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Introduction to Heaters
with Circulating Heat Transfer Fluid


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Thermal Oil Boiler

Thermal Fluid Boiler
Hot Oil Unit

Diathermic Oil Heater


Boilermaker AB&CO (Copenhagen, Denmark)
Models Oil/Gas-fired  & Electrical heated
Net Heating Capacities 23 - 10,000 kW (20 - 8,600 Mcal/h)
Operating Temperatures Standard applications up to 310°C
Special applications up to 400°C
Operating Pressures Atmospheric (up to 310°C)
Pressurised (up to 400°C)
Energy Source
(Fuel / Electric)
Natural gas, LPG, bio-gas, light and heavy fuel oil, combinations hereof, - and electricity (electrical heating).

Utilisation of energy for burner versions - depend on actual working loads and operating temperatures
86 - 91% (based on LCV) without economiser i.e. basic efficiency in normal boiler design

92 - 98%
(based on LCV
) including economiser i.e. extra flue gas cooling for instance external water heating

99% on Electric heated models
The Heater Unit General Description of Heaters
The Heating System Introduction to Thermal Oil Heating
General Sketch / Design Horisontal & Vertical Options
Data & Dimensions Total Data Survey - Heaters
Heat Recovery Features Economisers for Flue Gas
Introduction to
Other Heaters & Boilers

Electric Steam Boilers  (Resistant)

Steam Generators (Water-Tubes)

Steam Boilers (Fire-Tubes)

Heating Methods Using Thermal Oil for Heating

Thermal Fluid VS Steam Heating

Steam Generators VS Boilers

Thermal Oil Introduction HTF (Heat Transfer Fluid)
Assistance & Service Design & Trouble Shooting

Service & Supervising
Circulation Pumps Data & Sizes - Pumps
Chimney / Flue Gas Outlet Design & Lay-out - Stack
Flue Gas Data How to Calculate - Flue Gas
Thermal Oil Properties
(Heat Transfer Fluid)

Table Values

 Total Data Survey - HTF



Heater Models
with Oil/Gas-burners

  Vertical Types  < 3,500 kW

  Horisontal Types < 4,700 kW


Data Sheet


Assistance & Tools
of References

                                         Oil- or Gas Burner

                          Electrical Heated                            

Water and steam are typically used as heat carriers in heating systems. But at high temperatures, water and steam requires a corresponding high operating pressure. In thermal oil heaters, a special oil (HTF) is used instead and it means that you can achieve operating temperatures up to 300°C at atmospheric pressure.

The thermal oil heaters are widely customised and special designed thermal oil units, - respectively with fuel oil or gas-burner, or electrical heated (see above sketches).

AB&CO is a well-known Danish manufacturer of industrial boilers and heaters established in 1988, who have specialised in small and medium-sized units in individual designs - including complete skid-mounted solutions and container solutions.




The key argument for buying and investing in an AB&CO solution. Low total life-cycle cost (LCC) is guaranteed, since LCC include not only purchase, but also operation and maintenance cost, - and a total operation time of many years. On top of this comes the environmental features that meet the future demands - Sustainable Process Heating.


PI-diagrams for your choice of scope of supply :
Illustrated Options (PID)
for the Scope of Supply
according DIN4754/Fig.A1
 Electric Units Units with Burners
Illustrated Options (PID)
for the Scope of Supply
according DIN4754/Fig.A2
 Electric Units Units with Burners
 Lay-Out Examples
400 kW 600 kW 1000 kW




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Thermal Oil Heaters (also called Thermal Oil Boilers, Thermal Fluid Boilers, Hot Oil Units or Diathermic Heaters) are developed and designed especially for demanding process heating  operations where no compromise on quality are accepted and where reliability is the key word.

The heater solutions from AB&CO are not characterised by low purchase price - but instead low over-all costs including low maintenance and operational costs
, and long life-times.

Oil & Gas-fired Unit

Consequently the AB&CO heaters are made of first class material and components, without any compromise on quality. The pressure part are designed as standard between 10 bar and up to 40 bar pressure (although operation often are atmospheric and unpressurised).

The heaters for thermal oil or heat transfer fluid (HTF) are delivered as complete and fully equipped units with all necessary armatures, instrumentations and safety features.

The thermal oil heaters are delivered as oil-fired, gas-fired, for dual fuel or electrical heated. Other customised options are numerous and include free choice of horizontal alternatively vertical types. The models are designed for minimum 10 bar pressure with capacities in normal range up to 3000 kW, - and up to 10.000 kW on request.



Water and steam are typically used as heat carriers in heating systems. But at high temperatures which is often required in process heating systems, water and steam requires a corresponding high operating pressure. Consequently high temperature heating systems with water and steam can be very controversial and expensive - both in relations to personal safety issues, economy, design and

In Thermal Oil Heating Systems, this special oil-based Heat Transfer Fluid (HTF) is used instead of steam or water as the heat carrier. With this fluid it is quite simple to achieve operating temperatures up to 300°C even at atmospheric pressure. Comparing this to water and steam, it would require a pressure above 85 bar to obtain 300°C operating temperature.


There are several advantages by using the oil-based heat transfer fluid - often you compare Thermal Fluid versus Saturated Steam. Here the most obvious advantages are:


   High operating temperatures up to 300°C at atmospheric pressure
Optional temperature level set-points
No pre-treatment of boiler feed water
No heat loss due to hot condensate and flash steam
No risk of corrosion and no risk of freezing damages.
   Low maintenance costs
   Quiet in operation (no steam stroke and flash steam noise)
   Easy to operate (does not require
steam boiler certified staff)



Why  thermal fluid
instead of steam ?

 Read more




The thermal
oil heaters can be delivered in horizontal design (with low height), or in vertical design (occupying limited floor space).

See the sketches below.

The heater units are delivered thermal insulated and with heat shields. The enclosure is stainless steel cover sheets. The unit design includes burner (or electrical heating elements), armatures, instrumentation, safeties and control panel - and with full documentation including the necessary certificates.

Accessories can be chosen in order to complete the thermal oil heating system. Many of these accessories can be build on a complete skid-mounted unit  - or the can be build into a container-solution.

AB&CO also deliver complete thermal fluid heating systems (read more about the system solutions in column on the right column).


Beside the standard execution, the heaters can be delivered in e.g. following variations:

   Electrical heated
   Flue gas heated
   Steam heated
   Combination of heat sources
   Skid-mounted units
   Complete heating systems
   Parts made in stainless steel
   Customised design - special requests



The oil/gas-fired heaters from AB&CO are made with an internal arrangement of winded-up tube coils made of 3.1/PED-certified steel pipes.
The oil-based Heat Transfer Fluid (HTF) is heated gradually to the operational temperature during the circulation inside the coiled up tubes - while the heat is transferred to the HTF. Firstly it is done as radiant heat in the combustion chamber, where the inner cylindrical tube coil and a flat tube coil forms the chamber wall and the bottom respectively (consequently refractory concrete is avoided). Secondly the heat is transferred as convection heat from the combustion gasses (consequently being cooled) through the tube wall to the HTF. This takes place in the outer surrounding convection part i.e. the outer tube coil, as the gasses pass the space gab between the two tube coils, and thereafter the outer tube coil and shell. The thermal design ensures both a defined and constant velocity of the thermal oil, but also a modest volume of the thermal oil relative to the size of the heater. The coil design allows unlimited thermal expansion caused by the high fluid temperature.

The heater and the complete thermal oil heating system in general are constructed and equipped according to the recognised and comprehensive German standard DIN 4754, while atmospheric vessels and pressure vessels are made according the European regulations including the pressure vessel code PED 2014/68 EU, and according to all relevant EN-norms.

Container Solutions - closed type container


Container Solutions - open type container

Safe High Temperature Application
Skid-mounted Unit

Vertical Skid-mounted Unit


The electrical heated thermal oil heaters are build-up as a battery of one or several pressure vessel modules equipped with with conventional tube type or pull-out ceramic heating elements.



Electrical Heated Unit


Henning_G_VTOsyst2.jpg (40529 bytes)

Electrical Heated Skid-Mounted Solution

These electric thermal oil heaters are not only designed as heaters, they are also calculated and designed as heat exchangers involving - for each project/application/task - individual heat transfer and flow calculations on the heater elements, and their surfaces and geometry.

This is done in order to control the surface temperature of the electrical heating elements and thereby preventing critical temperature in both heating elements and the oil film of the HTF (preventing element blow-off and oil cracking respectively).

All thermal oil heaters are carefully checked, controlled and functional tested prior to dispatch from the workshop.



The oil/ and gas-fired thermal
oil heaters are equipped  only with international recognised  high-quality burner brands - adapted to operation on gas, light fuel oil, heavy fuel oil or combinations (dual fuel) and even fish oil and solid fuel. Individual demands for special burner designs can be meet and adapted in the heater design - for instance burners for combustion of bio-gas and other low caloric or even corrosive fuel gasses.

The electrical heated units are delivered in two options. The conventional electrical tube heating elements are used typically for low temperature applications and the costs for these are correspondingly low. The elements are especially designed for demanding tasks which means a.o. using the special and extreme high temperature resistant Incoloy 825 alloy.

For other applications, low and easy maintenance might be an important demand. In that case it can be recommended to use full welded steel tube, where the inner heating wires place in ceramic elements can be pulled out, without emptying the heater for oil.

The engineers at AB&CO calculate the optimum heat load in the heater individually i.e. for each task, - ensuring that the film temperature of the thermal oil is never exceeded  even during stop and start.



Whenever it might not be possible to use electrical heating using electricity from wind power, nuclear power or water power, - the environmental challenge is to use oil and gas-fired solutions, but at the same time to reduce the fuel consumption and thus the emission.

In this context the low flue gas temperature  is the key word. The heat recovery is done by utilisation of  the quite large amount flue gas heat recovered in flue gas coolers also known as economisers. AB&CO offer complete systems that involves high efficiencies and at the same time very low pay-back times.


Economisers can be delivered as build-on to the oil or gas-fired heater, for heating external colder fluid e.g. water. Alternatively the thermal oil heater can be made as a flue gas heat exchanger - a recuperator where the external heat source is hot flue gas.



Heat recovery - for instance as described above - and other environmental considerations have always been a standard topic at AB&CO, long before these became a official worldwide protocol. A lot of money is available by thinking "green". For that reason AB&CO offers consultant services. This offered arrangement has become very popular worldwide. See more about this at AB&CO · CONSULT



The circulating heat transfer fluid, also known as the thermal thermal oil needs to be correct chosen. Compromise on this issue can easily result in huge problems - but economical and with respect to ensure personal safety. It cannot be emphasized strong enough how fatal bad, old and and inappropriate thermal oil can be for the thermal fluid heating system.

Use a quality brand of thermal oil and make sure that this fluid correspond how the heating system is operated.

An important maintenance detail is that every 6 - 12 months, a sample should be taken from the system and send to a qualified laboratory (make sure that they know about thermal fluid systems and what traces they should look for in the sample) or directly to AB&CO.

Due to this important issue, AB&CO has established a department dealing with thermal fluid supply and sample analysis:

It is recommended to use a non fouling, high temperature resistant thermal oil for the applications as decribed here as the  AB&CO Heat Transfer Fluid.



A correct and safe system design is very important for high temperature systems. This can be ensured by using
AB&CO as system supplier of the whole system.

The engineers of AB&CO will design the heating system in a close dialog with the customer and/or the contractors. Visits on site can be provided in connection with kick-off meeting, commissioning and start-up.

AB&CO have 20 years experience with thermal oil heating (hot oil) solutions world-wide - both new installations or modified systems.

Thermal Fluid Heating Systems with heaters, heat exchangers, tanks, pumps, armatures, instrumentations and piping can be delivered as package solutions and as skid-mounted units - individually designed according DIN4754.

Consulting Assistance on design of systems and parts hereof, modification and trouble-shooting within thermal oil heating systems, can provided on request by AB&CO · CONSULT, who have more than 25 year experience working with thermal oil heating.

More info about
  the 'R&D Workshop'

Dimension Sketches

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Type with Burner
Max. Capacity[kW]

Max. Heat















70 60 1800 1400 1300 1100 1700 ø150 400 450
140 120 2100 1700 1400 1200 2100 ø150 500 600
235 200 2500 2000 1600 1300 2100 ø210 650 800
350 300 2800 2300 1700 1400 2200 ø210 1000 1200
600 520 3100 2500 1900 1600 2400 ø355 1700 2200
1,000 860 3900 3300 2200 1800 2600 ø400 2400 3200
1,500 1,300 4300 3700 2500 2000 2800 ø500 3600 4800
2,000 1,720 4800 4100 2600 2100 2900 ø560 4000 5500
2,350 2,000 5300 4600 2800 2200 3100 ø560 4300 5800
2,750 2,365 5400 4700 2900 2300 3300 ø630 4500 6600
3,000 2,580 5500 4800 3000 2400 3400 ø630 4800 6900
Up to 10,000 Up to 8,600

click to get all sizes on PDF

All below figures and data are estimations for guidance only. The exact capacity appears from the quotation. Exact sizes to be determined as part of order effectuation and adjustment of the customised design.

Since most AB&CO models are customised units, the above figures only represent some of the available sizes. Above models are all oil or gas-fired, while dimensions on electrical heated models can be provided on request.

For using above table you should choose the size above your required heat capacity, to get the dimensions and weight for the unit.