Centrifuge

CENTRIFUGE, a laboratory machine designed for the rapid separation of suspended or muddied particles from a liquid, or for separating liquid from bodies that impregnate it. The centrifuge belongs to centrifugal machines. The operating principle of such a machine is based on the creation of a significant centrifugal force, in the field of action of which the processed object is placed. Centrifugal force can be expressed as follows: P = m v² / r, where m is mass, v is peripheral velocity in cm/sec, and r is radius. From this it is clear that at the same radius and speed, particles with a greater mass will experience an absolutely greater centrifugal force and will arrange themselves in the liquid at the periphery of the circle, while lighter ones will be closer to the center. Peripheral velocity in cm/sec can be expressed through the number of revolutions per minute n as follows: v = 2πrn / 60. Substituting this expression into the formula for centrifugal force, we obtain: P = m (2πrn / 60)² / r = m 4π²r²n² / 60²r = 4π² / 3600 m r n².

FIG. 1.

Analyzing this expression, we have: 1) at constant mass and number of revolutions, the centrifugal force is stronger the greater the radius; 2) at constant mass and radius, the centrifugal force increases proportionally to the square of the speed. There is a huge number of different systems of centrifugal machines serving various purposes. In all these machines there is a vertical rotating axis that carries a greater or lesser number of metal sleeves (Fig. 1), into which glass tubes of various shapes can be inserted: rounded, conical, pointed, with lids, with graduations, etc. (Fig. 2). A centrifuge can work well if the tubes with liquid located at the ends of the diameter are balanced with each other; this achieves an uniform load on the axis, ensuring a uniform and smooth run of the centrifuge. Special scales are often used for this balancing (Fig. 3).

Centrifuges can be divided into several groups: firstly, according to the number of revolutions they produce per minute, and secondly, according to the volume of liquid that can be subjected to centrifugation simultaneously. 1) Centrifuges producing 2,000 to 3,000 revolutions per minute; the volume of liquid in each tube is 15 cm³, with a total of 2-4 sleeves. Rotation in these centrifuges is often performed by hand (Fig. 1). 2) Centrifuges producing from 3,000 to 4,000, and in exceptional cases up to 5,000 revolutions per minute; the volume of liquid varies: from 4 tubes of 15 cm³ to 4 tubes of 50 cm³ (there are also models with a larger number of sleeves or a volume of up to 100 cm³ each). Rotation is performed mostly by an electric motor. These centrifuges are currently manufactured in Moscow, Leningrad (Fig. 4), and Saratov. Saratov centrifuges (3,000 revolutions per minute) resemble in shape the centrifuge depicted in Fig. 5, but are suspended in a special stand on four legs. The suspension of the Kollatz centrifuge is made to a special wall bracket or to a special massive leg (Fig. 6). Figure 5 depicts a connection with a hematocrit (an instrument for determining erythrocyte volume). While the centrifuge produces 3,000 revolutions, the hematocrit rotates at a speed of 10,000 revolutions per minute. 3) Centrifuges producing from 3,000 to 4,000 revolutions, but designed for a larger volume of liquid—from 6 tubes of

Figure 6.

Figure 8.

of 50-100 cm³ up to 8 tubes of 1 liter (there are models producing up to 6,000 revolutions with a relatively small amount of liquid—6 tubes of 50 cm³). In these centrifuges, the axis is for the most part not directly connected to the motor (Fig. 7). Often in biochemical laboratories it is very convenient to replace the slow process of filtering large volumes of liquids with rapid centrifugation. For serological purposes, an insert allowing the centrifugation of a large number of tubes simultaneously can be used in the centrifuge (Fig. 8). Recently, the Kollatz firm has released ultra-speed centrifuges (Ecco-Super-Rapid) producing 10,000 revolutions per minute. These are small machines for 4 tubes of 1.5 cm³ each, which will probably find great application in research biochemical laboratories.

Derviz.