Thermal Fluid  VERSUS  Saturated Steam


What is the must efficient and economical way to establish an appropriate central system to distribute process heating? Steam is a well-known heating medium, but is this technology becoming a kind of old fashioned. Thermal fluid (thermal oil) is known for offering a lot of advantages, but is it always the best solution - why are steam still chosen in many set-ups?

This is an introduction to the these two main alternative heating mediums, that is normally considered in process heating systems today. It is also a guide for you that might make your considerations more easy. Remember process heating involves a lot of energy and money.

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Steam Heating and High Pressure

Water and steam are typically used as heat carriers in industrial heating systems.

In industrial heating systems a high temperature level is often a necessity for achieving the required high efficiency.

When using steam (or water) these high process temperatures are achieved by pressurising the steam (or water ).

Dry saturated steam is both very efficient to carry heat and to deliver and transfer the heat. Saturated steam offers a very high specific heat content (kJ per kg steam) and thereby deliver a large amount of heat by a modest steam flow. It also very efficiently find its way to the heat surfaces, and even more efficiently transfer the heat to the heat surfaces (large heat transfer coefficient also called the k-value). This makes indeed steam very attractive to use for process heating, and on top of this, it is also a well defined medium from which engineers worldwide have a lot of experience working with.



Steam Boiler (oil/gas-fired

Steam Boiler (electrical)
 

These advantages must be considered up against some significant challenges when dealing with this high temperature steam.
 

First of all, dry saturated condition steam temperature correspond and steam pressure as it appear from the steam tables :

AB&CO Steam Table


High temperatures correspond high pressure, and sometime this pressure needs to be extremely high.

A solution for this is the so-called steam generators, where the feed water evaporates while passing through the piping of a multiple coils. With this design it is technical possible to get up to 300°C which require a steam pressure of 85 barg and a design pressure of nearly 100 barg. The challenge here is not the steam generator, but the steam system after. Steam generators in this high pressure design are not available as electrical heated model, although they are often called "electric steam generator" :
 


  

     

   Steam Generator Boiler
Horisontal & Vertical models
(Oil & Gas-fired only)

 

 


 

Oil Replaces Water and Steam

So the pressurised steam or water systems easily becomes very controversial and expensive. However is that has been the situation for centuries - and has thus been general accepted to deal with - technically and economically.

There are actually alternatives - high  temperatures does not necessary need high operating pressures!

In thermal oil heaters a special oil - a so-called heat transfer fluid (HTF) are used instead of steam as heat carrier, and is operating at atmospheric pressure up to abt. 300°C. For comparing this to water and steam, it would require a steam pressure of 85 bar to obtain this temperature.

There are several advantages by using a thermal fluid (HTF) compare to saturated steam.
The most obvious advantages are:

  High operating temperatures - up to 300°C at atmospheric pressure.
  Adjustable set-points of operating temperatures.
  No equipment for pre-treatment of boiler feed water to maintain.
  No heat loss due to hot condensate and flash steam - high efficiency and better operation economy.
  No risk of corrosion and no risk of freezing damages.
  Low maintenance costs
  Quiet in operation with no steam stroke or flash steam / condensate noise
    Easy to operate (does not require
steam boiler certified staff)

 

An important detail in the thermal oil heating systems is the heat transfer fluid (HTF) which is often an oil-based fluid (based on natural ingredients or synthetic created). It is like the blood in your your venes - and thus it plays a major role in the total shape of the system.

Like any products, this fluid are available on the market in many qualities. The fluid must be safe and reliable, not only as a brand new fluid, but also after years of service. It takes an expert to evaluate the options in choice of fluid since potential side-effects are numerous especially with cheap HTF. A recommended fluid is this low-fouling and non-toxic ABCO NF fluid. Opposite fluid distributed by most oil companies, which are mostly paraffin based fluid, - this fluid is partly naphthenic based. This offers two very important features for a HTF :  First it is more heat resistant (stabile) to high film temperature (contact to hot heat surfaces) than pure paraffin based fluid. Secondly the fouling when it eventually arise, does not lead to deposits on the inside of the tube. Instead the cracked molecules stays in the fluid and can be filtered out in strainer or similar on-line cleaning arrangement. Synthetic HTF are available on the market also, although they are very stabile at temperatures up to 400°C - they are normally quite toxic, and must be handled accordingly.

 

Design of Thermal Oil Heaters

Thermal oil heaters can be delivered in horizontal execution (with low height), or in vertical execution (occupying limited floor space). They are  delivered complete, insulated and equipped with burner, armatures, instrumentation, safeties and control panel - and with full documentation including necessary certificates.


Thermal Oil Heater (oil/gas-fired)

 

The oil/gas-fired models are made with winded-up coils made of boiler-certified tubes. The thermal fluid is heated during the flow through the tube coil. The heat is transferred to the HTF as radiant heat in the combustion chamber, where the inner cylindrical tube coil and a flat tube coil forms the chamber walls and the bottom respectively. Consequently refractory concrete can be avoided. The combustion gasses are hereafter cooled in the outer convection part, as the gasses are led between the space of the two tube coils and the outer boiler shell - in another two pass.

The thermal design with tube coils ensures a modest volume of the thermal fluid relative to the size of the heater, and it also allows unlimited thermal expansion due to the high fluid temperature.

 





 

The alternative electrical heated models are also designed allowing a controlled and precise flow rate of thermal fluid. This is done be multiple long vessels with electrical elements welded in. The compartment with the elements are furnished with baffle plates to ensure high velocity and turbulent flow.


Thermal Oil Heater (electrical)


As with any other industrial heaters and boilers today, the electrical alternative is much more attractive today than previously. Electricity is clean and cheap in almost any way. Taxes and duties is another story - a political thing only. In practical the capacities are typically below 1,5 MW since the electrical installation rapidly becomes huge - for instance 1 MW requires a 1,600 A electrical installation with voltage 3 x 400 V.

 

Ensuring a Safe System

Thermal oil heaters and the complete thermal oil heating systems should be designed and equipped according DIN 4754.

Both DIN 4754 and different EU codes (incl. PED 97/23/CE and EN-standards) stipulate that the heaters must follow different regulation including being designed as pressure vessels - and consequently manufactured accordingly. This is required despite the installation and operation are actually considered pressureless (atmospheric) and therefore not subject to all the special pressure vessel operation requirements and maintenance practices & procedures.

Still the heater units must be provided with international recognised and high quality main components, such as the electric heating elements or the burner for natural gas, light fuel oil, heavy fuel oil or combinations (dual fuel).

All thermal oil heaters will be carefully checked, controlled and function tested prior to dispatch from the workshop and are delivered with a full set of documentation comprising drawings, diagrams, data sheets, specifications, part list and instruction manuals. In EU the units will be CE-marked accordingly.

A correct and safe system design is very important for any high temperature system. Therefore the requirements for the above codes and standards. But safety must also be ensured by using only experienced suppliers for other material, parts and components - and of course also the contractors for building-up the whole system on site. The engineers specialising in thermal oil systems, knows about all those details - and they will know about the features, updated technical knowledge of possibilities, options, but also the threats particular when dealing with these high temperature process heating systems.

Even when the heaters are of supreme quality in any details, the HTF inside is still an oil. At high operation temperatures and insufficient design, the HTF can be fatal if not treated correctly. It must be anticipated that the local fire regulation codes must be strictly followed. It is highly recommended that the local chief of the fire department is consulted prior to projecting the thermal oil heating system - or changes in this. Generally fire-preventing authorities can always shut-down any projects, even though all regulations are followed !

Considering all above, it is therefore strongly recommended that the chosen brand is from a experienced and well reputable manufacturer, and that the system is made together with a professional and experience supplier of these systems.

Do never compromise in quality, it can be both fatal and expensive !

 

Optional Designs & Features

Beside the standard design, thermal oil heaters can also be delivered in different special executions - for instance following variations:

  Electrical heated and EX-design (ATEX)
  Flue gas (exhaust gas) heated
  Steam heated thermal oil
  Combination of heat sources
  Container solutions
  Open trailer (mobile) solutions
  Complete skid-mounted units
  Complete unassembled "Lego" system
  Material in stainless steel
  Any customised design


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Important Legal Announcement !

The above discussion and guidelines, including all illustrations, are made by AB&CO and must be considered legally as property of AB&CO. It must not be copied in part or in whole without written permission by AB&CO.

All information given must be considered as general only, and they are given without any responsibilities to AB&CO or employees hereof. Using the recommendation is consequently completely on your own risk. For assistance on considerations relevant to the above, it is strongly recommend that AB&CO is fired as a consultant, enable you to make an optimum solution based on the actual circumstances.

Latest revision :
Copenhagen, 30th November 2016
by Arvid Blom, Senior Engineer


 

'Thermal oil versus Steam'


 
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