Pumping chocolate: It’s no easy job
Ever since its discovery in the 1500s, chocolate has arguably been the nation’s favourite vice. It quickly became the core ingredient in a multitude of confectionary products and the cornerstone of numerous business empires. Richard Charlton of AxFlow explains the philosophy behind the company’s approach to pumping this liquid gold.
The first thing to recognise is that pumping chocolate isn’t straightforward due to several factors related to its very peculiar mixture of properties. Chocolate is extremely sensitive to temperature; too hot and it will degenerate or even burn, too cold and it solidifies. Chocolate’s structure is delicate as the cocoa butter and cocoa solids can easily separate if subjected to high degrees of shear. Chocolate can be abrasive, and it will coat the internal surfaces and sealing areas of a pump. In addition, not all chocolate products are the same; they can have different viscosities and structures and may contain additional ingredients such as nuts, fruit and dairy products. And many chocolate processes require hygienic certified designs.
As a result, you need to have a range of pump types in your portfolio to ensure that you can offer a suitable and cost-effective solution for all applications. At AxFlow, we can provide this through the availability of three main pump types: vane, eccentric disc, and external circumferential piston pump.
Vane pumps
Vane pumps are normally our starting point as they usually represent the lowest-cost solution. Of course, we are not talking about any vane pump, but only those with very particular features that are not found in all designs. Designed to handle difficult liquids, this pump has been adapted to meet the specific requirements of cocoa and chocolate processing and can be found in many plants around the world.
The key feature is that these pumps do not rely on centrifugal forces to expel the vanes outwards from the rotor. This is because the pump needs to be able to run slowly in order to reduce shear as much as possible. Heating and cooling jackets must be available to keep the product in the correct state, and FDA materials must be an option. Fortunately, due to chocolate’s preponderance to coat the internal surfaces of a pump, cast iron is an option for those less demanding applications, which goes a long way to reducing costs.
The vane pump contains a number of vanes that are free to slide into and out of slots in the pump rotor. When the drive mechanism starts the rotor, centrifugal force, push rods and/or pressurised fluid causes the vanes to move outward in their slots and bear against the inner bore of the pump casing forming pumping chambers. As the rotor revolves, fluid flows into the areas between the vanes when they pass the suction port. This fluid is transported around the pump casing until it reaches the discharge port.
An influential factor in specifying pumps for chocolate processing is the requirement for constant performance, including priming and draining, over extended periods. The vane pump provides automatic adjustment of key performance characteristics, with flow rates being maintained without the need for any increase in pump rotation speeds.
However, their vanes will wear over time, they struggle with pressures of more than 12 bar, they have a limited choice of seal and they cannot be considered truly hygienic. So, at AxFlow, we have two other product types in our portfolio: external circumferential piston (ECP) pumps and eccentric disc pumps.
Although ECP and eccentric disc pump technologies comply with the requirements of 3A and EHEDG and offer lower shear pumping actions than vane pumps, they display several different characteristics so that between them AxFlow can cover the full range of chocolate transfer applications.
ECP pumps
The ECP pump, also known as the rotary piston pump, is well-established within many sectors of food processing. The operating principle employs arc-shaped rotary pistons or rotor wings that travel in annular-shaped cylinders machined in the pump body. Because the rotors produce a scooping action, they do not squeeze nor compact the medium being pumped. The ECP’s long sealing path reduces slippage and produces a smooth product flow without destructive pulses or pressure peaks and without the need for valves or complex parts. ECP pumps have the lowest NPSH requirement and work well with auger feeders should the need arise.
Eccentric disc pumps
For products that are sensitive to shear, ECP pumps cater for most requirements as they can run at speeds as low as 50 rpm. However, when it comes to products with additional ingredients such as fruit then the peristaltic action of eccentric disc pumps offer the gentlest option. This intrinsically gentler action also enables, in some circumstances, eccentric disc pumps to run at speeds faster than ECP pumps, to satisfy higher flow requirements.
The eccentric disc pump uses an oscillating eccentric piston that produces the pumping performance of a rotary positive displacement pump. However, its action is far more sensitive, making it ideally suited to applications where shear and potential leakage are major issues, as it does not agitate the pumped liquid. The eccentric movement pump consists of a cylinder and a pumping element mounted on an eccentric shaft. As the shaft rotates, the pumping element creates chambers with the cylinder, effectively increasing the size of the intake port and drawing fluid into the pumping chamber. The fluid is transported to the discharge port, where the size of the pumping chamber decreases, moving the fluid out into the discharge pipework. The pump does not require either mechanical seals or magnets, whilst providing leak-free performance.
Both types of pump are good at high pressure, but above 15 bar, it is the ECP pump that comes out on top as it possesses the ability to reach pressures of 35 bar. ECP pumps are also better at transferring viscous fluids because eccentric disc pumps are limited to about 10,000cP while ECP pumps can cope with viscosities of up to 200,000cP.
When it comes to sealing, ECP pumps are normally fitted with a special double O ring seal that utilises a food-grade lubricant between the two O rings. This seal arrangement is far superior to a mechanical seal as it doesn’t seize should hardened deposits form, nor will it burn the chocolate because it is well lubricated. Eccentric disc pumps avoid the whole sealing dilemma altogether as they are considered seal-less pumps. And if low maintenance is a particular concern, eccentric disc pumps have the fewest number of moving parts and, therefore, have the lowest maintenance requirements.
