Without interruption for prolonged periods

Dr. Dietmar Hunold, Jürgen Kröger

The old thermal oil plant consisted of two bottom-fired heaters, built in the 1960s, for heating different consumers (reactors, agitator tanks, etc.) via mixer distributors and secondary circuits. The old air preheaters were no longer in service, so that the energy efficiency of the thermal oil boilers was correspondingly low. The plant was used to heat twelve secondary circuits for regulating the temperature of various consumers individually. There was a poor control characteristic at the secondary circuits, owing to a very small pressure difference in the primary circuit. A mixture of 40% Santotherm 66 and 60% diphenyl-diphenyloxide (VP-1) was used as the heat-transfer medium and also had to be envisaged for the new boiler plant.

A so-called „chemical-type design“ was chosen for the thermal oil heater. Its main characteristics are as follows:

The flue gas side takes the form of a single-pass system with a cylindrical, upright com-bustion chamber (1st pass), whereby it was decided to employ a bottom-firing principle with a duplex-wound radiant burner. The convective heating surfaces adjoining the radiant burner are constructed as cross-flow, pull-out tube banks.

The first tubes at the inlet of the convective section (also known as shield tubes) are unfinned on account of the relatively high radiation component at the heat-transfer surface, while those farther downstream have fins. These are followed by another bank – the air preheater – which is a plain-tube bank with condensate draining and an ash pan.

The recirculating pumps for the thermal oil are designed with a magnetic clutch, which is required to be extremely robust in view of the stipulation regarding starting at temperatures as low as -15 °C. The design and operating data is shown in Table 1. This data was the basis for the choice of the PN 40 valves, GSC-25 material, flanges with expansion bolts and Spiraltherm gaskets. A Weishaupt combined burner (natural gas/distillate heating oil) was chosen for the plant. This burner preheats the air to a maximum of 250 °C and has an adjustable capacity in the range 1:3. A forced-draught admixture facility was also installed for regulating the air preheaters. Compliance with the values specified by the German Clean Air Code was achieved by means of an exhaust-gas return system. The burner at the bottom of the boiler is protected against the effects of the weather by a shell skirt. This area also accommodates the gas float and the control valves for the heating oil supply. An additional fan was installed for cooling the burner – essential owing to the preheated combustion air. The redundant design of the primary-circuit pump – a twin pump assembly with magnetic clutches – is also worthy of mention. The driving power is 75 kW per pump. All the components are easily accessible thanks to a free-standing stair tower with three platforms erected next to the heater. As a result, it is possible to walk on the heater up to a height of around 17.5 m, so that both the waste heat banks and the preheater bank can be pulled out for cleaning without difficulty. The heater was assembled on the site from the prefabricated components within a period of four weeks. An initial trial run was then carried out by short-circuiting the plant. The latter was subsequently disconnected and reconnected (to the twelve existing secondary circuits), so that the complete plant could be returned to service after an interruption of just two weeks.

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Further information cpp-204