Pumps twin-headed

The actual cylinders and pistons of a two-headed pump are constructed in a very similar manner to the single piston pump with a sapphire piston and a stainless steel cylinder. Each cylinder is fitted with nonreturn valves both at the inlet and outlet. The cams that drive the two pistons are carefully cut to provide an increase in flow from one pump while the other pump is being filled to compensate for the loss of delivery during the refill process and thus, a fall in pressure. A diagram of a twin-headed pump is shown in figure 10. 

The Waters pump was one of the first twin headed LC pumps to be produced and was probably the major factor in establishing Waters fine reputation in the field of liquid chromatography instrumentation. 

In the single-headed reciprocating pump shown in the figure, the mobile phase is being delivered to the column for only half of the pumping cycle. During the drive stroke of the piston, the flow rate is not constant (because the speed of the piston changes with time). The output of the pump is shown in Fig. 2.2e (z). Use of a twin-headed pump with the two heads operated 180° out of phase... 

Modern twin-headed pumps use two pistons driven by a cam or gear that is shaped so as to make the piston speed constant. Ideally, the output of one such head should be as shown in Fig. 2.2e (Hi), operation of two heads 180°out of phase producing a pulseless flow. In practice, on each drive stroke the change in flow rate is not instantaneous giving an output shown in (iv). To overcome this, the driving cam is arranged to make the piston travel faster on the refill than on the drive stroke, producing an output shown in (v). The flow rate, which is the sum of the output of both heads, is constant. 

The use of twin-head reciprocating pump (/. e., having the two heads operated 180° out of phase) functions in such a manner that while one head is pumping, the other is refilling as could be seen in Figure 30.3. 

Figure 30.3 Working of A Twin-Head Reciprocating Pump.

In Figure 30.3, the flow rate of a twin-head reciprocal pump has been plotted against time. The stage-A depicts the drive while the refill zone is vacant while the stage-B evidently shows the two-heads functioning simultaneously thereby the drive and the refill both zones could be visualized. 

Single head HPLC pumps create an unsteady flow with regular pulses, which, at best, reduces the retention achievable but, at worst, is a source of errors. Twin headed pumps that create a pulse free steady flow should be used. 

Fig. 2. A typical twin-headed reciprocating pump. Reproduced from W.J. Lough I.W. Wainer, High Performance Liquid Chromatography, 1996, first published by Blackie Academic Professional.

An alternative approach to the removal of pump pulses, which is probably the more successful but, as one might expect, the more expensive, is the use of twin pump heads. In a two-headed pump, one... 

LKB manufacture a reciprocating twin-pump system with pump heads made of KEL-F. Gradients are formed by a low volume mixer which is located on the low pressure side of the pump, thereby... 

In the case of the two-component systems, one component is usually a formulated resin system and the other a formulated hardener. The two-component systems are often supplied in twin cartridges, which pump out the two parts of the adhesive through an efficient mixing head. Thus the adhesive can be easily mixed and applied to the substrate in one operation. 

Twin-screw dump extruder with gear pump and strainer head... 

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