which can be used in fighting scale, ranging from electronic devices through chemical softeners and as a last resort, acidizing, or flushing the heating coil and pressure hose with an inhibited acid solution. Cleaning chemicals can also contribute to scaling.

In many cases the bulk of scale deposits will be made up of cleaning chemical deposits. Using a cleaning chemical that reduces this buildup goes a long way toward reducing the need for descaling procedures. A cleaning chemical formulation can include water softening agents which reduce calcium and silicate deposits. A mechanical water softener can also reduce scale buildup. Some machines are designed with built-in water softening systems. Such systems can be installed on other equipment types as well.

Proper care of equipment including use of scale reducing cleaning chemicals and a water softening system may be the most economical approach to solving or controlling scale problems.

Scale Removal

Running an acid solution through the coil is a solution for scale build up developed in the early days of steam cleaning. Since water in a steam cleaner is heated to 325 degrees Fahrenheit and beyond, impurities precipitate out faster, and scale can be more of a problem than in a hot water high pressure cleaner. This process can be time consuming and expensive and involves potentially dangerous chemicals. If done improperly, acidizing can literally eat up a coil.

Under normal operating conditions scale can be removed periodically by short-term circulation of acid, sometimes called "proofing." This won’t remove major scale buildup from cleaning chemicals. Scale can be a serious problem in a hot water machine in hard water areas, and the choice may be between scale removal or coil replacement.

Important Things To Know About Descaling
If Scaling has progressed to the point where scale removal is necessary, there are important points to remember.
· Descaling should be done with great care and attention paid to all precautions. After all, even if it’s diluted, it’s still acid.
· If possible, a descaling pump rather than the machine’s water pump should be used to circulate water through the coil. Many older steam cleaners used diaphragm pumps because they are good acidizing pumps with little susceptibility to acid damage. If the water pump is used, the parts that come into contact with the acid solution should be checked for damage and replace if necessary.
· The proper kind of acid should be used. Generally this is a self-inhibited acid, or one that does not attack the metal b ut confines its activity to the deposits inside the coil. A seven percent hydrofluoric acid solution is considered "heavy duty". Acid purchased from a swimming pool supply house is not inhibited and should not be used. Stainless steel coils need special treatment since even inhibited hydrochloric or hydrofluoric acid attacks a stainless surface.
· The acid should be neutralized with a good alkaline, and never just dumped down the drain. One should dress for the occasion, wearing protective clothing and goggles to keep acid from contact with skin and eyes. Always descale in a well-ventilated area. Descaling requires the use of strong acid and alkaline chemicals. Protect eyes and exposed skin areas from splash and splatter. If chemicals come in contact skin or eyes, thoroughly flush with water and see a doctor immediately.
· Proofing a coil by running a gallon of acid solution through the coil simply widens the narrowest spots in the scale deposit but does little to solve the insulation problem.

Sooting

Carbon deposits on the coil from smoky combustion can also reduce heat exchange efficiency. The soot acts as a insulator, making it harder to heat water in the coil. A one eighth inch soot deposit can seriously impair heat exchange efficiency.

Smoke need not be visible for sooting to result. Flue gasses that do not include visible smoke can cause heavy sooting. Soot can be removed by washing the outside of the coil. This is likely to require removal of the coil wrapper if not the coil itself and can be a very dirty job. Heavier carbon deposits, which build up from flame impingement on metal or other combustion problems, may require special treatment.

Heavy sooting is generally the result of a burner adjustment problem. The type of fuel used can contribute to sooting problems as well. The heavier the fuel oil, the more likely heavy sooting will occur. Kerosene, on the other hand, has fewer carbon molecules than fuel oil and will generally produce fairly clean combustion.

Corrosion and Rust
Corrosion and rust can also damage the coil. Condensation in the ceramic insulation blanket can cause external corrosion, which will eventually lead to coil faillure. Condensation occurs when hot air containing water vapor comes in contact with the colder coil surface. When this happens the vapor turns into water droplets adhering to the colder coil surface. Combustion byproducts, including sulfur from diesel fuel, may combine with the condensed water, forming sulfuric acid which speeds corrosion.