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Sorting and Grading of Tea

Sorting is a mechanical process. In the early days of the industry, it was believed that sorting did not require the same degree of management as other operations of tea manufacture. But, it has been found that in order to get good prices at the auction, proper sorting is essential for all grades of tea in general, and broken grades in particular.

One of the objectives of sorting is to classify tea according  to the size of the particles. In the past, the object of sorting was merely to remove foreign mater and big leaves. This process was carried out by hand, with the only implement used being a small pincer used to pull out long stalks. The process of grading was known as sifting at that time. Today, no distinct division can be made between sorting, cleaning and grading since the processes are carried out simultaneously and procedures are common to all. All processes are based on the four important characteristics of made tea. These include:

• size,
• shape,
• specific gravity, and
• surface characteristics of the particles.

.2 Modern methods of sorting

Modern methods of sorting were introduced along with mechanisation in the 1870s. Mechanical sorters were developed to utilize power from steam engines and other sources. In 1880, George Reid introduced a mechanical breaker into the sorting operation to reduce the size of leaves. In one stroke, a very tedious process was greatly simplified.

More modern sorting machines incorporate different sieves with meshes of size 8,10,12,16,24,30 or greater. Tea passed over a number 8 mesh is cut and sifted again, and so on. As the tea passes through the various meshes, it is classified into grades such as whole leaves, broken leaves, fanning, and dusts.

In modern factories a Myddleton machine is used at the first stage of the operation for orthodox teas. For CTC manufacture, an electrostatic stalk extraction unit is used first. Usually, in orthodox manufacture, the spillover from the Myddleton is passed through a ghugi or rotary hexagonal sifter; from the trays, the leaf is passed into the Arnott and Macintosh sorters. But in CTC and leggcut manufacture, the ghugi is not used. Thereafter, the process incorporates the Britannia Tea Sorter (which works very well in duel manufacture), the Macintosh sorter and the wind tunnel. A number of hand sievings are also essential to produce some quality grades. In wind tunnels and waterfalls, fibre and dust grades are separated according to the specific gravity of the particle. Tea breakers and cutters are extensively used in modern sorting operations to reduce particle sizes for subsequent classification.

In orthodox manufacture, the first classification of tea takes place at the time of sifting. BOP, BOPF and OP grades are produced from the first and second fine teas. The coarse leaf that is first broken or cut goes into the production of BP, PF  and souchong grades. In CTC and leggcut manufacture, sorting is based wholly on the size of the leaf particles.

.2.1. Precaution to be taken while sorting

Sorting should not be carried out directly on hot particles discharged from the dryers; this can destroy the bloom of the made tea. After drying, the particles should be cooled on the floor or on a wooden platform. The tea should not be bulked hot. At the same time, care should be taken to see that the leaf does not cool to the point where it can pick up atmospheric moisture.

.2.2. Grades of tea

Broadly speaking, there are four grads of tea. These whole leaf, broken leaf, fannings and dust. Each of these grads again have various sub-grades. Of the four, only the latter three can be produced via CTC and leggcut manufacture.

.2.2.1 Orthodox whole leaf grades

.2.2.2 Orthodox broken grades

.2.2.3 Orthodox fanning

.2.2.4

.2.2.5 CTC grades

CTC Broken

Fannings

CTC dust

.2.2.6 Leggcut grades

Leggcut fannings

Leggcut dust

.2.3 Percentage of various grades

The types and percentages of various grades produced in a tea factory depend upon market demand. Also, but to a lesser extent, plucking standards and the quality of leaf play a role. In orthodox tea manufacture, the following percentages of different grades are normally produced:

From the 15 % whole leaf grades, the following three sub-grads are extracted.

Similarly, the broken, fanning and dust grades are processed further into:

It should be noted here that failure to produce whole leaf grades will result in reduced liquor colour and strength in broken and fanning grades.

In CTC tea, primary broken and fanning grades constitute about 20 and 50%, respectively. Secondary siftings of the same two grades yield about 9% each. Dust accounts for about 18% of the total produt, leaving a residue of 3%.

In comparison, leggcut manufacture yields fewer grades, making it simpler than orthodox and CTC manufacture. In leggcut manufacture, only fanning and dust grads are produced, in the following percentages:

In green tea manufacture, various grades are produced in addition to the common fanning and dusts. These include:

.2.4 Stalk extraction

The extraction of stalk from made tea is one of the major problems in the sorting process. Stalk-free tea can be produced through fine plucking, light rolling, and avoiding drastic treatment to the leaf during processing. Unfortunately, such procedures are not always economical. Red stalk and fibre often appears and their removal becomes necessary. At present, the Myddleton is the machine most commonly used for stalk extraction. In this machine, there are two trays with bosses on the surface. During processing, the particles are hopped through these bosses and are thus separated. Java tunnels and waterfalls, which operate on the varying specific gravity of tea particles, are also useful in stalk removal. There is also the more recently developed Andrew Breaker, with its flute rollers which break leaf up but leave stalks intact. Japanese tea technologists have also invented a system based on the variation of moisture levels between leaf and stalk particles. Though the machine works well in principle, its output is low to cope with the needs of  a modern factory. This, however, is true of most stalk extraction systems. The presence of stalk and fibre is controlled at best, not eliminated. A fine balance needs to be maintained between the level of control, efficiency and market price.

.2.5 Greying of tea

In the buyers' market, tea is valued for colour, strength quality and briskness. In order to obtain fine tea with all the above desirable characteristics, quality must be inherent in the leaf when it isfirst brought to the factory. No skill can produce high class tea out of poor quality leaf. By the same token, poor manufacture can easily turn good leaf into bad tea.

One defect, arising from flawed manufacture, is greyness or the loss of bloom. This accounts for significant losses for the tea industry. But, before dealing with details of the subject, let us understand the terms 'bloom' and greyness'.

Bloom

The bloom sought in tea leaf is achieved through the development of a varnish-like film on its outside surface. Failure to produce proper bloom leads to dullness in made tea. Teh bloom can be damaged through faulty sorting.

Greyness

Grey is perhaps the most undesirable colour in dry leaf. greyness is caused when the gummy or varnished surface of the leaf, which plays an important part in its liquoring properties, is abraded or rubbed off. greyness is particularly likely to occur during sorting and may be caused by one of the following factors.

• Tea particles get caught in sagging meshes in the sorting machine and are abraded through the movement of the trays.

• Blunt knives in the cutter  and some older types of machine break, rather than cut, the tea. This not only produces excess dust, but also causes greying. Fast running cutters too tend to cause greyness.

• Excessive handling of the product also creates greyness.

• The tea readied for sorting should have a moisture content of between 3 and 5%. Moisture above 5% causes greyness.

• Over-sorting quickly causes greyness because of the continuous rubbing and striking action of the particles against the trays. A leaf's varnish is easily removed in this manner.

• Excessive rolling may also cause greyness.

• Tea leaf fed into a sorter from an incorrectly placed hopper is subject to striking action that also removes the outer varnish.

.3 Sorting Procedure

Every tea estate has its own method of sorting. This is because each company has to develop its own criteria, depending on the market it services. But there are some common procedures that most gardens would follow for CTC and orthodox manufacture.

In CTC manufacture, tea is initially passed through an electrostatic fibre extractor. Once the fibre and stalks are removed, the tea is fed to the Myddleton machine. From here, the tea passes to one or two successive Arnotts, a Macintosh machine for final grading, and then it is cleaned in a Java tunnel or waterfalls. The spillover from the Myddleton is passed through the Andrew Breaker and recycled for grading.

The first step in orthodox sorting also involves the involves the Myddleton. Teh output from its the two trays is passed into the Macintosh, where the teais graed through various meshes. The spillover from the Myddleton is passed to the Ghugi, or rotary hexagonal sifter, where the TGFOP, GFOP and FOP grads are extracted. But the sequence of grading varies widely from estate to estate. For example, in some estates, fine tea from dryers is fed directly to a Ghugi fitted with number 14,12 and 10 meshes and sorted in the following manner through hand sieving.

6.3.5 Sorting of Leggcut tea

Leggcut tea is easier to sort than orthodox and CTC teas. The bulk of the work is done in the Myddleton and Macintosh machines, where the output from the former passes to the latter to produce fanning and dust grades.

.4 Sorting Machines

.4.1 Myddleton

The Myddletontea stalk extractor is a common machine in all types of tea manufacture. It is used widely in almost all tea growing areas as an initial sorting machine. It performs the operations of grading and making tea free of stalks.

.4.1.1 Important parts

Trays

There are two trays in the Myddleton, the top and bottom trays. The top tray is 9' x 4' and the bottom tray is 8'5 inch x 3'10 inch. The trays are made of thick aluminum sheet, with lines of small bosses, each with a perforation at the top. The trays are easily removable and sieves of different specifications can be fitted into the frames to suit the tea being treated. One of the Myddleton's two sieves is normally has 3/16 inch perforations and 1/8 inch high bosses. Two additional trays are also available; one with 7/32 inch perforations and 1/8 inch high bosses, and the other with 17/64 inch perforations and _inch high bosses.

Pulley

There is a 10 inch pulley with a 3 inch face, suitable for a 2.5 inch drive. This can be attached directly to a motor.

Crank shaft

The crankshaft imparts reciprocating motion to the machine via connecting rods. The crankshaft is made of very strong steel bars.

Grafton Phillips Attachment

The Grafton Phillips Attachment is used for removing dust and fluff from the tea. It has two superimposed trays, fitted with brass wire mesh number 16 on the top and number 32 at the bottom.

The tea discharged from the top tray of the extractor is passed to the top tray of this attachment. An oscillating motion causes the tea to be thoroughly sieved as it passes over the gauge. The finer parts of the tea and the dust fall onto the lower tray. On this tray, a similar operation takes place - the cleaned fine tea passes over the end of the tray while the dust falls through into a collector underneath. The tea discharged over the ends of both trays is clean and free of dust and fluff.

.4.1.2 Working principle

An oscillating motion is imparted to the sieves wit the help of a crankshaft, which revolves at 200rpm. Leaf is fed to the Myddleton machine at spread thicknesses of approximately 1.3 to 1.9 inches. The tea being sorted in the Myddleton moves down a slopped tray between the bosses. This imparts a slight disturbance to the mass and causes the leaf to hop through the perforations. The stalk and fibre slide over the bosses pass over the end of the tray, and is thus separated.

The Myddleton can also be used for grading by treating fine and coarse teas separately. The following arrangement of trays would then the required.

.4.1.3 Power requirement

The Myddleton operates on 1 hp of power

.4.1.4 Capacity

The machine is capable of processing 300 kgs of leaf an hour.

.4.2 Rotary sifter

The rotary hexagonal sifter, or Ghugi, is one of the most important machines for orthodox operations. This machine has gained popularity for its efficient working. Normally, leaf sorted at the uppermost level of the Myddleton is fed to the Ghugi for selection into other grades.

.4.2.1. Important parts

Screens

The important components of the Ghugi are three screens of different mesh combinations - such as 14,12 and 10 or 10,8 and 6 - forming a hexagonal body of 5 ft and 3 ft at two ends. The sifter is set on an incline, which can be adjusted according to requirements. The sifter is usually about 10 ft long.

.4.2.2. Working principle

The sifter can be run at 25 to 30 rpm with the help of a motor or a line shaft. Due to the rotation of the sifter at an inclined position, the tea moves from the feeding point to the sifter's discharge end. The tea passes through various separating zones in the sifter according to the size of the particles. Thus, the grading of tea is carried out.

.4.2.3 Power requirement

The Ghugi requires 3 hp of power

.4.2.4 Capacity

It processes about 1,000 kgs (or 2,200 lbs) of leaf an hour.

.4.3 Electrostatic Fibre Extractor

The Electrostatic Fibre Extractor (EFE) is a relatively new machine used for stalk extraction after firing. In this machine, the principle of static electricity is used for extracting fibre and stalk. It can be used in continuous manufacture with the help of a conveyor system after the second firing. The machine is normally 10-12 ft long and 3-4 wide and is considered one of the best for extracting fibre in CTC manufacture.

.4.3.1 Different parts

Main frame

The main frame of the EFE is made of mild steel, and is rigidly built. In some motor-driven models, cast iron wheels are attached to the frame so that the machine can be easily moved within the factory.

Roller

The machine has seven PVC rollers, placed in two stages of four and three rollers, respectively, Each roller has a diameter of about 4 inch, and is designed to rotate at speeds of 2,000 to 2,500 rpm. The rollers rotate smoothly on heavy duty bearings housed in plumber blocks.

Motors

The motor used in this machine is of 1400/1500 rpm, 400/440 volts 3 phase 50 cycles.

.4.3.2 Working principle

This machine works on the principle of static electricity. There are seven PVC rollers arranged in two rows that run at speeds of 2,000-2,500 rpm. Tea fed into the machine via a conveyor system comes into contact with the PVC rollers, causing friction between the two. As a result, static electricity is produced with the fibre and rollers developing opposite charges. This causes the particles and rollers to attract each other. But the fibre is turned over through centrifugal action and collected in aluminum collecting boxes fixed over the conveyors between rollers. From there it is transferred to other containers.

.4.3.3 Power requirement

The EFE uses 3 HP.

.4.3.4 Capacity

The EFE process 400 to 450 kgs (880 to 990 lbs) of made ta an hours.

.4.4 The Macintosh tea sorter

The Macintosh tea sorter has several outstanding features. It is widely used for orthodox. CTC and leggcut manufacture. The unique feature of this machine is that it makes uniform size grading possible. In today's tea market, this if of great importance.

.4.4.1 Important parts

Crankshaft, Flywheel and Bearing

The crankshaft used in a Macintosh is known as a single throw crankshaft, and is made of a round mild steel bar. The crankshaft is fitted with a heavy-duty ball bearing which, in turn, carries a balance flywheel. This ensures vibration-free operation.

Trays

The machine has five trays with seven discharge outlets for various grades of tea. The trays slide into the frame and are each securely fastened with four band-operated screws. All trays are interchangeable, and the mesh used depends on the type of tea to be sorted. The selection of trays too depends on th grades to be produced, but the following broad guidelines may be considered for sorting leaf.

In the case of broken grade teas, the arrangement of trays can be made as follows

.4.4.2 Working principle

Tea is fed to the Macintosh through a hoper attached at one end. The rate of feed is controlled by adjusting a mechanically agitated flap valve. A satisfactory rate of feed has to be ensured to suit the different types of tea being sorted in the machine.

The gradient trays are adjustable and quickly interchangeable so that grading can be changes as desired. The machine's frame is connected by a connecting rod to a single crankshaft fitted with heavy duty ball bearings.

The crankshaft rotates at 250 to 270 rpm and transmits a straight oscillatory motion to the trays with the help of spring legs. Due to this motion, tea particles pass through holes according to their size, and are thus classified into various grades within 8 to 10 seconds.

.4.4.3 Power required

The Macintosh sorter uses 2 hp of electric power.

.4.4.4 Capacity

The  Macintosh sorter processes approximately 220 kgs of tea an hour.

.4.5 Balanced Tea Sorter

.4.5.1 Introduction

The most attractive feature of the Balanced Tea Sorter is that it can be operated efficiently even while maintaining the best bloom of tea. It is considered most suitable for the dual type of manufacture. Various  sieves with different mesh sizes sort tea particles into the required grades. However, the drawback of the Balanced Sorter is that its output is lower than that of other comparable sorting machines.

.4.5.2 Important parts

Hopper

A hopper fitted at one end of the Balanced Sorter contains no wearing parts, ensuring even input of tea. A flap easily regulates the thickness of spread.

In the lower part of the hopper, there is a rubber diaphragm which is subjected to the slight impulse of the beater bar during the upward movement of the trays. The required impact distance is very small, with not more than 1/16 inch being needed for CTC tea and little more for orthodox tea. The action of the beater bar can be adjusted easily while the machine is running. The impact should be kept to a minimum, as this will prolong the life of the diaphragm.

Two handles on both sides of the hoper are used to control the shutter. Two small glands grip the spindle of the shutter-opening device just tightly enough to prevent self-movement, and thus eliminate the use of any clamping or locking devices.

While the hopper can be fed through a conveyor; it can also be hand fed. Since the hopper is stationary in this machine, greying can be avoided.

Oscillating links

Ideally, the links that impart Jigger type motion to the machine, are set at an angle of 15°. The links are fitted with 'silent block' bushes, which do not require any lubrication during operation.

Bearings

The ball bearings attached to the machine are properly enclosed to ensure protection from dust and moisture for at least six months at a time.

Trays

This machine comes equipped with five trays, with the bottom-most used for producing dust grades. Half this tray's length is fitted with number 40 mesh, and the rest with number 30 mesh. The other trays are normally fitted with number 8,10,12, 20, 30 and 40 meshes.

Since a simple spring clip automatically holds the trays in position, no clamping device is required to remove or the refit trays. This can easily be done manually.

.4.5.3 Working principle

The Balanced Tea Sorter receives power from a motor or line shaft belt. A barrel with a cambered portion runs at speeds of 300 rpm. This cambered portion serves as a fixed pulley and imparts Jigger-type motion to the machine's fitted screens via links.

The lower slope of the machine's feed hoper is made of a rubber diaphragm forms. As tea passes over it into the machine, it is subjected to the impulse of the oscillating trays' beater bar. This ensures a uniform feed into the machine.

.4.5.4 Power

The total power required for this sorting machine sis 0.25 HP. but, a 1 HP motor is used to over come the initial torque.

.4.5.5 Capacity

The Balanced Tea Sorter can process up to 200 kgs (440 lbs) of tea an hour.

.4.6 Benton Density Sorter

The Benton Density Sorter is a small machine that works on the density variations of particles. A small fan that consumes only 30 Watts of electricity powers this machine. Four wheels fitted to the machine make it easy to move the machine to a desired location anywhere within a factory.

The Benton Density Sorter is used for the following operations:

• Separation of sand from dust tea.

• Separation of fibre and flaky leaf from other grades.

• Separation of large foreign matters from dust tea.

• Separation of stones and broken grades.

.4.6.1 Important parts

Fan and Motor

The Benton Density Sorter is powered by a small fan driven by a totally enclosed 0.125 HP motor that consumes approximately 30 Watts.

Feed shutter

A feed shutter is used to admit leaf into the machine; the shutter can also control the rate of feed. The rate of feed is controlled at thickness of 0.25 to 0.5 inch.

Chute

The chute is a polished aluminum sheet on which the leaf slides into the machine. It can set at various angles to suit the needs of different particles.

.4.6.2 Working principle

As soon as leaf is fed into the hopper, it is admitted to the chute through the feed shutter at 0.25 to 0.5 inch thickness. The chute is set at such an angle that the leaf accelerates rapidly down its surface in such a manner that individual particles separate out. At the point where the chute terminates in the gap, the leaf mass is subjected to a current of air supplied from beneath. The heavier particles are least affected by the current of air and pass into the side delivery number 1. Lighter particles float across the gap on the cushion of air and pass into side delivery number 2. Similarly, the lightest particles are carried upward for a short distance and pass into the end delivery.

The chute can be angled from 30° for heave grades to 70° for powdery grades. This change is possible merely by altering the position of a clamp on its rack. The angel of the fan duct can not be changed relative to the angle of the chute; but there is a provision for altering the angle of the fan duct separately to optimise the airflow through the gap. Normally, the machine is operated with the fan duct in the lowest position. The volume of air used in this machine is very small, but that can be adjusted through orifice plates of varied diameter. Orifice plates of 9, 6 or 4 inch diameters can be used in conjunction with the two available fan speeds.

.4.6.3 Capacity

The Benton Density Sorter can process up to 10 kgs a minute.

.4.6.4 Power required

The machine runs on a small 0.125 HP motor.

.4.7 Circular Vibro Screen

The circular Vibro Screen was introduced into tea industry for the classification of various grades of  tea in an efficient and economical way. This machine is designed to work in a continuous manufacturing situation. Essentially, it comprises a circular gyratory screen that is subjected to three-component - horizontal, vertical and tangential - multiplane mechanical vibrations. The vibration is created by a motor with a double extended shaft fitted at both ends with eccentric weights. Both motor shaft and weights are mounted on the screen assembly. The drive assembly, in turn, is mounted on a circular base fitted with rugged springs. The springs not only allow the unit to vibrate freely, but also prevent the vibration from reaching ground where the machine is installed.

.4.7.1  Working principle

A three-component multiplane mechanical vibration is induced by a motor mounted on a vertical axis. The motor has double extended shafts fitted at both ends with eccentric weights. It is suspended from the screen assembly, which is supported on a circular base by rugged springs at its periphery. When leaves are fed into the assembly through a hopper, oversized particles leaves are immediately discharged through a tangential outlet. Smaller particles pass on down to the next screen to produce finer grades.

The machine's horizontal and vertical motion depends on the eccentric weights fitted at the top and bottom. Any increase in the top eccentric weight increases horizontal motion, which increases the machine's output. similarly, more bottom eccentric weight increases vertical motion, which keeps particles in circulation.

The tangential motion, meanwhile, is controlled by the weights' angular position. It is at a minimum when the top and bottom weights are lined up; there is then virtually no tendency for the material to spiral. But if the weights are not lined up, the particles spiral from the center out to periphery. In this manner the leaf is sorted, with the finest particles spiralling furthest out.

.4.8. Hammer Mill

The Hammer Mill

The Hammer Mill is used to produce dust teas in a continuous manner. It is normally installed apart from other sorting machines so that the dusty atmosphere can be minimized in the sorting room.

.4.8.1 Important parts

Grinding chamber and wheel

The grinding chamber, which holds the grinding wheel, is made of thick steel. The latter is made of a cluster of carbon steel strikers running at very high speeds of up to 4,500 rpm. The size of the machine is expressed according to its grinding wheel - i.e., 6, 8, 10, 15, 18 or 20 inch.

Collecting chamber

This is a cylindrical chamber made of steel plate supported by three rods. Dust tea outlets through a vent at the chamber's bottom.

Motor

The motor of the Hammer Mill runs at 3,000 rpm.

.4.8.2 Working principle

When the coarse product of other sorting machines is fed into the Hammer Mill's grinding chamber through the hopper,  it comes into contact with the grinding wheel revolving at 4,500 rpm. The leaf is reduced into a fine power by the striker's sharp cutting edges. The dust tea so produced is directed by a suction fan into a collecting chamber and is directly discharged into the container. The velocity of the dust at the collecting chamber's inlet can be minimized by enlarging the duct through which it travels.

.4.8.3 Capacity and power required

The capacity of the Hammer Mill varies according to its size. The average capacities of and power required by various Hammer Mills are:

.4.8.4 Advantage

The greatest advantage of the Hammer Mill is its simplicity and versatility.

.4.8.5 Disadvantage

Its high power requirement is the Hammer Mill's biggest drawback.

.4.9 Andrew Breaker

In 1948, a machine was developed for breaking tea. It is called the Andrew Breaker, after its developer H.C. Andrew. Today, this machine is an integral of the modern tea sorting room. Earlier machines used for breaking tea could not solve a critical problem -i.e. when leaf is broken into small grades, so is the stalk and this complicates its removal. The Andrew Breaker eliminated this problem in sorting.

.4.9.1 Important parts

Roller

The most important part of the Andrew Tea Breaker consists of a pair of fluted rollers. The rollers - of 11 inch diameter and 17.5 length - run in mesh but are not in contact. The rollers rotate at 60 rpm and are spring loaded. A lever is provided to adjust both the gap between the engaged teeth of the rollers as well as the pressure applied to the tea particles  passing through.

Three types of rollers are used in the Andrew Breaker. The first kind of roller is the longitudinal fluted roller. These are sub-divided into two types - the 8 and 16 fluted. types. Eight-fluted longitudinal rollers are general purpose rollers. The increased number of closer flutes in the 16-fluted longitudinal rollers are used in CTC manufacture for even grading and appearance. The third type of roller is also 8-fluted. But here the roller is fluted circumferentially because of which it is known as the corrugated type. This is a special purpose roller used for the treatment of broken teas or stalky coarse residue.

.4.9.2 Working principle

When rough tea is fed to the Andrew Breaker (fitted with longitudinal fluted rollers) through a conveyor system, it is compressed between the rollers rotating at 60 rpm. The amount of tea broken depends on the degree of pressure applied and the speed at which it is applied. Care must be exercised here to optimise the speed at which pressure is applied; that way there is no damage to the natural bloom of the made tea.

After extracting all normal, worthwhile grades from the bulk, stalky coarse residue can be fed to the Andrew Breaker fitted with corrugated rollers. In these rollers, the flutes run around the periphery of the rollers. These rollers cause a rubbing action that removes the skin from stalk and tends to disintegrate the tough and flaky particles that previously escaped breakage. The speed of the rollers here is maintained at 180 rpm.

.4.9.3 Capacity

An Andrew Breaker can deal with 3,000 kgs of rough tea per hour with thick and continuous feed.

.4.9.4 Power requirement

The machine requires 3 HP to run.