Heat exchanger temperature responses

HEAT EXCHANGER TEMPERATURE RESPONSE FOR DUTY-CYCLE TRANSIENTS IN THE NGNP/HTE... 

Heat exchanger temperature response for duty-cycle transients in the NGNP/HTE... 

Vilim, R.B. (2008), Heat Exchanger Temperature Response for Duty-Cycle Transients in the NGNP/HTE, ANL-GenIV-114, September. 

Fluidized combustion of coal entails the burning of coal particles in a hot fluidized bed of noncombustible particles, usually a mixture of ash and limestone. Once the coal is fed into the bed it is rapidly dispersed throughout the bed as it bums. The bed temperature is controUed by means of heat exchanger tubes. Elutriation is responsible for the removal of the smallest soHd particles and the larger soHd particles are removed through bed drain pipes. To increase combustion efficiency the particles elutriated from the bed are coUected in a cyclone and are either re-injected into the main bed or burned in a separate bed operated at lower fluidizing velocity and higher temperature. 

Distance-Velocity Lag (Dead-Time Element) The dead-time element, commonly called a distance-velocity lag, is often encountered in process systems. For example, if a temperature-measuring element is located downstream from a heat exchanger, a time delay occurs before the heated fluid leaving the exchanger arrives at the temperature measurement point. If some element of a system produces a dead-time of 0 time units, then an input to that unit,/(t), will be reproduced at the output a.s f t — 0). The transfer function for a pure dead-time element is shown in Fig. 8-17, and the transient response of the element is shown in Fig. 8-18. 

Figure 55. Geothermal response test with combined heat extraction/heat injection and different energy levels, shown are the borehole heat exchanger flow and return temperatures and the calculated heat flux. Borehole flow rate is not shown in the graph...

The Geothermal Response Test as developed by us and others has proven important to obtain accurate information on ground thermal properties for Borehole Heat Exchanger design. In addition to the classical line source approach used for the analysis of the response data, parameter estimation techniques employing a numerical model to calculate the temperature response of the borehole have been developed. The main use of these models has been to obtain estimates in the case of non-constant heat flux. Also, the parameter estimation approach allows the inclusion of additional parameters such as heat capacity or shank spacing, to be estimated as well. 

As each reactor was insulated, the heat liberated during treatment was sufficient to raise the reactor temperature above 28°C. Surplus heat was removed in response to a controller by a heat exchanger, supplied with tap water, at a constant pressure. 

Consider again the temperature control system fitted to the heat exchanger in Fig. 7.1. Suppose that the temperature of the cold stream decreases. Then, clearly, the temperature at Y, i.e. 0, will also begin to fall. In response to this the controller will open the control valve further in proportion to the error—where the error is given by equation 7.1. In order to maintain this new steady state, i.e. with the increased rate of flow of the hot stream, a constant additional output must be applied to the control valve by the controller. This additional output can exist only if there is an... 

FIG. 8-50 The response of a heat exchanger varies with flow in both gain and dynamics here the PID temperature controller was tuned for optimum response at 50 percent flow. 

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