Bosch TKA 865. Operator's Manual download pdf (Page 44)

Exhaust system

Properties of exhaust gas water preheaters

Exhaust gas water preheaters are frequently subject to considerable wear resulting from corrosion on the

exhaust gas side.

The cause of this is that they are charged with aggressive, acidic exhaust condensation at low wall temper-

atures in the partial load range.

This wear can be countered by the use of suitable materials, but the use of special steel means high costs.

Consideration is therefore to be given to whether it is worth making such investments to extend the service

life of the respective unit or whether it is more economical to exchange the pipe cluster at certain intervals.

To clean and replace the pipe cluster, the heat exchanger must be fitted with detachable flange caps.

The smoke tube internal diameter must not be smaller than 22 mm to prevent the tube from prematurely

clogging up and, consequently, to prevent considerable increases in the exhaust backpressure.

Wear resulting from corrosion can be limited if precipitation of condensation from the exhaust gases is pre-

vented. This can be achieved if the exhaust gas temperature at the outlet from the exhaust gas system is

not less than 180_C.

Exhaust gas temperature at the exhaust gas outlet w 180_C

The best results are achieved if the systems always run at high load, i.e. at high temperatures, and if the

preheaters are fully charged.

It must be possible to switch off the exhaust gas water preheater via hot gas slides or butterfly valves. A

bypass is then necessary for the exhaust gas ducting. This prevents both the heat exchanger from being

charged with exhaust gases at low exhaust gas temperatures (e.g. when the system is being started) and

the formation of condensation.

As contamination increases, so does the exhaust backpressure of exhaust gas heat exchangers. For this

reason, continuous monitoring of the exhaust backpressure is compulsory.

Ascertaining the usable exhaust gas heat

The amount of usable heat contained in exhaust gases is calculated with the following formula:

Q + c

m Dt

Key:

Q = Amount of usable heat in the exhaust gases in kJ / h

m = Exhaust−gas mass flow in kg / h

= Specific heat capacity of the exhaust gases in kJ / kg  degrees

Dt = Usable temperature drop in the exhaust gases in Kelvin

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Bosch TKA 865. Operator's Manual Page 44