Measuring transient temperature of the medium in power engineering machines and installations

Abstract

Under steady-state conditions when fluid temperature is constant, there is no damping and time lag and temperature measurement can be made with high accuracy. But when fluid temperature is varying rapidly as during start-up, quite appreciable differences occur between the true temperature and the measured temperature because of the time required for the transfer of heat to the thermocouple placed inside a heavy thermometer pocket. In this paper, two different techniques for determining transient fluid temperature based on the first and second order thermometer model are presented. The fluid temperature was determined using measured thermometer temperature, which is suddenly immersed into boiling water. To demonstrate the applicability of the presented method to actual data, the air temperature which changes in time, was calculated based on the temperature readings from the sheathed thermocouple.

Measurement of Transient Fluid Temperature in the Heat Exchangers

“…In turn, the industrial thermometers, which are designed to measure the temperature of the fluid of high pressure, are characterised by a considerable delay of indications in reference to the actual changes of fluid temperature. For such thermometers, the second-order thermometer model, allowing for modelling the signal delay, is more adequate [14].…”

“…Further analysis of the heat exchange between the housing of the thermometer and the temperature sensor omits the heat transfer by radiation [14]. This is possible when the gap between the thermowell and the temperature sensor is filled with a non-transparent substance or if one of the two emissivities ε w and ε T is near to zero.…”

“…The cross section of the temperature sensor placed in the massive housing is shown in Figure 1. This example will be analysed to show that the thermometers can be modelled as second-order inertial systems [14]. Air gap can occur between the outer thermowell and the temperature sensor (Figure 1).…”

“…Based on these assumptions, the mathematical model of the thermometer is the differential equation describing the inertial system of the first order [14]:…”

“…Some thermometers may have a time constant of 3 min or more, which makes the implementation of a single temperature measurement requiring about 15 min [13].On the other side, some designs of thermometers need more than one time constant to describe the unsteady response of temperature sensor inserted into the thermowell. Measuring the fluid temperature in a controlled process may require the knowledge of two or three time constants, which describe the transient response of the thermometer [14].…”