Feed water flow

The feed water flow through an RO system should be dictated by the water source, as described in Chapter 9.1. The cleaner the water source, the higher the feed water flow may be, resulting in smaller systems and lower overall cost of operation. 

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Kehlhofer explains that the pre-heating loop must be designed so that the heat extracted is. sufficient to raise the temperature of the feed water flow from condenser temperature T to Ta (see Fig. 7.6). The available heat increases with live steam pressure Ipf), for selected 7 b(= Ta) and given gas turbine conditions, but the heat required to preheat the feed water is set by (Ta — T. ). The live steam pressure is thus determined from the heat balance in the pre-heater if the heating of the feed water by bled steam is to be avoided but the optimum (low) live steam pressure may not be achievable because of the requirement. set by this heat balance. 

Feed water flow stops Same as above... 

For technical and safety reasons a number of other parameters must be controlled pressure of circulating gas, reactor pressure, nitrogen flow, temperature of the cooling water for the UV lamps, quantity of circulating gas and of circulating paraffin, extract level, S02 pressure, 02 pressure, S02 flow, 02 flow, off-gas flow, feed paraffin flow, feed water flow, and backwater flow. 

In the case of Hollosep, it may be possible to employ a simplified module model because of its low concentration polarization, uniform feed water flow and low pressure drop. 

Some designers push the RO feed-water flow rate through MM filters to as high as 15 gal/sq ft/min. This is generally too high and can lead to poor filtration. Also, the resultant cost savings due to a reduced filter size is seldom warranted because the filter is a relatively minor cost as a proportion of the entire RO scheme. A better maximum is 9 to 10 gal/sq ft/min, with 5 to 6 gal/sq ft/min being preferred. 

Typical design considerations include feed water flow rates, water and air temperatures, tower feed and discharge systems (gravity feed or type and location of pumps), and influent contaminant concentrations. In addition, requirements for effluent water contaminant concentrations and restrictions on air emission should be considered according to the various federal and state regulations. 

Scale is caused by precipitation of dissolved metal salts in the feed water on the membrane surface. As salt-free water is removed in the permeate, the concentration of ions in the feed increases until at some point the solubility limit is exceeded. The salt then precipitates on the membrane surface as scale. The proclivity of a particular feed water to produce scale can be determined by performing an analysis of the feed water and calculating the expected concentration factor in the brine. The ratio of the product water flow rate to feed water flow rate is called the recovery rate, which is equivalent to the term stage-cut used in gas separation. 

Feed water source also influences the design array of the RO unit. This is because the feed water flow and concentrate flow rates are also determined based on feed water quality. Higher feed water quality allows for higher feed flows and lower concentrate flows to be employed. Higher feed water flows and lower concentrate flows reduce the number of membrane modules required in the RO system. 

Tables 9.2 and 9.3 list the recommended feed water and concentrate flow rates, respectively, as functions of feed water source quality.1 Higher feed water flow rates result in water and its contaminants being sent to the membrane more rapidly, leading to faster rates of fouling and scaling. As Table 9.2 shows, an RO operating on a well water source can have a feed flow rate as higher as 65 to 75 gpm per pressure vessel, while a surface water source RO should not exceed 58 to 67 gpm per pressure vessel. The well water RO would require 12% fewer pressure vessels than the surface water RO.

Table 9.2 Recommended feed water flow rate as a function of feed water source for brackish water membranes, as adapted from Dow Water and Process Solutions.1...

Table 9.2 listed the recommended feed flow rates as a function of water source.1 At higher feed water flow rates, contaminants such as colloids and bacteria that may be present in the source water, are sent to the membrane more rapidly, resulting in faster fouling of the membrane. This is why lower flow rates are recommended for water sources that contain high concentrations of contaminants. 

Design of an RO system has a great effect on the potential for fouling or scaling the membranes. As discussed in Chapter 9, feed water flow, concentrate flow, water flux, and recovery all affect the ability of the membranes to foul and scale. Flow rates affect the concentration polarization boundary layer where fouling and scaling occur (see... 

Another method for dealing with high reactor temperatures is to generate steam, as shown in Fig. 4.19. Here we allow the coolant to boil and thereby provide a constant jacket temperature. The secondary loop controls pressure in the boiler drum by venting steam. Fresh boiler feed water is added by level control. A potential problem W ith this arrangement is the possibility for boiler swell that results in an increase in the level due to increased vaporization in the jacket. The increased level due to swell reduces the intake of boiler feed water when in reality it should be increased. This problem can be overcome by providing a ratio controller between the steam flow and the feed water with the ratio reset by the steam drum level controller. Boiler feed water flow will now change in the correct direction in response to load. 

Pressurized feedwater enters the tube through an end fitting which seals the membrane to the tube and prevents cross contamination of the product water. The feed water flows down the length of the tube and product water permeates through the membrane and weeps through the tubular pressure vessel into a collection basin. The reject flows through an end fitting and is routed to additional tubes in series or to waste. 

A hollow-fiber membrane module similar to the one described in Example 2.14 is used for water desalinization. The feed water flows on the shell side at a superficial velocity of 5 cm/s, 298 K, 70 bar, and 2 wt% NaCl. The permeate flows in the fibers lumen at a pressure of 3 bar and a salt content of 0.05 wt%. For this particular membrane, a water permeance of 1.1 x 10-5 g/cm2-s-bar, and a salt rejection of 97% have been measured. 

FEED WATER LINE - The piping leading to a system through which the feed water flows. 

Reactor vessel height / diameter Primary coolant systems Primary coolant sodium mass Inlet / outlet reactor temperature Primary coolant flow rate Primary coolant flow velocity Secondary coolant systems Secondary coolant sodium mass Inlet / outlet IHX temperature Secondary coolant flow rate Secondary coolant flow velocity Water - steam systems Feed water flow rate Steam temperature (turbine inlet) Steam pressure (turbine inlet) Type of steam generator Refueling system... 

The changes in process parameters resulting in decrease in core inlet temperature causing reactivity transient were studied. The required inlet temperature change to cause the two transients were estimated to be -3.1 and -4.9 C respectively. Extensive tests on the influence of changes in primary, secondary and feed water flow and steam pressure were studied at 9.5 MWt power and the results are given in Table 1. 

Feed water flow change 13.4% increase results in increase of power by 200 kWt after a time delay of 200 s. 

To increase the power to 20% of the rated power to start the turbine generator system, the reflector is periodically lifted up at a speed of 1mm per 15 minutes in automatic mode. Periodic operation is needed to stabilize the system heat balance. At the same time, the pressure of the water steam separation tank is decreased to generate steam and to reduce the re-circulation flow. After this, the power is increased to fiill power by lifting up the reflectors and increasing the feed water flow. 

Regular power operation is attained by moving the reflector upward at a constant speed of Imm/day to compensate for the reactivity decrease due to the bum-up of the core. Since no feedback system or control system are used, the reflector speed remains constant and the electric output is adjusted by varying the feed water flow rate to control die core inlet temperature. The controllable range of the power level by the water flow is 10% at the rated power, which is limited by the steam generator heat balance. Beyond this range, a back-up control mechanism to adjust the reflector position is installed in the driving mechanism. 

During reactor operation SG got isolated on waterside due to disturbance in control power supply making heat sink not available. Reactor did not trip on low feed water flow due to zero error in flow transmitter and reactor was shutdown manually. In order to prevent recurrence of incident, protection circuit was modified to trip the reactor in case of isolation of SG directly and threshold setting of low feed water flow was increased to 50% of nominal flow. 

In operation, feed water flows along the outer surfaces of the membrane envelope under pressure in a direction parallel to the permeate collection tube. Water that permeates across the membrane enters the channel created by the permeate spacer and flows perpendicularly to the feed (a crossflow contacting pattern) towards the permeate collection tube from which the product water is removed. 

The module assembly is placed within a case that possesses manifolds to direct feed water along the membrane surface, withdraw product water from the collection tube, and remove the rejected water. A rectangular case might be used but a more compact unit is obtained by rolling the envelope and feed spacer around the permeate collection tube the feed spacer defines a channel for feed water flow between successive layers of the envelope. A cylindrical pressure vessel is used to hold the spiral wound module thus produced. 

Feed water flow rate to the first element of each stage is the same, <10%... 

Case study I. The process flow diagram of a small SWRO plant is shown in Figure 3.31. The plant design is typical of a SWRO plant. The operating conditions are as follows Feed water flow rate 2200 m /d at 70 bar g Permeate flow rate 1000 m /d Product water recovery 45%... 

Feed water flow rate 1400 m /d at 75 bar g Permeate flow rate 490 m /d Product water recovery 35%... 

Pre-treated RO feed water combines with RO reject recycle water and flows to the RO skid through a 5.0 pm pore size (nominal) cartridge filter. During RO operation only a small portion of the RO feed water flows back to the holding tank continuously. 

Chemical treatment consists of three weU-known treatment processes for ensuring rehable operation of RO membranes. First, a 20% sodium bisulphite (SBS) solution is injected by the chemical dosing pump (one pump is on standby). The iiyection rate is proportional to the RO feed water flow rate and is controlled by the PLC based on the chlorine concentration monitored by a chlorine analyser downstream of the in-line mixer. SBS like sodium sulphite and sodium metabisulphite is a reducing agent commonly used to dechlorinate RO feed or lower the chlorine concentration to less than 0.05 mg/1 in RO plants that use polyamide aromatic membranes. It takes 7.33 mg/1 of 20% NaHS03 solution to remove 1 ppm of residual chlorine in water on a stoichiometric basis. 

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