Heaters phase changes

The time variation of the mean and maximum heater temperature is presented in Fig. 6.39. The mean heater temperature (i.e., the average temperature of the whole heater) changed in the range of ATav = lOK. The maximum heater temperature changed in the range of ATmax = 6 K. Comparison between Figs. 6.37,6.38 and 6.39, shows that the time period (frequency) is the same for the pressure drop, the fluid temperature at the outlet manifold, and the mean and maximum heater temperature fluctuations. It also allows one to conclude that these fluctuations are in phase. 

Four classes of this kind of equipment are considered heat exchangers without phase change, steam heaters, condensers, and vaporizers or reboilers. These arc grouped together with descriptions in Figures 3.8-3.11. Where applicable, comments are made about the utility of the particular method. In these heat... 

There are many different types of home heaters that work in many different ways. Some use phase changes to produce heat. The sort of heat you use in your home depends a lot on where you live and how cold it gets in the winter. 

A heat pump is perhaps the simplest example of a heater that uses phase changes. Often seen in areas with mild winters, a heat pump is nothing more than an air conditioner in reverse. Instead of moving heat from the inside to outside, they move heat from outside to inside. 

I Condensers 1 1 1 Liquid-to-vapor Heaters Coolers phase-change exchangers t Chillers... 

The transfer of heat is one of the most basic and best-understood unit operations. Heat can be transferred between the same phases (liquid-liquid, gas-gas) or phase change can occur on either the process side (in the case of condensers, evaporators, and reboilers) or the utility side (in the case of steam heater) of the heat exchanger. 

Assuming that the temperature and composition dependence of the heat capacity and density of the process streams is not significant, that there is no phase change in the heat exchanger, and that the material holdup in both legs of the heat exchanger and in the heater is constant, the model of the process can be written as... 

Heat transfer to boiling liquids occurs in a number of operations, for example, distillation and evaporation. Heat is transferred by both conduction and convection in a process further complicated by the phase change that occurs at the heating boundary. When boiling is induced by a heater in contact with a pool of liquid, the process is known as pool boiling. Liquid movement... 

Condensers Liquid-to-vapor phase-change Heaters Coolers Chillers... 

The formation and subsequent growth of a vapor bubble on a heated wall covered with a liquid is controlled by the forces arising from the excess pressure inside the bubble, the surface tension forces at the liquid-vapor interface and at the contact line formed by the interface at the heater surface, and the inertia forces resulting from the motion of the flow as well as the interface. The resistance to the phase change process at the liquid-vapor interface is quite small in comparison to the... 

In a power compensation differential scanning calorimeter (DSC), the base of the sample holder unit is in direct contact with a reservoir of coolant (Figure 2.12). The sample and reference holders are indiv idually equipped with a resistance sensor, which measures the temperature of the base of the holder, and a resistance heater. Upon detection of a temperature difference between the sample and reference, due to a phase change in the sample, electrical power is supplied so that the temperature difference falls below a threshold value, typically <0.01 K. 

If you leave your car parked outdoors in the winter, you may find frost on the windows in the morning. If you then start the car and let the heater warm the windows, after some minutes the windows will be dry. Describe all of the phase changes that have occurred. 

Figure 21.6 Unregulated Heat Exchanger Using Utility Streams with a Phase Change (a) Process Heater and (b) Process Cooler...

The contrast ratio of the phase change dichroic display is found to decrease with an increase in temperature (see Fig. 22). This decrease is basically caused by the reduced absorption in the quiescent state that results from the decrease in the order parameter and pitch of the dichroic mixture at higher temperatures. The cells show faster switching speed and lower memory at higher temperature. The switching becomes slow at lower temperature due to an increase in the viscosity of the mixture, and a heater is usually required for low temperature operation. 

Based on service, heat exchangers can be classified as cooler, heater, condenser, and vaporizer such as evaporator, boiler, or reboiler. Cooler is used to cool a stream and heater is used to heat a stream. The stream can be vapor or liquid. Condenser is ased to condense a vapor stream partially or totally. Vaporizer is used to vaporize part of the liquid to vapor. There (s no phase change for cooler and heater, but there is phase change for condenser and vaporizer. 

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