Transfer and Charge Temperatures

Transfer Temperature. Transfer temperature is the process fluid temperature at the heater outlet. It is usually connected to a Temperature Controller (TRC) that adjusts fuel into the heater to maintain the desired outlet temperature. Better temperature control results if the TRC is connected as the master in a cascade to a fuel flow controller (FRC). The FRC compensates for changes in fuel header pressure resulting in more stable operation than would occur with TRC control alone. Flow controllers also give quicker, more positive control valve action. (Figure 600-1 shows such a cascade arrangement.)

Charge Temperature. Charge temperature is the inlet feed temperature. It is used in a particular advanced control strategy that maintains constant heat input regardless of fuel heating value changes. It is also used in feed-forward control to minimize process temperature upsets.

Charge and transfer thermocouples should be installed at the following locations:
• the inlet line
• the combined transfer line from the fired heater, with enough mixing distance to get an average temperature
• the header pass points from the convection to the radiant sections
• the outlet from each pass (Pass outlet thermocouples are used to monitor the results of pass flow distribution. With even flow distribution among the passes, the pass outlet temperatures will all be approximately the same. With uneven distribution, the process fluid in the pass with the least flow will be hotter than the other passes, encouraging faster coking.)

Typical flanged thermowells are shown in Drawing GB-J1197; comparable wells from commercial manufacturers are acceptable. Typical piping installations are shown in Drawing GB-J1198. (See the Instrumentation and Control Manual for instrumentation drawings.)

Pass-balancing Flow/temperature Controller Arrangement

01. May 2018 by sam
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