Suction pressure maintaining constant

At a constant speed, a constant volume of gas (at suction conditions of pressure and temperature) will be drawn into the cylinder. As the flow rate to the compressor decreases, the suction pressure decreases until the gas available expands to satisfy the actual volume required by the cylinder. When the suction pressure decreases, the ratio per stage increases and therefore the discharge temperature increases. In order to keep from having too high a discharge temperature, the recycle valve opens to help fill the compressor cylinder volume and maintain a minimum suction pressure. 

A suction pressure throttling valve can also be installed to protect the compressor from too high a suction pressure. This is typically a butterfly valve that is placed in the suction piping. As flow rate to the compressor increases, the valve will close slightly and maintain a constant suction pressure. This will automatically limit the flow rate to exactly that rate where the actual volume of gas equals that required by the cylinder at tlie chosen suction pressure setting. It will not allow the suction pressure to increase and the compressor cylinder to thus handle more flow rate. 

Figure 6-31 illustrates control schemes for the single stage unit which allow greater stability in performance. As the load changes for a fixed suction pressure, the process fluid is replaced by an artificial load (usually air Figure 6-31, item 1) to maintain constant ejector operation. An artificial pressure drop can be imposed by valve (2), although this is not a preferred scheme. Wlten the addi-... 

Figure 6-32 illustrates ejector systems with large condensable loads which can be at least partially handled in the precondenser. Controls are used to maintain constant suction pressure at varying loads (air bleed), or to reduce the required cooling water at low process loads or low water temperatures [2]. The cooler W ater must not be throttled below the minimum (usually 30%-50% of maximum) for proper contact in the condenser. It may be controlled by tailwater temperature, or by the absolute pressure. 

But, suppose we must maintain a constant pressure in the wet-gas drum. The pressure in this drum may be controlling the pressure in an upstream distillation column. To hold a constant pressure in the drum, we will have to resort to spillback suction pressure control, illustrated in Fig. 28.5. 

Regenerator pressure is controlled by throttling the flue gas (305). A manually operated valve is sometimes used for rough adjustment and supplemented by an automatically controlled valve in a by-pass line for precise regulation (330). The automatic control may be arranged to maintain constant differential pressure between the reactor and the regenerator (68). Reactor pressure is controlled by holding a constant pressure at the suction side of the compressors that remove uncondensed vapors from the fractionator (202,330). 

SUCTION PRESSURE CONTROL VALVE - Device located in the suction line which maintains constant pressure in evaporator during running portion of cycle. 

A second valve then controls compressor suction pressure by allowing gas to recycle from the discharge of the compressor. Good control maintains a constant differential... 

Air vessels are also incorporated in the suction line for a similar reason. Here they may be of even greater importance because the pressure drop along the suction line is necessarily limited to rather less than one atmosphere if the suction tank is at atmospheric pressure. The flowrate may be limited if part of the pressure drop available must be utilised in accelerating the fluid in the suction line the air vessel should therefore be sufficiently large for the flowrate to be maintained approximately constant. 

The sampling of a suction lysimeter is initiated by applying a vacuum (approximately 40-50 cm of mercury) through the vacuum/pressure line with a hand pump or electric pump. The valve on the sampling line must be closed. A constant vacuum may be maintained on the lysimeter using an electric pump. The time required before collecting a sample from a lysimeter will depend on the method of vacuum application, the moisture content of the soil, and the soil type. 

A cooled mixture of 200 g. (109 cc.) of concentrated sulfuric acid (2 moles) and 300 cc. of water is now added slowly with careful cooling (Note 2), the pasty mixture being stirred with a thermometer and the temperature not being allowed to rise above 35°- Sodium sulfate crystallizes during this addition. The mass is now shaken vigorously with 400 cc. of ether and allowed to stand for some minutes. The ether is then decanted as completely as possible and the residue filtered with suction. The sodium sulfate is now shaken with six successive 300-cc. portions of ether, the ether solutions being subsequently employed for extraction of the filtrate. This latter requires ro-r4 such extractions (each with 300-400 cc. of ether) for the satisfactory extraction of the /3-hydroxypropionic acid. The combined ethereal solution is dried over 50 g. of anhydrous sodium sulfate and the ether distilled from a water bath, the temperature of which is not allowed to rise above 50°. The product is then concentrated under reduced pressure from a water bath maintained at 55-60°. The residue should have attained constant weight after four to six hours of this treatment it consists of a sirupy liquid of pale straw color which contains 75-80 per cent of /3-hydroxypropionic acid (by titration) (Note 3). The yield is 120-225 g. (28-31 per cent of the theoretical amount). 

Water was used as the liquid phase. The liquid delivery system consists of a feed tank, pump, and a flow indicator. Water is recycled, as well as added if necessary, to maintain a constant suction head at the pump. The in-house air system was used as the gas delivery system through a rotameter. Air enters the setup at a point below the packing. Two taps are mounted at the inlet and outlet of the packed column for pressure drop measurement. 

Figure 3.15. Control modes for the upper sections of fractionators, (a) Pressure control by throttling of the overhead vapor flow. The drawbacks of this method are the cost of the large control valve and the fact that the reflux pump operates with a variable suction head. The flow of HTM is hand set. (b) Applicable when the overhead product is taken off as vapor and only the reflux portion need be condensed. Two two-way valves can replace the single three-way valve. The flow of HTM is hand set. (c) How rate of the HTM is regulated to keep the pressure constant. One precaution is to make sure that the HTM, for example water, does not overheat and cause scaling. The HTM flow control valve is small compared with the vapor valve of case (a), (d) Pressure control is maintained by throttling uncondensed vapois. Clearly only systems with uncondensables can be handled this way. The flow of the HTM is manually set. (e) Bypass of vapor to the drum on PC ...

---