Hot oil heaters and thermal fluids: the complete guide - Pirobloc
According to the nature of the supply of heat to the boiler, we can consider three types of configuration:
- By means of traditional fuels, whether liquid or gaseous.
- Heating by means of electric resistive heating elements.
- The energy is provided by the recovery of sensible heat from gases originating from the combustion of a furnace or from a production process.
Basic requirements
- It must be designed in accordance with the appropriate codes for this type of equipment (ASME design codes, AD-Merkblätt), not only with regard to calculations, but also for materials and their traceability, testing (hydraulic tests, non-destructive testing, etc.), execution (welding processes), safety and environment.
- The documentation and records generated in the design and manufacture of the equipment, should be formed into a file that will allow the final certification of the equipment in accordance with the international regulations – ASME, CE marking, etc. -.
- Adapt to the technical requirements specified by the user, especially with regard to power supplied and maximum operating temperature.
- The performance and energy efficiency values should be optimized and, therefore, the fuel consumption should be moderate.
- It must be highly reliable and easy to maintain. It must be remembered that, in general, the boiler is a critical part of the equipment in almost all production systems and a stoppage or breakdown can imply costly production stoppages.
- It must be sufficiently flexible to be able to work with the widest possible range of heat transfer oils, allowing a long useful life of the fluid used, due to a correctly designed heat exchange – see characteristic temperatures of heat transfer fluids -.
- Obviously, another requirement is a competitive price, which not only allows easy marketing by the manufacturer but also relatively short repayment terms for the user.
The fulfilment of these basic requirements to a great extent determines the design of the equipment, in as much as some of the not so obvious technical and functional parameters are important but not critical.
Within these functional parameters, requirements to be considered include convenient and fast access to the heating equipment, whether these be burners or electric resistive elements, inspection inside of the boiler, the possibility of emptying the boiler, sufficient thermal insulation to allow safe temperatures on the outside surfaces without risk of burns.
Under the more technical headings and for boilers with traditional fuels, the correct dimensioning of the combustion chamber, which allows, on the one hand, high heat transfer by radiation without exceeding the maximum permissible temperatures in the materials of the chamber and, on the other hand, allows the installation of low index NOx burners, requires a well-adjusted and exact design.
Furthermore, losses in the charge, whether in the flue gas or in the heat transfer fluid circuits, should not be high, thereby allowing the use of standard burners and pumps with low electrical consumption.
Boiler with liquid fuels: fuel oil, gas, natural gas or LPG
This is undoubtedly the type of boiler that you will most frequently come across and its design, with some variations and details, is very similar among many of the manufacturers in the industry.
Its assembly can be vertical or horizontal, depending on the needs of the user, but, in both cases, the concept and, therefore, the design are the same.
A boiler with a horizontal assembly allows it to be located in rooms that are relatively low in height and permits convenient and easy access to the burner and to the various parts of the equipment. By contrast, the greater needs for floor space with respect to a vertical boiler, can sometimes be a decisive element in the final decision.
As can be deduced from the previous paragraph, the location of the heat transfer fluid boiler is key in the selection of its orientation. The reason for this is that heat transfer fluid boilers, in almost all circumstances (only boilers with aromatic synthetic heat transfer fluids are excluded from this premise – see heat transfer fluids), can be installed, in accordance with the regulations, as close as the user wants to the consumer appliances, offering the possibility to avoid long and expensive installations.
Figure 1. PIROBLOC heat transfer fluid boilers with liquid or gaseous fuels.
Horizontal or vertical assembly
Description
The basic diagram for a heat transfer fluid boiler using liquid and gaseous fuels is shown in Figure 2.
The most common design is that of two concentric coils (8) and (9), within which the temperature of the heat transfer fluid increases by absorbing the energy supplied by the burner (1), attached to the lid of the boiler (17). With just a single coil (and, therefore, two smoke passes) it is difficult to obtain good performance due to insufficient heat exchange surface, whereas using three or more coils, while guaranteeing high energy efficiency, also implies a high economic cost. Thus, the equipment with two coils and three smoke passes can be considered as the “optimum design” that combines satisfactory yields with a moderate cost.
The inner coil performs the contour functions of the combustion chamber (5), establishing its diameter. The burner flame is projected from the burner to the combustion chamber, reaching, depending on the combustion adjustment, to just make contact with the ceramic tile (rear closure of the combustion chamber (13)) which delimits the length of the hearth. This is what is colloquially known as the first smoke pass.
Upon reaching the rear closure of the combustion chamber, the gases change direction and circulate at high speed and turbulence between the two concentric coils (second smoke pass (6)) to the front cover, where they change direction again until evacuated by the flue (14), through the passage between the outer coil and the inner casing (11) (third smoke pass).
In the vast majority of cases, the two coils are connected in series. Only specific designs for large flows and low heat differentials, require the coils to be connected in parallel (see technical concepts).
In order to achieve the airtightness of this smoke circuit, which is necessary to ensure the anticipated energy yields of the boiler, there are closures (13) and (18) which force the combustion gases to travel the path planned initially during the design of the equipment.
To promote heat exchange, the circulation of the heat transfer fluid is initially through the outer coil to then pass to the inner coil, thus being a counter-current exchange of temperatures with respect to the flue gases and achieving excellent energy yields.
The entire assembly is thermally insulated (10), (12) and (16), in order to minimize structural energy losses into the atmosphere, while avoiding possible burns by inadvertent contact with the surface of the boiler.
Figure 2. Heat transfer fluid boiler for liquid or gaseous fuels. Basic diagram
Key
1.- Burner
2.- Fuel supply
3.- Heat transfer fluid Output to consumer/system points
4.- Heat transfer fluid Return from consumer/system points
5.- Combustion chamber. Combustion gases, first pass
6.- Combustion gases, second pass
7.- Combustion gases, third pass
8.- Heat transfer fluid Interior coil
9.- Heat transfer fluid Exterior coil
10.- Thermal insulation of the boiler body
11.- Inner casing
12.- Base of the boiler
13.- Combustion chamber bottom closure. Ceramic tile/refractory concrete
14.- Chimney/flue
15.- Output of combustion gases
16.- Thermal insulation of boiler and combustion chamber
17.- Boiler cover
18.- Combustion chamber top closure
Heat exchange
For the purposes of heat exchange, the configuration described can be divided into three parts in accordance with the heat transfer method and in relation to the technical constraints that are required at each point, in order to achieve the energy efficiency and durability results from the heat transfer fluid charge and from the equipment materials. (see Heat transfer).
In Figure 3, the three zones are clearly distinguished:
1. Radiation
It encompasses practically the entire combustion chamber, more specifically, the inner face of the interior coil, with it being decisive in this area, from a technical point of view, to know the exact values of the maximum temperature reached by both the heat transfer fluid and the material of the coil because, although it is the area with the greatest exchange capacity, it is also at risk of exceeding the maximum permitted values. – Figure 4 -.
Figure 4. Areas of the boiler according to heat transfer method. In relation to the mass and film temperatures reached –see Temperatures-.
The characteristics of the thermal fluid used, the fuel, the combustion regulation, the flame diameter, the exchange requirements, the minimum circulating flow of heat transfer fluid required and, therefore, its velocity and the diameter of the coil tube are all parameters that determine what must be considered as critical in the design – the dimensioning of the diameter and the length of the chamber.
A too small diameter for the combustion chamber would allow an optimum transfer of heat but would jeopardize the useful life of the charge of heat transfer fluid as well as of the boiler itself and would also cause a loss in the smoke circuit charge which may be an excessive burden for a standard burner.
On the other hand, a combustion chamber with an over-sized diameter, will decrease the energy efficiency of the equipment.
The length of the combustion chamber is also of great importance with respect to the reliability of the equipment. A combustion chamber that is too short for the power required would involve unusually high temperatures in the bottom closure and in the upper closure of the chamber, which could lead to the partial destruction of these elements.
2. Transition zone
This comprises the inner faces of the ends of the inner and outer coils. Depending on the adjustment of the burner, it may partially include the outer face of the inner coil. In this area, radiation and convection coexist as heat transfer processes and, therefore, with regard to the heat, both the precautions for exchange by radiation and the constraints due to exchange by convection must be taken into account.
Particular attention should be paid to the design for the change in direction of the combustion gas circuit in the bottom closing of the combustion chamber, since complete airtightness must be achieved (otherwise the combustion gases would pass directly from the 1st pass to the flue outlet, giving very poor performance and worse, with extremely high temperatures in the flue that could cause its destruction) together with a low loss of charge in the change of direction of the flue gases.
3. Convection zone
This corresponds to both faces of the outer coil and the inner face of the interior coil.
Although there may a slight risk of exceeding the maximum temperatures of use of heat transfer fluid and materials (see Figure 4), the main concern when designing this area is that of achieving a high level of heat transfer by means of a considerable velocity of combustion gases but without producing significant contamination risks in smoke passes 2 and 3 owing to under sizing in these passages or a high loss of charge in the smoke circuit (known as boiler overpressure) making it difficult to use standard market burners.
Figure 3. Distinct areas in a heat transfer fluid boiler for heat exchange purposes
In addition to all of the parameters discussed above, the coils should also be carefully designed so that, from the hydraulics point of view, the heat transfer fluid circuit charge losses are not high, which would result in non-standard pumps and high electricity consumption and which, at the same time, guarantees sufficient heat transfer fluid velocity in order to provide satisfactory heat transfer coefficients – see Figure 5.
Figure 5. Heat transfer fluid velocity / heat transfer coefficient. Values for BP Transcal N. heat transfer fluid Temperature 290°C. Other factors are excluded for a better understanding of the importance of velocity
Heat differential. Passes in the coils
Heat differential also known as heat jump, is the maximum increase in temperature of the heat transfer fluid that a boiler is able to obtain in its nominal heat power, at the design flow rate of heat transfer fluid.
The most common thermal jumps are 20°C and 40°C, although these values have some margins depending on the heat transfer fluid used and the operating temperature, thus, we should actually talk about intervals of between 18-22°C in the first case and 36-42°C in the second case.
It is important to keep in mind that one boiler is not better or worse than another boiler with the same heat power but a different jump. With the correct design, both types of boiler will have similar energy performances and similar operating functions.
The reason for having boilers with different heat differentials is to obtain the best adaptation of the boiler to the characteristics of the production process and, more specifically, to the system’s consumer appliances.
Initially, a boiler with a 20ºC heat jump can give a greater uniformity of temperature in the consuming appliances due to having a greater circulating flow, although with an initially more expensive installation due to a larger pipe diameter, more heat transfer fluid capacity in the system and a higher electrical consumption in the main pump. However, a boiler with a 40°C heat differential can also achieve the same results by means of recirculation circuits with secondary pumps which provide a greater flow rate in consumer appliances and, thus, greater uniformity. In the latter case, however, the installation cost of the heat differential boiler is considerably higher which is not a positive factor.
Heat differentials higher than 40 or 50ºC are not common given that the useful life of the heat transfer fluid is affected by such high and abrupt changes of temperature and the design of the boiler must anticipate measures for absorbing additional expansions, which makes the design more specialized and more expensive. However, in applications for solar thermal power plants, heat transfer fluid boilers with heat differentials of 100°C can be found.
Our recommendation is that the user contact the boiler manufacturer, authorized installer or in-house or external engineer to discuss what heat differential would be the most suitable for their process.
We have already seen that determining the heat differential, basically by the characteristics of the consuming devices, determines the circulating flow rate of heat transfer fluid required in the system. But this flow must also meet certain requirements marked on the boiler.
The velocity of the heat transfer fluid in the coils must be high enough to ensure a good heat exchange while not exceeding the film temperature of the heat transfer fluid used in order to avoid its rapid degradation. But these high circulation speeds that are required also imply significant charge losses (pressure losses) since the charge loss is proportional to the high velocity squared, with the possibility of having to resort to very large pumps with inordinately high electricity consumption in order to achieve hydraulic stability in the circuit.
Reconciling the factors of high velocity and acceptable charge losses is only possible with a precise heat and hydraulic study of the coils, the diameter of their tubes, the length of these and their connection.
With the help of the diagrams in Figure 6 and a short example, we will try to clarify a little all these issues. We have simplified the possible hydraulic options exclusively in these three cases. In reality, the parallel passes of the coils can be from only 1 pass or up to 6, 7 or 8.
The operating temperature T1 and its kW heat output are the same in all three diagrams in Figure 6. Also, the total length of the component pipe of the coils is the same – 4L.
The differences relate to the boiler inlet temperatures (return temperature from the consuming appliances after supplying the required energy), T2, T3 and T4. The circulating flow rates Q, Q1 y Q2 and the charge losses ΔP1, ΔP2 and ΔP3 are also different.
Real numeric example
We have a heat transfer fluid boiler with 40ºC heat differential and with 1100 kW of heating power. Its exchange surface is 54 m2 with yields in the order of 86-89%, depending on operating temperature.
Its design outline is A) in Figure 6, with two coils in series and two parallel passes per coil. The design flow rate for these conditions is 52 m3/h, with a charge loss of 2.37 bar at 260ºC operating temperature.
If we try to operate this boiler with a heat jump of 20°C, the flow rate would have to be 104 m3/h and the expected charge loss at the same temperature as before, 260°C, would be 8.17 bar. We would have to resort to very sophisticated and expensive pumps, with very high electricity consumption.
On the other hand, if we use design outline B) in Figure 6 (two coils in series with three parallel passes per coil) with the same flow rate, 104 m3/h, and exchange surface, 54 m2, the charge loss would be 2.62 bar, which is acceptable for conventional pumps.
This type B) design outline would not be feasible for a boiler with a 40ºC heat differential since, with the low flow rate required, 52 m3/h, there would be no problems of pressure drop (only 0.71 bar) but, instead, the problem would be overcoming the fluid film temperature, since this would be approximately 44°C higher than the operating temperature.
As can be seen in Temperatures, the maximum film temperature is usually in the order of 10-20°C above the maximum operating temperature so, in this hypothetical case, we would either suffer a rapid degradation of the heat transfer fluid charge or we would be forced to work at low temperatures, which may not be acceptable for our production system.
Design C), with two coils connected in parallel, each one with three passes of heat transfer fluid, corresponds to a fairly unusual assembly and one typical of boilers requiring very small heat differentials, in the order of 10 or 15ºC. Under these conditions, the flow rate, 205 m3/h, is very high and if this configuration were not chosen, the heat transfer fluid charge loss would be excessively high, even with the three-pass configuration in design outline B), given that it would be around 8.45 bar.
Figure 6. Types of coil connection. A) In series, two passes per coil in parallel. B) In series, three passes per coil in parallel. C) In parallel, two passes per coil in parallel
We can see, therefore, that the heat jump required greatly influences the design of the boiler and it must, therefore, be considered as a key factor in the installation project of a heat transfer fluid system.
Even though in many countries the cost of electricity is much higher than that of liquid or gaseous fuels, the absence of air pollution together with the absence of a need for a combustion gas flue means that heat transfer fluid electric boilers are often used in laboratories or in companies located in urban environments, as well as in companies in which strict respect for the environment is part of their business philosophy.
Other important advantages are the absence of fuel installations, which can sometimes require a significant amount of space, as well as the fact that they do not require a burner and therefore avoid the maintenance related to these, a particularly important point in companies with limited maintenance services.
Even with these advantages, the use of these types of heat transfer fluid boilers is limited to relatively small heat outputs since, due to the high economic cost of electric energy, they also require the availability of contracting the necessary power.
Figure 7. Electric heat transfer fluid boiler
Its configuration is simple (see Figure 7), consisting of heating elements (5) welded to a flange (3) which serves to connect to another flange (4) of the cylindrical housing (2).
The heat transfer fluid (which is heated when it passes between the heating elements) flows within this housing and through inlet and outlet pipes (7) and (8). The junction or terminal box (1) is placed remotely in order to avoid the high temperatures. The housing is thermally insulated to avoid losses into the atmosphere and the risk of burns by inadvertent contact.
Image 1. Vertical assembly heat transfer fluid boiler mono-bloc unit. On the right you can see the terminal box positioned further away to avoid overheating
It is usually assembled horizontally to enable lower temperatures in the terminal box and to allow easy access to the same and to the elements for maintenance operations. However, occasionally, due to space requirements in the plant, they may also be assembled vertically (Figure 8); in these cases, the remote distance of the terminal box is greater.
If the power required is high, several groups of resistive elements, connected in series or parallel, can be assembled – see Figure 2.
The electrical resistive elements that provide the energy to the heat transfer fluid must be those specified for this type of operation. A correct determination of the specific surface charge (W/cm2) of the heating elements is essential.
Factors such as heat transfer fluid characteristics and process parameters including minimum flow, inlet and outlet temperatures, maximum temperature regarding heating elements and the type and number of fastening rings (heat deflector plates) mounted on the element’s surface.
All of this is intended to avoid deterioration of the heat transfer fluid charge due to exceeding the film temperature (see Temperatures), as well as overheating of the resistive elements. The deflector plates ensure a good circulation and hence a more uniform dissipation of the transferred energy.
The usual specific charges of heat transfer fluid in electric boilers are around 1.5 – 3 W/cm2 in the design known as the “container design”, which is the most common and the configuration of which we have described.
In another type of design, known as the “tubular design”, used for very specific processes, where the heating element is inserted in a tube and, therefore, the heat transfer fluid can acquire high speeds, specific charges of up to 6 W/cm2 are used.
This requires special care at the beginning of the process, as the heat transfer fluid is more viscous at low temperatures and a high specific charge can cause overheating.
As a comparison, for other types of fluids such as water, it is usual to work with charges of up to 12 W/cm2.
Image 2. PIROBLOC heat transfer fluid electric boiler, consisting of three groups of resistive elements
We can distinguish several types within the group known as heat recovery boilers, which involve complex and very diverse designs.
There are, however, characteristics common to all of them and these are what determine the basic principles of this type of boiler.
These general principles are:
- The combustion occurs in equipment that is external to the heat recovery boiler.
- A large amount of the energy is transferred by convection.
- The equipment cleaning system is a point to be considered as critical.
The origin of the gases that provide the energy to the boiler allows us to distinguish:
- Originating in a furnace by wood, pellet or some type of waste combustion.
- Derived from a conventional boiler.
- Is the result of some kind of reaction within our productive system.
In this section we do not include those heat exchangers, commonly known as batteries, that use the combustion gases from the boiler itself to preheat the combustion air of the burner – see Figure 8.
Figure 8. – Combustion air preheating battery
Key
1.- Heat transfer fluid boiler
2.- Duobloc burner combustion chamber
3.- Battery/heat exchanger
4.- Heat transfer fluid return from consumer appliances
5.- Output of heat transfer fluid from the boiler to consumer appliances
6.- Output of combustion gases from boiler
7.- Chimney. Gases into the atmosphere
8.- Duobloc burner fan/blower
9.- Combustion air intake at room temperature to the battery
Although there is obviously a recovery of energy, the fluid that is heated is not that belonging to the system (heat transfer fluid) but an auxiliary one (the air that will be used in the combustion) and we should not consider this equipment to be a heat recovery boiler but, instead, treat it as a useful and beneficial accessory of the conventional boiler and its burner.
This burner cannot be conventional, with the built-in fan or blower (usually called a monobloc burner) because the high temperature of the air requires specific materials as well as special designs to achieve the necessary turbulence in the preheated air in order to attain good mixtures in the combustion chambers.
The blower is located on the outside of the burner frame and that is why these burners are known as duobloc, since their components are separate.
An estimate of the improvement in performance of this accessory is shown in Figure 9. It can be seen that, with a standard combustion adjustment (an excess of air of approximately 1.2), the performance with air at room temperature (20°C) is 87%, while with a preheating to 170°C, a performance of around 92.5% could be obtained.
However, being an unconventional burner and, therefore, more expensive, the use of this type of configuration must be carefully evaluated, depending basically on the capacity of the boiler (duobloc burners only exist for average/high power requirements), the operating temperature, operating time and schedule, fuel and price of fuel, etc., to determine whether the system would offer satisfactory cost recovery.
Figure 9.- Estimate of the improvement in performance with preheating of air according to combustion adjustment. Fuel: natural gas, operating temperature: 300ºC
Now speaking strictly about heat recovery boilers, their basic principle is the same, regardless of the origin of the gases, and is that shown in Figure 10.
Figure 10.- Heat recovery boiler diagram
Key
1.- Heat recovery boiler
2.- Gases produced by furnace, process or boiler
3.- Intake of heat transfer fluid to the heat recovery boiler
4.- Output of heat transfer fluid
5.- Chimney/flue
6.- Safety Flue
7.- Bypass
Image 3. Double surround heat recovery boiler. High gas temperatures. Heat transfer fluid connection in the top front. Gas output at the right. The intake is at the top. The cleaning doors can be seen at the bottom
Heat is recovered from the gases produced by a furnace, a boiler or from a production process, by means of a heat recovery boiler (1) installed in the flue (2). The heat transfer fluid (3) and (4) is heated therein and the gases, once the heat has been transferred, are released into the atmosphere through a chimney/flue (5).
In this diagram the installation of a bypass (7) is essential, to allow adjustment of the heat supply to the heat recovery boiler, thus allowing the heat transfer fluid to reach the desired temperature.
If this temperature is reached, assuming that the activity that provides the energy via the gases cannot be stopped (because in most cases it would cause a serious loss of production), these gases are then diverted through the bypass to the safety flue.
Depending on the accuracy of the operating temperature that is required, the bypass can be either an all or nothing action or a modulated control.
Image 4. Integrated bypass heat recovery boiler. For frequent cleaning Intermediate gas temperatures
Other auxiliary components of this configuration not indicated in the basic diagram are the silencers and expansion joints. It is important to remember that in some processes the temperature of the gases can be in the order of 1000°C, therefore, considerable expansions are expected to be absorbed.
The great diversity in the characteristics of the heat recovery system, such as gas composition, aggressiveness of these gases, amount and type of ash, the required temperatures of the gas and of the heat transfer fluid, gas flow, heat differential of the heat transfer fluid, gas pressure at the intake of the heat recovery boiler and, therefore, the pressure that it can control, etc., etc., means that the design of heat recovery boilers is extremely varied and practically unique in each situation.
Image 5. Inline heat recovery boiler Specifically for boiler gases
In images 3, 4 and 5, we can see various designs of heat recovery boilers: Double surround (suitable for high gas temperatures), battery-type with integrated bypass (medium power and temperature, but with significant amounts of ash), and inline chimney fan (specifically for using the flue gases of conventional boilers).
Figure 13 shows the two possible configurations of a combustion gas, heat recovery boiler taken from a conventional heat transfer fluid boiler.
In diagram A), the heat transfer fluid that flows through the heat recovery boiler belongs to the main system, by means of which the heat recovery boiler becomes an “attached coil” or a “third coil” of the conventional heat transfer fluid boiler.
Under these conditions a bypass regulator is redundant because the temperature control is performed via the usual safety features of the boiler, with there being no flue gases to be recovered if the burner is in the rest position after having reached the operating temperature and with there being no possibility of overheating.
Figure 13. – Inline heat recovery boiler
Key
1.- Heat transfer fluid conventional boiler
2.- Burner
3.- Heat recovery boiler
4.- Heat transfer fluid return from consumer appliances
5.- Output of heat transfer fluid from the conventional boiler to consumer appliances
6.- Output of combustion gases from boiler
7.- Chimney. Gases into the atmosphere
8.- Heat transfer fluid inlet to conventional boiler
9.- Secondary line heat transfer fluid return
10.- Heat transfer fluid output to secondary line
11.- Bypass
12.- Safety flue
In diagram B), the recovery of heat makes it possible to make use of a network of heat transfer fluid independent from the main one, obviously, at an operating temperature lower than that one. In this scenario, the bypass regulator and a second flue are necessary.
The bypass can act exclusively as a safety device, performing temperature regulation and automatic valve functions for this secondary circuit.