Proper vacuum receiver selection impacts central system operation.
* Types of receivers
* Basic receiver features
* How receivers operate
* Troubleshooting procedures
Vacuum receivers, also called vacuum chambers, vacuum stations, or just stations, are integral parts of a central conveying system. Think of them as intermediate holding points for resins conveyed by vacuum. Vacuum receivers have one primary job: separate material from air. They accept resin from a source that is pulled by vacuum, separate the material from the air flow and transfer that resin to a destination.
Vacuum receivers come in several designs with all sorts of features aimed at offering a variety of advantages in conveyance, maintenance, and ease of use. Some receivers are designed for pellets, some for regrind, and others for powders.
Some receivers mount atop hoppers or bins; others are mounted directly to the throat of a processing machine.
All receivers have a sequencing device. Some receivers require a separate sequencing valve to connect to the vacuum line; others have a built-in fill valve. The sequencing device allows vacuum to pull material into the receiver and while the bulk of the material falls to the bottom of the receiver, the vacuum pulls the air up through the top of the receiver and back to the vacuum pump.
Most dust and fines are trapped by a filter.
Receivers come in various capacities, from small (1.5 lb.), to medium (75 lb.), to large (114 lb. and more). The multitude of designs, capacities, and feature sets is what makes it possible to specify a vacuum receiver that perfectly matches the needs of a plastics conveying system.
MAJOR PARTS OF A RECEIVER
1 Receiver lid--allows access to the receiver chamber. Access is necessary to clean and/or change filters, if present, and perform other maintenance within the receiver. The lid can be hinged or completely removable, and typically can be rotated to the orientation required by the layout of the conveying system.
2 Vacuum breaker or sequencing valve--at each receiver directs vacuum to that receiver. The vacuum draws resin through the conveying lines to the vacuum receiver. Sequencing valves can be either a "T" valve located above the receiver in the vacuum line or an external fill valve (EFV) located right on the receiver lid.
3 Receiver chamber or body--is usually a cylinder connected to cone-shaped lower section--that collects resin from the conveying system before discharge. The slant of the cone helps material slide down to the discharge valve.
4 Machine-mounted receivers--typically have a glass hopper and are connected directly to the throat of the process machine. often referred to as J.I.T. receivers.
5 Filter or some type of filter-less system --separates powders, dust, fines, and other contaminates from the resin entering the receiver. The filter-less receiver shown below uses a cyclonic action to separate material from the air flow. The material is heavier than air so it falls into the body of the receiver while vacuum and cyclonic action pulls the air into the vacuum stream.
6 Material level switch--detects the amount of resin in the machine-mount receiver and controls the discharge of the resin. The mechanism to detect the level of material in the receiver can range from simple, non-automated devices to sophisticated electronically controlled sensors.
7 Discharge valve--allows collected resin exit the receiver chamber. The resin can be dispensed to a hopper or some other type of container. Machine-mount receivers typically gravity-feed directly to the throat of the process machine
RECEIVER FEATURES TO CONSIDER
* Receiver selection is determined by the flow rate required and the type and bulk density of the material transferred.
* Large receivers are better than small ones; their increased capacity reduces the number of fill-cycle times, which reduces the wear and tear on pumps and valves. Also, large receivers are better suited for the increased capacity of today's vacuum pumps.
* Powder receivers are used for powders (such as PVC) and dusty regrind. These receivers include special filters and blowback systems to avoid clogging and to maintain energy efficiency.
* Machine-mount (JIT) receivers are used on process machines.
* Stainless steel is the most durable material for most applications.
* Receivers for operations with abrasive materials require custom options, such as thick-walled wear plates, electro-less nickel plating, and interior ceramic coatings. (See Delay Abrasive Wear As Long As Possible)
* Receivers having a tangential inlet with flapper and gasket exhibit minimal material degradation while also eliminating the damaging effects of direct impact on receiver bodies.
* The discharge valve should allow material to exit the receiver quickly. Larger-diameter discharge outlets are preferred for regrinds or difficult-to-flow materials.
* Receiver and related connections must be air and leak-tight. Neoprene seals on a dump throat ensure against vacuum loss.
* "No-tools access" in receivers makes cleanout and other maintenance easy and fast.
* Machine-mount receivers typically come with height-adjustable photoelectric level sensors.
Receivers are designed to operate within the specifications of the conveying system.
That means the receiver must be matched to the line size of the conveying system (tubing diameter and vacuum pump), controller voltage and both the throughput (capacity) and conveying distance. A system may have many vacuum receivers. The limiting factor is the amount of vacuum which--is provided by one or more central vacuum pumps.
Problem: Poor or No Conveying
1 SEQUENCING VALVE OPERATION
Vacuum "T" valves or a built-in sequencing valve isolates the vacuum conveying power of the pump to one receiver at a time for conveying. Each valve in the system must close off air flow when it is NOT in operation, allowing other receivers to receive full vacuum. One 'stuck' valve can ruin the vacuum supply for the entire system. Check that each valve operates in response to its receiver's turn in the vacuum system. Each valve should open for loading and close when loading is complete. The extended shaft of the T valve's cylinder is a good indication of valve operation. Where the valve is built into the receiver lid, the valve operation can viewed through the vent or there may be an indicator light.
2 RECEIVER DISCHARGE FLAPPER STUCK OPEN
If the flapper valve is stuck open or does not fully close, conveying cannot take place. A problem receiver can be easily checked for proper, free movement of its flapper valve:
* If conveying is not triggered when the flapped is closed, there is an issue with the electrical demand switch.
* If the flapper does not swing nearly shut by its own weight, there is a pivot point (hinge) or counterweight issue.
* If the flapper is 'stuck' in the open position, there is a material contamination issue with the pivot point (hinge) of the valve.
3 INLET CHECK VALVE STUCK OPEN
Many receivers are equipped with swinging check valves on their material inlets. A check valve that is stuck open, either by hinge wear or a trapped pellet, will leak valuable vacuum air, decreasing vacuum capability at other receivers or even preventing conveying throughout the system.
Rule of thumb: On systems that convey material from one source to multiple receivers via a common material line; If only one receiver in the system conveys correctly, the problem is probably at THAT receiver.
CONVEYING CONTROLS NOT PROPERLY (RE) PROGRAMMED
Central material conveying systems that include a network of pumps, receivers and material sources provide high efficiency and a multitude of flexibility. But often, new requirements are not completely programmed after material or system configuration changes. Items to check:
* Is the new material source further away than the previous source? More conveying time and/or purge time might be required to accommodate this difference in distance.
* Is the new material as free-flowing as the last material? Does the material have a tendency to clog the conveying lines, or simply convey slower due to weight or shape? Changes to load/purge times as well as material pick-up tube changes may be required.
* Has the receiver been assigned to the proper vacuum pump? The proper material valve?
* Has system piping and or wiring been modified to accommodate this new configuration for conveying?
CONFIRM VACUUM BREAKER OPERATION (See System Vacuum Breaker Valve--pg.40)
CHECK THE MATERIAL SOURCE (See Material Source Issues--pg.40)
FEED TUBE/TAKE-OFF BOX AIR SETTINGS (See Feed Tube/Take-pff Issues--pg.40)