Heat Exchange

For the purposes of heat exchange, the configuration described can be divided into three parts according to the heat transfer method and in relation to the technical constraints required at each point to achieve the energy efficiency and durability results from the heat transfer fluid charge and the equipment materials.

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In Figure 3, the three zones are clearly distinguished:

1. Radiation

This encompasses practically the entire combustion chamber, more specifically, the inner face of the interior coil. It is important to know the exact values of the maximum temperature reached by both the heat transfer fluid and the coil material. Although it is the area with the greatest exchange capacity, it is also at risk of exceeding the maximum permitted values.

The characteristics of the thermal fluid, the fuel, the combustion regulation, the flame diameter, the exchange requirements, the minimum circulation flow of the heat transfer fluid required, its velocity, and the diameter of the coil tube are all parameters that determine what must be considered as critical in the design of the diameter and the length of the chamber.

A too small diameter for the combustion chamber would allow optimum heat transfer but would endanger the useful life of the heat transfer fluid charge and the boiler itself. It would also cause a loss in the smoke circuit charge, which may be an excessive burden for a standard burner.

Conversely, a combustion chamber with an oversized diameter will decrease the energy efficiency of the equipment.

The length of the combustion chamber is also of great importance concerning the reliability of the equipment. A combustion chamber that is too short for the required power would involve unusually high temperatures in the bottom and upper closures of the chamber, potentially leading 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 burner adjustment, it may partially include the outer face of the inner coil. In this area, radiation and convection coexist as heat transfer processes. Therefore, both the precautions for exchange by radiation and the constraints due to exchange by convection must be considered.

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. Complete airtightness must be achieved to prevent the combustion gases from passing directly from the 1st pass to the flue outlet, giving poor performance and potentially causing its destruction due to extremely high temperatures.

3. Convection Zone

This zone corresponds to both faces of the outer coil and the inner face of the interior coil.

Although there may be a slight risk of exceeding the maximum temperatures of use of the heat transfer fluid and materials, the main concern when designing this area is to achieve a high level of heat transfer by means of considerable combustion gas velocity. This must be done without producing significant contamination risks in smoke passes 2 and 3 due to under-sizing in these passages or a high loss of charge in the smoke circuit, making it difficult to use standard market burners.

In addition to all the parameters discussed above, the coils should also be carefully designed to ensure that, from the hydraulics point of view, the heat transfer fluid circuit charge losses are not high. This would result in non-standard pumps and high electricity consumption. At the same time, sufficient heat transfer fluid velocity must be guaranteed to provide satisfactory heat transfer coefficients.

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 can obtain at its nominal heat power and the design flow rate of heat transfer fluid.

The most common thermal jumps are 20°C and 40°C. These values have some margins depending on the heat transfer fluid used and the operating temperature, thus, we should actually talk about intervals between 18-22°C in the first case and 36-42°C in the second case.

It's important to understand 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 boilers will have similar energy performances and 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.

A boiler with a 20ºC heat jump can provide greater temperature uniformity in the consuming appliances due to a greater circulating flow. However, this comes with a more expensive installation due to larger pipe diameters, more heat transfer fluid capacity in the system, and higher electrical consumption in the main pump. A boiler with a 40°C heat differential can achieve the same results through recirculation circuits with secondary pumps, which provide a greater flow rate in consumer appliances and, thus, greater uniformity. 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 rare because the useful life of the heat transfer fluid is affected by high and abrupt temperature changes. The design of the boiler must also incorporate measures for absorbing additional expansions, making the design more specialized and expensive. In solar thermal power plants, heat transfer fluid boilers with heat differentials of 100°C can be found.

Our recommendation is to contact the boiler manufacturer, an authorized installer, or an in-house or external engineer to discuss which heat differential would be most suitable for their process.

We've seen that determining the heat differential, based on the characteristics of the consuming devices, determines the circulating flow rate of heat transfer fluid required in the system. 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 good heat exchange while not exceeding the film temperature of the heat transfer fluid to avoid its rapid degradation. These high circulation speeds imply significant charge losses (pressure losses) since the charge loss is proportional to the square of the high velocity. This may necessitate very large pumps with high electricity consumption to achieve hydraulic stability in the circuit.

Reconciling the factors of high velocity and acceptable charge losses is possible only with a precise heat and hydraulic study of the coils, their tube diameter, length, and connection.

With the help of the diagrams in Figure 6 and a short example, we will try to clarify these issues. We have simplified the possible hydraulic options exclusively into these three cases. In reality, the parallel passes of the coils can range from 1 pass to 6, 7, or 8.

The operating temperature T1 and its kW heat output are the same in all three diagrams in Figure 6. The total length of the component pipe of the coils is also 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, and Q2 and the charge losses ΔP1, ΔP2, and ΔP3 are also different.

Real Numeric Example

We have a heat transfer fluid boiler with a 40ºC heat differential and 1100 kW of heating power. Its exchange surface is 54 m2 with yields of around 86-89%, depending on the 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 need to be 104 m3/h, and the expected charge loss at the same temperature of 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, using 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 because, with the low flow rate required of 52 m3/h, there would be no problems with pressure drop (only 0.71 bar). However, the problem would be overcoming the fluid film temperature, as it would be approximately 44°C higher than the operating temperature.

As shown in Temperatures, the maximum film temperature is usually around 10-20°C above the maximum operating temperature. In this hypothetical case, we would either experience rapid degradation of the heat transfer fluid charge or 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 with three passes of heat transfer fluid, corresponds to a rare assembly typical of boilers requiring small heat differentials, around 10 or 15ºC. Under these conditions, the flow rate of 205 m3/h is very high. If this configuration is not chosen, the heat transfer fluid charge loss would be excessively high, even with the three-pass configuration in design outline B), at around 8.45 bar.

Therefore, it's clear that the required heat jump greatly influences the boiler design and it must be considered a key factor in the installation project of a heat transfer fluid system.

Hot Oil Boiler

How do the hot oil boiler designs from Sigma Thermal work? Typically, water acts as a heat carrier for industrial heating systems. At higher temperatures, water requires high operating pressures, which may not always be desirable from an installation or safety standpoint. When considering industrial heating systems, high-temperature levels are often a great advantage when high-temperature process output is the required result.

In hot oil boiler designs, synthetic and oil-based thermal fluids are used as the heat carrier instead. This oil operates at nearly atmospheric pressures that reach up to 300C. Yet it will work just as any other heating fluid as it is cooled and heated while passing through the system.

Sigma Thermal hot oil boiler designs (with superior technology) are proven with years of use in the field. We offer 2 hot oil boiler models (the HC-1 and HC-2) that you can learn more about here, or call our home office at 770-427-5770 and speak with a representative that serves your surrounding area.

What are some of the applications best suited to Sigma Hot Oil Boiler Systems?

• Natural gas heating
• Jacketed vessel heating
• Indirect steam generators
• In-line liquid heating
• In-line gas heating
• Heated molds or dies
• Crude oil heating
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• Autoclaves … and much more!

Sigma Thermal are leaders in industrial heating

People recognize us as leaders in the heating industry because we are able to troubleshoot all types of industrial hot oil boiler systems. We didn’t get this way by treating every job just like the last. We give every project our best by treating it as an important step-by-step process. Our consulting agents listen and understand how to blend your demands with the right equipment into efficient application designs. Contact us now with any specific questions.

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