How to Improve Oil Yield with a Twin-Screw Oil Press
The previous section mainly introduced the structure and application of single-screw oil presses. A detailed analysis of the structure of single-screw oil presses reveals several issues in cold-pressing high-oil-content peanuts, including an excessively small length-to-diameter ratio of the pressing chamber, an insufficient total theoretical compression ratio, an overly short conveying screw coverage length, and insufficient conveying capacity. The pressing chamber structure of existing single-screw oil presses is not suitable for cold-pressing oil extraction. Therefore, to achieve cold-pressing of oilseeds, it is necessary to enhance the conveying and propulsion capability of the material conveying screw, increase the pressing force, and extend the pressing time.
Key Insight: Traditional single-screw oil presses are limited in cold-pressing applications, particularly for high-oil-content materials such as peanuts. The twin-screw design addresses these limitations by providing greater mechanical advantage.
1. Equipment Features
The twin-screw oil press combines intermeshing and non-intermeshing principles, implementing multi-stage compression, relaxation, and thin-layer pressing within the pressing chamber. Compared with traditional single-screw oil presses, the SSYZ50 twin-screw oil press has the following characteristics:
- Unique Twin-Screw Counter-Rotating Conveying Structure
In the first stage of the pressing chamber, the left and right screws intermesh with each other, and the oilseeds between the two screw grooves are semi-enclosed. After entering the pressing cage, the material particles are subjected to friction from the screw threads, the inner surface of the pressing cage, and between the particles themselves. Their motion can be decomposed into rotational motion along the axis and axial motion. Due to the low friction coefficient of the material, the primary motion is rotational along the axis.
When the material moves counterclockwise relative to the intermeshing area between the screws, it undergoes mutual extrusion and friction, disrupting its rotational motion about the axis and significantly increasing axial motion, thereby generating a strong axial pushing force. Because the screw grooves are divided into isolated C-shaped chambers at the intermeshing points, the material is propelled forward along the C-shaped chamber paths by the screws, resulting in mild pressing.
- Second Stage Pressing Mechanism
In the second stage of the pressing chamber, the outer diameters of the left and right screws are tangent to each other, and the screw grooves are interconnected. The material primarily moves forward due to the screws, but mutual extrusion and friction still occur at the tangent points.
Thus, the twin-screw and pressing cage form an ideal automatic cleaning mechanism for pressed material, providing a robust self-cleaning function that fundamentally addresses the high-viscosity and residue-accumulation issues commonly encountered in oilseed processing.
② Performance Specifications
As shown in Table 3-5, the twin-screw oil press design achieves a theoretical compression ratio of up to 23. The powerful radial compression force exerts great mechanical pressure on the oilseeds. The length-to-diameter ratio of the pressing chamber is 11.5:1, and the pressing time is extended to 180 seconds, resulting in a more thorough pressing process. At the same time, it enables long-distance thin-layer pressing, significantly reducing the oil flow distance and allowing sufficient pressing time for the oilseeds during the high-pressure stage. The actual high-pressure oil extraction time can exceed 80 seconds, thereby increasing the oil yield.
Comparison with Single-Screw Oil Presses
In comparison, the existing ZX-10 and ZX-18 single-screw oil presses have length-to-diameter ratios of 6.3 and 6.72, respectively. The ZX-18 single-screw oil press has a theoretical compression ratio of 13.2 and a pressing time of 150 seconds.
| Model | Type | Length-to-Diameter Ratio | Theoretical Compression Ratio | Pressing Time (seconds) |
| Twin-Screw Press | Twin-Screw | 11.5:1 | Up to 23 | 180 |
| ZX-18 | Single-Screw | 6.72 | 13.2 | 150 |
| ZX-10 | Single-Screw | 6.3 | N/A | N/A |
③ Mechanical Structure
One end of the twin-screw shaft is connected to the output shaft of the dual-shaft gearbox via a coupling, while the other end is supported by a spherical sliding bearing and the cake discharge adjustment screw. The sliding bearing uses a solid lubricant for self-lubrication. During operation or under no-load conditions, the two screw shafts can rotate concentrically, with the screws always remaining at the center of the pressing chamber. This reduces asymmetric wear on both the pressing chamber and the screws.
④ Feeding Mechanism
The feeding mechanism uses a combination of horizontal and vertical augers to supply material. The feeding rate is adjusted via frequency-conversion speed control, enabling stepless regulation over a wide range.
2. Oil Press Structure
The twin-screw oil press enables oilseeds to generate powerful axial propulsion and conveying capacity within the pressing chamber. Any oilseeds after dehulling or decortication can be pressed smoothly. Compared with single-screw oil presses, twin-screw oil presses have a higher length-to-diameter ratio and a higher theoretical compression ratio in the pressing chamber, resulting in higher oil yield and lower residual oil content in the dry cake.
The twin-screw oil press consists of components such as the main power motor, transmission device, feed inlet, feed motor and auger, forced feed motor and vertical auger, screw pressing shaft, pressing cage, discharge (cake) outlet, machine base, and oil outlet. The oil press specifications include daily processing capacities of 5 t/d, 10 t/d, and 50 t/d. Some are twin-screw spiral oil presses, suitable for cold-pressing vegetable oilseeds or for hot-pressing after steaming and roasting.
Adopting the twin-screw principle can significantly enhance the conveying screw’s propulsion capability, thereby addressing the pressing challenges posed by high-oil-content materials that are highly plastic and prone to slippage.
The twin-screw oil press is composed of the oil press base frame, main power gearbox, feed motor, horizontal feeding device, vertical forced feeding device with roller-mounted motor, cake adjustment device with adjustable cake outlet clearance, screw pressing shaft, and pressing cage assembly. The key component of the twin-screw oil press is the screw pressing shaft. Its design employs a combined pressing scheme that alternates between intermeshing and non-intermeshing types. As shown in the twin-screw oil press structure diagram in Figure 3-16, the first-stage screws on the left and right intermesh with each other, meaning the thread of one screw inserts into the thread groove of the other screw, leaving a certain clearance around it. This generates powerful axial propulsion capability for the material and easily achieves multi-stage compression and relaxation, as well as thin-layer pressing in terms of structure. The pressure and its distribution within the pressing chamber can be conveniently adjusted. The characteristics of this scheme include a strong axial propulsion force and the ability to adjust the pressure in the pressing chamber and the compression ratio of each section of the screw and cage rings.
The pressing cage and screw shaft are the heart of the oil press. Addressing the shortcomings of the single-screw oil press, namely the small length-to-diameter ratio and total theoretical compression ratio of the pressing chamber, in designing the pressing principle and structure of the twin-screw oil press, the total length of the screw shaft is increased, adopting a combined form of multiple screw sections and cage rings. By inserting multiple conical cage rings between the two screws, multi-stage compression and relaxation are achieved, increasing the total theoretical compression ratio and length-to-diameter ratio. In the main pressing section, while the screw root diameter gradually increases along the longitudinal direction of the shaft, the depth from the screw tooth crest to the root gradually decreases, realizing thin-layer pressing of the material, shortening the oil drainage path, and facilitating higher oil yield. The pressing cage is axially split, using bar rows for oil discharge, which is the same as the pressing cage structure of existing single-screw oil presses. The pressing cage and screw shaft together form the pressing chamber of the twin-screw oil press. The pressing chamber of the oil press consists of two cages with different diameters, the so-called two-stage pressing type. The outer diameter of the first-stage pressing chamber screw is larger than that of the second-stage screw. Within the two stages of the pressing chamber, the screw root diameter increases from small to large, and the screw pitch decreases from large to small, achieving the purpose of gradually reducing the pressing chamber space and increasing the compression ratio.
The two main screw shafts rotate in opposite directions. Based on force analysis and oil discharge method, the rotation direction is determined to be reverse, rotating from the inside to the outside. During operation, the two screw shafts rotate with floating automatic centering and must maintain centering rotation even under no-load conditions.
The oil press uses an end-cake discharge method. The cake pusher head and the tail shaft of the screw shaft rotate synchronously and can also achieve axial displacement. The adjustment of cake thickness is accomplished by an adjustment device composed of screws, nuts, etc. To adjust the oil press output, in addition to using a combination of horizontal and vertical augers for feeding, the horizontal auger drive motor also employs frequency-conversion control, enabling stepless adjustment of the feeding rate over a wide range.
The SSYZ50 twin-screw oil press (Figure 3-17) consists of the mechanical transmission system, screw shaft, pressing cage, feeding device, cake-discharge device, residue-discharge device, frame, and electrical control system. The mechanical transmission system primarily comprises the motor, V-belt and pulley, reducer, dual-shaft gearbox, and coupling. The screw shaft is composed of main shaft bolts and conical nuts. The pressing cage consists of a pressing cage plate, connecting plates, bar rows, and pressure plates. The pressing cage is arranged axially, and the bar rows are arranged in four sections. The feeding device mainly consists of the hopper, gate, horizontal conveying auger, vertical conveying auger, horizontal auger drive motor with cycloidal pinwheel reducer, and vertical auger drive motor with cycloidal pinwheel reducer. The horizontal auger drive motor is controlled by a frequency conversion control cabinet. The cake discharge device mainly consists of parts such as the cake discharge plate, cake pusher head, mandrel, spring, steel ball, adjustable threaded push rod, nut, and beam. During no-load operation, the two main screw shafts can still rotate concentrically. The residue discharge device mainly consists of the horizontal conveying auger, auger blade shaft bearing seat, and sleeve roller chain and sprocket. The electrical control system primarily includes strong electrical control for starting and stopping the main motor and the two motors of the feeding device, as well as weak electrical control for frequency-conversion adjustment of the horizontal auger drive motor.
Table 3-5 Technical Parameters of SSYZ50 Twin-Screw Oil Press
| Name | Quantity / Specification | Name | Quantity / Specification |
| Total screw shaft length / mm | 1800 | Production capacity (processing capacity) / (t/d) | 40~50 |
| First-stage cage inner diameter / mm | 160 | Main motor power / kW | 37~55 |
| First-stage screw outer diameter / mm | 154 | Vertical auger shaft speed / (r/min) | 60~88 |
| Second-stage cage inner diameter/mm | 140 | Vertical auger motor power / kW | 2.2 |
| Second-stage screw outer diameter / mm | 136 | Horizontal auger shaft speed / (r/min) | Frequency conversion stepless speed regulation |
| Screw shaft speed / (r/min) | 10~18 | Residue discharge auger shaft speed / (r/min) | 17.7~32.0 |
| Compression ratio | 23:1 | Overall dimensions (L×W×H) / (mm×mm×mm) | 3900×2500×1800 |
| Length-to-diameter ratio | 11:1 | ||
| Pressing time / s | 180 |
3. Production Application of Twin-Screw Oil Press
After cleaning and drying treatment, the raw material is adjusted to a suitable moisture content and fed into the twin-screw oil press for pressing. During cold pressing, the frequency converter is used to gradually increase the horizontal auger shaft speed, thereby increasing the feed rate and generating pressure within the pressing chamber. Attention is paid to changes in the main motor current of the oil press. From production observations, when dehulled peanut kernels are pressed in the twin-screw oil press, oil flows smoothly, the pressed cake has good formability, forming a cake with moderate hardness, and material propulsion is good. Due to peanuts’ high oil content, the residual oil in the cake after two pressings is 6%-8%. When equipped with solvent extraction equipment, the residual oil in the pressed cake can be increased. Peanut cake can also be crushed to suit the requirements of the solvent extraction process. After extraction and defatting, the residual oil rate in the peanut meal reaches 0.5% to 1.0%, and the crude protein content is above 46%. The extracted crude oil is refined to obtain finished mechanical peanut oil and solvent-extracted peanut oil. The cake and meal can also be crushed into peanut meal of different particle sizes. If the peanut raw material has already been dehulled, these peanut products can be used as food ingredients.
The twin-screw oil press can be used for cold-pressing peanuts, hot-pressing peanuts for oil production, and processing other high-oil-content materials for oil production.
When peanuts are cold-pressed using a twin-screw oil press, there is no need to add peanut shells as auxiliary pressing material, thus yielding better peanut kernel pressed cake. The entire process uses low-temperature pressing, with a short duration and a maximum temperature below 80°C. Under these conditions, high-quality peanut oil can be obtained. After crushing, the cold-pressed peanut cake yields a high-quality crude protein powder with 50%-60% protein, laying the foundation for further conversion into food-grade protein.
Cold-Pressing Process Flow: Peanut kernels → Cleaning and impurity removal → Conditioning → Cold pressing → Cold-pressed crude oil → Coarse filtration → Sediment removal → High-quality cold-pressed oil → High-quality peanut cake → Crushing → Peanut crude protein powder.