An Introduction to Coil Heat Exchangers
Coil heat exchangers in their simplest form, use one or more tubes that run back and forth a number of times. The tube separates the two fluids. One fluid flows inside the tube and another flows on the outside. Let us have a look at a heating example. Heat is transferred from the hot inner fluid to the tube wall via convection, it then conducts through the pipe wall to the other side and the outer fluid carries this away also through convection.
The Coil Type Heat Exchanger produced by metal industries are suitable to transfer heat in a wide variety of operating conditions and to refuse to accept decay for the longest period of time possible under the harshest operating circumstances. Coil-type exchangers are more efficient than shell and tube exchangers for low flow rates. Due to their simple construction, they are low in price and easy to clean on the shell side. Thermal efficiency approximates that of a true countercurrent flow type exchanger.
Condensers are used for condensation vapors cooling liquids. Condensers are made by fusing a number of parallel coils in a glass shell. Coil Type Heat Exchangers are artificial to special requirements as to dimensional tolerances, finish and tempers for use in condensers and heat exchangers.
Copper heat exchanger tubes are normally supplied in straight length in annealed half-hard temper. Coil Type Heat Exchangers shaped by are metal industries not only have stiff tolerances the most dependable dimensions throughout the tube length. The tube surface is clean both inside. Coils are made in different diameters using tubes different bores.
The Applications of Coil Heat Exchangers
When it comes to heat exchangers, coil heat exchangers specifically are ideal for applications including boiler air preheating, pulp dryers, unit heaters, condensing and cooling as well as high-pressure, air tempering, and dryer applications. Depending on the application, there are many types and styles of coil heat exchangers to choose from.
Some of these include stainless tube bundles, double-pip heat exchangers, stainless steel tube immersion coil, bare tube immersion cooler, gas to water cooler, copper coil heat exchangers, combination ambient air/chiller water cooler, or coil tube-in-shell design to name a few. Each of these may also come in a variety of sizes and each has specific advantages. However, coil heat exchangers, in general, can be expected to have many advantages overall as well.
The Advantages of Coil Heat Exchanger?
Some advantages of coil heat exchangers include high efficiency, flexibility, low-pressure drop, they require little maintenance, are compact and lightweight, and are also easy and inexpensive to install. Coil heat exchangers tend to have higher efficiency than other types because of the large number of closely aligned tubes. This design aspect enlarges the heat transfer area, which results in a higher heat transfer co-efficient overall.
This efficiency equates to higher production while using less energy and that means big savings both upfront and in the long run. The coil-type heat exchanger is also known for being compact and lightweight due again to the closely packed tubes. The exchangers’ compact, lightweight design as well as their unique vertical orientation also means that they will take up less space and will be easier and less expensive to install. Their lightweight and compactness also lend to the flexibility mentioned earlier.
Another advantage of these heat exchangers is that there are many options when it comes to model types and configurations, which means they can be used with a wide variety of temperatures, flows, and pressures.
This flexibility equates to greater value overall. In addition to the benefits I just mentioned, coil heat exchangers also have low-pressure drops and require little maintenance as the structure of the tubes allows for turbulent fluid flow, which minimizes fouling and scales build-up. They can also be easily removed from the piping system to be flushed if that is necessary. Saving time and money both on installation and maintenance can go a long way for your business, and allows for overall smoother operations and less hassle and frustration.
How Hot Water, Boosters and Reheat Coils Work
Hot water coils are a type of heat exchanger often called hydronic coils that use hot water from a boiler to heat or remove moisture from the air. The air moves through the fins of the coil which is hot from water flowing through the tubes. This is a type of heat exchanger mostly used in commercial and industrial air handler units, roof top units as well as ductwork installations downstream.
In parts of the country and world, you may be more familiar with radiant heat or baseboard heat. Those particular units do not have air moving through them but simply radiates the air near the units. Hot water coils on the other hand are used in forced air systems and booster coils or reheat coils are used in ductwork downstream to reheat the air that has cooled off.
Hot Water Coil Components Explained
Hot water coils are one of the simplest parts to an HVAC unit, but it can be important to know the parts so you know what type of connections and where they need to be located. Similar to chilled water coils, If and when a coil is to leak it usually happens in the copper u-bends. But with hot high pressured water, it will first leak at the weakest place which can be on a return bend or in the middle of the coil. When patching a leak in the middle of the coil it will significantly destroy that area causing the coil to lose a great deal of efficiency.
No matter how simple a hot water coil or reheat coil is, you still have options of what materials you should use.
All types of industrial plants require coils – chemical plants, water treatment plants, food processing plants … the list goes on and on. Regardless of the type of plant, corrosive materials (carbon and carbon compounds, chlorides, metal oxides, sulfates and sulfuric acid, etc.) are often found inside. These materials can lead to general corrosion and/or localized corrosion in coils, such as pitting and formicary corrosion.
When ordering chilled water coils and hot water coils for industrial plants, there are different materials to choose from. Selecting materials for water coils that are corrosion-resistant offer the most value, even if the initial investment is higher, because coils won’t need to be replaced as often. For example, stainless steel is best suited for use in oxidizing environments, where corrosion resistance is needed. We use stainless steel for tubes and fins.
Water is the most abundant medium for heating and cooling applications. Luckily there’s a water coil from SPC for just about any hot or cold water application.
We have over 30 years’ experience in the design and manufacture of coils. There are few, if any, design and specification issues that we have not addressed before. When you specify a coil from SPC, you’re sourcing it from the coil experts.
Efficient transmitters of heat energy
A water coil is a heat exchanger. It transfers heat energy between water and another medium (usually air) as quickly and efficiently as possible. Heating coils transmit energy from hot water to a stream of air, cooling coils extract energy from an air stream and add it to cold water.
The coils themselves consist of a matrix of copper tubes through which the water flows. The tubes run back and forth between two end-plates in an arrangement that lets the air flow perpendicular to the tubes. The tubes pass through – and are attached to – a layered array of thin metal plates known as fins. The fins are also perpendicular to the tubes, which means they lie parallel to the direction of air flow. The air flows between them. Energy either passes from the water via the tubes and fins to the air, or from the air via the fins and tubes to the water.
Economy Coil The efficiency of this arrangement depends on the types and thickness of metal used, the integrity of the bond between tubes and fins, the number and the layout of the tubes and fins, the air flow, and the water flow. Our skill lies in pulling all those elements together to make a coil that’s right for your application.
Types of water coil
Heating coils are characterized by the temperature of the water that passes through them.
Low-grade hot water (LGHW) – typically below 60°C. LGHW is associated with condensing boilers or waste water that has already lost much of its heat.
Low-pressure hot water (LPHW) – typically between 60°C and 110°C. LPHW is the most common medium for coil heat exchangers.
Medium-temperature hot water (MTHW) – typically between 110°C and 130°C. To keep water liquid above 100°C requires additional pressure. 120°C requires a pressure of 200kPa (2 bar).
High-temperature hot water (HTHW) – typically above 130°C. The most demanding condition for coil design. 140°C requires a pressure of 360kPa (3.6 bar).
When discussing heating coils, the terms ‘low-‘, ‘medium-‘, and ‘high-temperature’ are interchangeable with ‘low-‘, ‘medium-‘, and ‘high-pressure’.
Cooling coils are characterized by the amount and type of anti-freeze used.
Pure water is the most common medium. It’s used in applications that do not require the water temperature to fall below 0°C, or where there’s no risk of freezing.
The introduction of anti-freeze adversely affects the heat transferring properties of water. The increased viscosity slows flow rates through the coil. Anti-freeze concentrations above 30% require specialist pump equipment. They are not recommended.
Ethylene glycol solution (EGS) is the more efficient and the most widely-used of the two anti-freeze solutions.
Propylene glycol solution (PGS) is always used in the food industry because it is less toxic.
Cooling coils are supplied with a drain pan when condensation from latent cooling is anticipated. Where physical parameters constrain the surface area of the coil, moisture eliminators can be incorporated to prevent moisture carry-over.
Heat exchangers and electric heaters, of whatever design, serve a common purpose: to change the temperature of liquids or gases. However, there are some differences. The two most significant differences probably are: Electric heaters, as the name suggests, are for heating purposes only. Heat exchangers can be designed both for heating and for cooling.
While in heat exchangers a service fluid heats a process fluid without both having direct contact with each other, in electric heaters only the fluid to be heated is in the process. Here, heating is made by means of tubular heating elements immersed directly in the process fluid. Tubular heating elements consist of a coiled resistance wire centered in a tube and electrically isolated from the tube wall with highly compacted magnesium oxide, ensuring a high dielectric strength and excellent heat transfer from the wire to the tube and from the tube to the fluid being heated.
At first glance, this seems to be very interesting. Instead of heating in some way a service fluid, which then heats the process fluid, the required amount of heat is transferred from electric heating elements directly to the process fluid. Significantly lower heat losses through reduced piping, elimination of double temperature control (for service and process fluids) and maintenance-prone control valves.
At least here, a very appropriate question arises: do electric heaters intend to replace traditional heat exchangers? This question is easy to answer: NO. Each of the two heating systems offers some advantages over the other. But both also have their technical limitations, so they cannot be applied to every heating process. Therefore, it is worthwhile to make a closer comparison between both methods.
If a heating process requires very high final temperatures, heat exchangers are in disadvantage compared to electric heaters. Regardless of their efficiency, heat exchangers can heat the process liquids to just below the highest temperature of the service fluid. Operating fluids may be, as required, hot water (under pressure up to 150°C), steam (up to 375°C at 221 bar), mineral or synthetic heat transfer oils (up to 400°C). The final temperatures of the process liquids are thus always slightly lower than those of the operating fluid.
Electric heaters, on the other hand, can heat liquids up to 625°C (molten salt storage) and gases up to 750°C at process pressures from a few millibars to a few hundred bars. The process temperatures for air and gas are limited by the highest surface temperature at which the intended alloy of the tubular heater can be operated. For high-performance alloys this is in the range of 900°C, which guarantees good operational safety for air and gas temperatures of up to 750°C.
For liquids, the maximum surface temperature of the heating elements must be lower than that to which the liquid can be exposed without altering its properties. As an example, mention may be made of various liquid petroleum products which decompose at critical temperatures. This requires that not only the process temperature be controlled, but also the surface temperature of the tubular heating elements in direct contact with the fluid being heated.
In electric heating, the surface temperature of the tubular heater is limited by designing the correct watt density (W/ cm²). This is based on the properties of the medium to be heated, the flow rate and the temperatures to be achieved. The surface temperature of the heating elements can be predicted with absolute precision by thermodynamic calculation. This is done at Schniewindt with HeatR, a specially developed and verified software.
Depending on the design, heat exchangers have an efficiency ranging from 70% to 90%. And we must consider that heat exchangers require a primary heat source, the one that delivers the hot service fluid. This operating fluid is supplied by various sources, often steam boilers or thermal oil heaters, both of which are fueled by fossil fuels. Since the overall efficiency of the heating process depends on the heat exchanger and its heat source, it would go too far at this point to list every possible combination and the associated efficiency. The effective efficiencies are well known by everyone in the business.
It may sound exaggerated to say that electric heaters are 100% efficient and this is often contested by those who are less familiar with electric heating. It is not just a simple statement, but this efficiency corresponds to the facts if one considers only the heating element itself. Einstein and the immersion heater send greetings from physics lessons. No energy is lost. All electric energy is converted into heat. Of course, here too, the entire chain, including the generation of electric energy, must be considered. But if renewable energy can be used, then the subject is very interesting (see Power-to-Heat).
Almost universal application of electric heaters
Electric heaters can be used for almost every application where heating of stationary or flowing fluids is required, as long as it remains within the range of the temperatures already mentioned. For heating liquids such as water, oils of all kinds, acids or alkalis, as well as gaseous media such as air, natural gas, methane or nitrogen, just to name a few, electric heaters can be a very good, compact and usually also cost-effective solution. It is nearly impossible to list all known applications for electric heaters.
Electric heating as a primary source of heat is a clean process that does not generate combustion gases through open flames. Electric heaters can be installed directly into the process line where heat is needed without the need for additional steam or hot oil piping. No specialized staff is needed for the operation of an electric heater and they require almost no maintenance during normal operation. The system is controlled by contactors or thyristors, by means of which the temperature can be controlled very quickly and accurately.
Electric heaters, depending on the fluid to be heated and the final process temperature, can be built in a relatively compact design. If water is to be heated, high watt densities (W/cm²) can be applied to the heating elements. Schniewindt has built a 10 MW flanged immersion heater mounted in a circulation heater with a DN 800 (32”) nominal diameter and a total length of 3.000 mm. Of course, such a compact design can only be applied to a water heater. When liquids with high viscosity or low thermal conductivity must be heated or high temperatures are to be reached, such compact solutions are not always possible. But even in these cases, the dimensions are still comparable to those of conventional heat exchangers.
Electric heaters can also be installed in potentially explosive atmospheres, allowing their use in the chemical and petrochemical industries, as well as in refineries and offshore platforms. Here, the areas of zone 1 & 2 as well as the temperature classes T1 to T6 are covered.
But electric heaters also have their limits as far as the application is concerned. Not that electric heaters could not heat everything up. However, there are some areas for which any responsible manufacturer of electric heating systems will advise against using his products directly in the process.
This means that electric heaters are not always suitable for some particular fluids, as they would have to be designed in a way to ensure a high level of operational safety for the protection of the fluid to be heated, causing the price of this heater being beyond economic rationality.
Electric heaters combined with heat exchangers
Whenever heat exchangers are indispensable for heating processes, electric heaters are a very interesting alternative to provide the required hot service fluid. While oil- or gas-fired heat sources must be installed in separate rooms or areas, electric heaters can be mounted directly next to the heat exchangers. This not only reduces the unnecessary need for long piping with avoidable heat losses during transport of the service liquid but increases the overall efficiency of a heating process by the additional replacement of the less efficient fired heat source.
In summary, electric heaters are a good alternative to heating fluids, but they cannot replace conventional heat exchangers in all processes. However, they are also a serious option for new investments as a replacement for fired heat sources to achieve both higher overall efficiency and compliance with environmental regulations. The topic of CO2 savings (CO2 certificates) through the use of renewable energies should also be or become very interesting for many users and operators.
Solar Hot Water Heat Exchanger
While there is huge interest in solar water heating thanks to increased awareness of our impact on the environment and the surge in the costs of gas and electricity, there is often one major stumbling block – the hot water tank.
Dual coil hot water tanks are available, but they are expensive (as is the cost of removing the existing tank). It is of course possible to add a second hot water tank to a home for solar water heating, but this can also be expensive, complicated, and takes up space (which may not even be available).
In this article we will introduce an affordable heat exchange coil which can easily be retro-fitted in an existing standard hot water tank to enable indirect solar water heating.
Immersion Coil Heat Exchanger
The product pictured above is an immersion coil heat exchanger. Unlike a standard immersion heater element which is heated by electricity, heated fluid passes through this immersion coil (without coming into contact with the water in the tank – i.e. indirect heating).
If fluid (typically anti-freeze) heated to say 40 degrees Celcius is pumped through this coil of copper pipe, it will lose heat into a hot water tank filled with water at say 20 degrees Celcius. As the anti-freeze cools on its journey through the coil, the heat it loses is efficiently taken by the water in the tank.
The anti-freeze leaving the heat exchanger could well emerge at 25 degrees (having lost 15 degrees), thereby heating the tank (which contains a much larger volume of water) by a fraction of a degree. If hot anti-freeze is continuously pumped through the coil then the temperature of the water in the tank will continue to increase.
This type of heat exchanger is a commercial version of the type discussed in our article DIY Solar Water Heating Prototype. With this immersion coil heat exchanger, a solar water heating panel, a suitable circulation pump, and a simple pump controller, a very effective solar water heating system can be put together relatively inexpensively without the financial costs, trouble, and inconvenience of replacing the existing hot water tank.
The only disadvantage of the coil as pictured is the depth it can reach into the hot water tank – 800mm. The water at the top of a hot water tank can be very hot when the water at the bottom is still cool. A standard immersion element is always fitted at the bottom of a hot water tank so that it heats all of the water in the tank. Fortunately these heat exchange coils can be ordered in custom lengths as required so it is worth requesting a longer one to maximize heating efficiency..
Heat Exchangers for Solar Systems
Heat Exchangers for Solar Water Heater Systems
Solar water heater systems use heat exchangers to transfer solar energy absorbed in solar collectors to the liquid or air used to heat water or a space.
Heat exchangers can be made of steel, copper, bronze, stainless steel, aluminum, or cast iron. Solar heating systems usually use copper and, because it is a good thermal conductor and has greater resistance to corrosion.
Types of Heat Exchangers
Solar water heating systems use three types of heat exchangers:
A liquid-to-liquid heat exchanger uses a heat-transfer fluid that circulates through the solar collector, absorbs heat, and then flows through a heat exchanger to transfer its heat to water in a hot water tank. Heat-transfer fluids, such as antifreeze, protect the solar collector from freezing in cold weather. Liquid-to-liquid heat exchangers have either one or two barriers (single wall or double wall) between the heat-transfer fluid and the domestic water supply.
A single-wall heat exchanger is a pipe or tube surrounded by a fluid. Either the fluid passing through the tubing or the fluid surrounding the tubing can be the heat-transfer fluid, while the other fluid is the potable water.
Double-wall heat exchangers have two walls between the two fluids. Two walls are often used when the heat-transfer fluid is toxic, such as ethylene glycol (antifreeze). Double walls are often required as a safety measure in case of leaks, helping ensure that the antifreeze does not mix with the potable water supply. An example of a double-wall, liquid-to-liquid heat exchanger is the “wrap-around heat exchanger,” in which a tube is wrapped around and bonded to the outside of a hot water tank. The tube must be adequately insulated to reduce heat losses.
While double-wall heat exchangers increase safety, they are less efficient because heat must transfer through two surfaces rather than one. To transfer the same amount of heat, a double-wall heat exchanger must be larger than a single-wall exchanger.
Solar heating systems with air heater collectors usually do not need a heat exchanger between the solar collector and the air distribution system. Those systems with air heater collectors that heat water use air-to-liquid heat exchangers, which are similar to liquid-to-air heat exchangers.
Heat Exchanger Designs
There are many heat exchanger designs. Here are some common ones:
The heat exchanger is a coil of tubing in the hot water tank. It can be a single tube (single-wall heat exchanger) or the thickness of two tubes (double-wall heat exchanger). A less efficient alternative is to place the coil on the outside of the collector tank with a cover of insulation.
The heat exchanger is separate from (external to) the hot water tank. It has two separate fluid loops inside a case or shell. The fluids flow in opposite directions to each other through the heat exchanger, maximizing heat transfer. In one loop, the fluid to be heated (such as potable water) circulates through the inner tubes. In the second loop, the heat-transfer fluid flows between the shell and the tubes of water. The tubes and shell should be made of the same material. When the collector or heat-transfer fluid is toxic, double-wall tubes are used, and a non-toxic intermediary transfer fluid is placed between the outer and inner walls of the tubes.
In this very efficient design, the tubes of water and the heat-transfer fluid are in direct thermal contact with each other. The water and the heat-transfer fluid flow in opposite directions to each other. This type of heat exchanger has two loops similar to those described in the shell-and-tube heat exchanger.
A heat exchanger must be sized correctly to be effective. There are many factors to consider for proper sizing, including the following:
- Type of heat exchanger
- Characteristics of the heat-transfer fluid (specific heat, viscosity, and density)
- Flow rate
- Inlet and outlet temperatures for each fluid.
Usually, manufacturers will supply heat transfer ratings for their heat exchangers (in Btu/hour) for various fluid temperatures and flow rates. Also, the size of a heat exchanger’s surface area affects its speed and efficiency: a large surface area transfers heat faster and more efficiently.
For the best performance, always follow the manufacturer’s installation recommendations for the heat exchanger. Be sure to choose a heat-transfer fluid that is compatible with the type of heat exchanger you will be using. If you want to build your own heat exchanger, be aware that using different metals in heat exchanger construction may cause corrosion. Also, because dissimilar metals have different thermal expansion and contraction characteristics, leaks or cracks may develop. Either of these conditions may reduce the life span of your heat exchanger.
Your Best choice for Heat Exchange!