Primary loop

Example 10.1 Consider a simple cascade system as shown in Fig. 10.2a with a PI controller in the primary loop, and a proportional controller in the slave loop. For simplicity,... 

Now we solve for the closedloop transfer function for the primary loop with the secondary loop on automatic. Figure 11.3c shows the simplified block diagrana. By inspection we can see that the closedloop characteristic equation is... 

When requested by the user located at the point of use, the first exchanger on the secondary loop starts to cool the water. This one is automatically heated at minimum 85°C before the return in the primary loop. When the temperature asked for the cold point of use is reached, the sampling value is automatically opened. The users ask to stop the sampling. A safety timer must be installed to prevent the draining of the installation. Safety timing is adjustable (maximum 1 h) temperature of cold point 30°C 5°C. 

In the other design, PWRs have two closed loops of water circulating in the plant plus a third, external loop to remove the waste heat. Water is pumped through the reactor core in the primary coolant loop to moderate the neutrons and to remove the heat from the core as in the BWR. However, the reactor vessel is pressurized so that the water does not boil. Steam is necessary to run the turbines, so the primary loop transfers the heat to a secondary loop. The water in the secondary loop is allowed to boil, producing steam that is isolated from both the core and the outside. The water in the primary loop usually contains boron (as boric acid H3BO3 0.025 M) to control the reactivity of the reactor. The steam in the secondary loop is allowed to expand and cool through a set of turbines as in the BWR the cold steam condenses and is returned to the primary heat exchanger. A third loop of water is used to maintain the low-temperature end of the expansion near room temperature and remove the waste heat. 

Usually the dynamics of the secondary loop are sufficiently faster than those of the primary loop for G / (s) to be approximated by its steady-state gain. For the same reason it is possible to tune the cascade system by tuning first the inner loop and then the outer loop. 

In order for the cascade loop to be effective, the slave should be more responsive (faster) than the master. The slave s time constant should be one quarter to one tenth that of the master loop and the slave s period of oscillation should be one half to one third that of the master loop. The goal is to distribute the time constants between the inner (slave or secondary) and outer (master or primary) loops, while making sure that the largest time constant is not placed within the inner loop. When that occurs, such as in the case when the valve has a positioner (the slave) on a fast flow or liquid pressure controller (the master), stability will be sacrificed because the slave (valve)... 

Flow as a secondary cannot only overcome the effects of valve hysteresis, but also insures that line pressure variations or badly selected valve characteristics will not affect the primary loop. For these reasons, in composition control systems, flow is usually set in cascade. Cascade flow loops are also useful in feedforward systems. Flow controllers invariably have both proportional and integral modes. If their proportional band exceeds 100%, they must have an integral mode. 

The flow of heat across the heat-transfer surface is linear with both temperatures, leaving the primary loop with a constant gain. Using the coolant exit temperature as the secondary controlled variable as shown in Fig. 8-55 places the jacket ( mamics in the secondary loop, thereby reducing the period of the primary loop. This is dynamically advanti reous for a stirred-tank reactor because of the slow response of its large heat capacity. However, a plug flow reactor cooled by an external heat exchanger lacks this heat capacity and requires the faster response of the coolant inlet temperature loop. 

The flow of heat across the heat-transfer surface is linear with both temperatures, leaving the primary loop with a constant gain. Using the coolant exit rather than inlet temperature as the secondary controlled variable moves the jacket dynamics from the primary to the secondary... 

Multiple layers of low conductivity phenolic foam insulation and small temperature differences between the primary coolant and the ambient minimized the heat loss from the primary loop to the ambient. Also, the heat addition to this loop was minimized by using a recirculation pump with an extremely low heat dissipation rate, which was calculated from the pump curves supplied by the manufacturer. With the pump heat dissipation and the ambient heat loss being small fractions of the secondary loop duty, the test section heat load was relatively insensitive to these losses and gains. Local heat transfer coefficients were therefore measured accurately in small increments for the entire saturated vapor-liquid region. Additional details of this thermal amplification technique are provided in the paper by Garimella and Bandhauer [32]. 

The primary coolant circuit of a PWR is shown in schematic form in Fig. 36. In this particular circuit, there are four loops between the reactor and the steam generators. The pressurizer is also shown, which maintains the pressure in the primary loop at a sufficiently high value (typically 150 bar) such that sustained boiling does not occur and maintains the desired concentration of hydrogen in the coolant. The reactor heat removal system (RHRS) and the reactor water cleanup system are not shown. The general operating conditions in a PWR primary loop are summarized in Table 2. 

The carrier system has good compatibility with a high temperature gas reactor, because the regeneration process, which is relatively safe, can be applicable at tlie primary loop of the reactor. Off-peak power of 600 MWt for 8 h from a reactor can supply H2 for FC vehicles of 45 thousands, which is die similar number for conventional electrolysis system under the same thermal input from the reactor. The carrier system shows a new possibility of chemical energy carrier system. 

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