Introduction
If you have been involved with the sampling of bulk materials in some capacity, you have likely encountered the option of using a cross belt style sampler or a falling stream style sampler — particularly at the primary stage of sampling. This topic often transitions into a spirited discussion about which type of sampler is “better”, and frequently, there are some strongly held beliefs on either side of the conversation.
Instead of debating the specifics of each machine in an effort to establish which is better, it is likely more suitable (and productive) to frame the conversation in terms of what equipment better suits the requirements of a particular application — not necessarily which is simply “better” in general terms.
Cross Belt Sampler in a concrete sand application.
From the standpoint of a manufacturer that produces both cross belt and various falling stream type samplers, this issue really is not a matter of which type of sampler is “better”.
As a manufacturer, the goal is to develop the best sampling solution for a client once all of the particular nuances associated with their application have been considered. It is not really about the better type of machine — it is about providing the best solution to a sampling problem and then properly applying the equipment necessary to implement that solution.
Each sampling application is a puzzle to solve. The various types of sampling equipment available are simply pieces of the puzzle. There is flexibility in customizing the pieces to some extent, but the correct pieces located in the correct places create the overall picture of what the sampling solution should look like. The key is knowing where and how to locate each puzzle piece – or how to correctly apply sampling equipment to develop an effective sampling system.
Falling Stream Sampler in a coal application.
In terms of Cross Belt Samplers and Falling Stream Samplers specifically, both types of sampler have benefits and drawbacks to their use. The key is to appropriately match the benefits associated with a particular type of sampler to an application while minimizing the effects of any potential drawbacks.
It is worth noting that the ideal state for all the various considerations for a sampling system cannot always be attained, so we must know when and where to compromise to achieve the best possible outcome for a particular application.
Cross Belt Samplers
The Cross Belt Sampler typically is applied where a requirement for a reasonably accurate sampling solution combines with the need for cost effective, easily installed equipment with a relatively small installation envelope.
Cross Belt Samplers tend to get a bad rap with respect to increment collection. Specifically, the argument is that complete increments are not collected, particularly when fine particle sized materials are involved.
In reality, a properly designed Cross Belt Sampler with a conveyor belt support and contouring system (e.g. McLanahan Cut Zone) can do a good job of increment extraction when proper adjustment is maintained.
McLanahan Cut Zone System
This does not mean that it should be the first choice for sampler type in a high-precision application involving fine particle sized material. It simply means that if your application involves some fine material, it is possible to achieve good sampling results with a Cross Belt Sampler.
Other items that factor into using a Cross Belt Sampler involve system design, layout, and installation.
Cross Belt Samplers will typically collect a smaller increment mass than a Falling Stream Sampler given the same application. Therefore, samples collected by a primary sampler can often be processed using fewer pieces of equipment, often resulting in simpler system layouts. Less material usually means the equipment that is used can be sized smaller as well.
Cross Belt Samplers and Cross Belt Sampler-based systems also tend to require a smaller installation envelope (particularly height), which can help if space available for a sampling system installation is limited.
Cross Belt Samplers are relatively easy to install. They are typically supported by the existing conveyor structure and install around the conveyor belt without interfering with the conveyor belt. Smaller sized, standard units are often installed in a day or less.
For completeness, we will touch on maintenance. Cross Belt Samplers tend to have greater maintenance requirements than Falling Stream Samplers. This is primarily seen in adjustments to things like cutter wipers, belt seals and brakes.
Generally, some fugitive material will escape during normal operation, so some amount of housekeeping is sometimes required as well.
Cross Belt Samplers tend to be more commonly applied in industries like coal (mining, power generation), coke production, aggregates and industrial sand. That said, you will find Cross Belt Samplers in nearly every bulk material related industry, handling just about any material that you can think of, including metal bearing ores, concentrates, potash, bauxite, biomass, etc.
Falling Stream Samplers
Falling stream type samplers are difficult to beat in terms of simplicity of operation, effectiveness, reliability and maintenance requirements.
Why then, doesn’t everyone use falling stream type samplers?
This would be largely due to higher total costs, difficulties with system layout (e.g. lack of height), more invasive installation and the realization that, in many cases, acceptable results can be achieved with less expensive sampling options.
It would be difficult to argue the point that falling stream style samplers are capable of collecting samples with high levels of both accuracy and precision. This is primarily due to fact that it is easier to collect a complete increment, particularly with fine particle sized materials, with a correctly designed Falling Stream Sampler.
It is important to note here that a correctly designed falling stream type sampler considers the possibility of collecting any part of the lot being handled.
For example, if a falling stream type sampler is installed at the head end of a conveyor, but the cutter is not designed to capture fine material that is removed from the conveyor belt with a belt scraper, an important part of the lot has a zero probability of being sampled.
As the belt scrapings are generally of fine particle size and are often moist material, excluding this type of material can result in significant issues with sampling results.
Additionally, if the fine material removed by a belt scraper is not sampled, one of the key reasons for applying this type of sampler is effectively nullified.
The function of falling stream type samplers is usually simple and much less abusive in nature than a Cross Belt Sampler. Simple operation with less impact during sample collection, coupled with the fact that fugitive material is less of a concern with Falling Stream Samplers, generally lend these types of machines to lower maintenance requirements, which in turn can mean higher availability.
Falling Stream Samplers also tend to have a longer overall service life – again, due to the less abusive nature of sample collection with these machines.
Falling Stream Samplers collect a larger increment mass, which may or may not be a problem from a sampling standpoint, but it does often require that more equipment is incorporated into a system in order to produce a final save sample of reasonable size for laboratory processing.
Larger increment masses result in higher flow rates, which usually results in larger equipment selections – at least in the initial sampling stage(s).
Another interesting consideration is related to high-speed conveyors handling high flow rates. Falling Stream Samplers are typically better suited to reliably collecting consistent samples in these difficult applications.
From a layout perspective, Falling Stream Samplers will require more vertical clearance for installation and often will require modifications to existing structures and/or the discharge hood of the process conveyor on which it is installed.
Additionally, some clearance on either side of the conveyor head pulley is needed to park the sample cutter out of the material flow and to allow an appropriate acceleration or deceleration distance at either end of the cutter travel.
Falling Stream Samplers are the “original” style of sampler, and you can find some version of these machines in nearly every industry handling just about any material you can think of. Needless to say, they are versatile and widely accepted.
So, Cross Belt or Falling Stream?
Effective sampling solutions can be designed and implemented using either cross belt type samplers or falling stream type samplers as long as either type of sampler is correctly designed and applied.
There are many examples of successful sampling installations that incorporate either, or in some cases, both types of sampler.
In some situations, it is very clear as to which type of sampler would be best suited to a particular application.
In other situations the equipment selection is not quite as apparent, but when the complete set of circumstances involved with a particular application are considered, an appropriate machine selection will usually become apparent.
That said, situations still do exist where it is difficult to determine which type of sampler is “better” or “best”, and oftentimes the concept of “better” or “best” truly does fall back to the opinions of the people involved.
In these situations, it is likely best to focus on the less subjective idea of what is “better” or “best” for an application and/or customer, as opposed to a more subjective debate related to specific equipment types.
Regardless, a good course of action would be to discuss your application with a sampling equipment manufacturer and experienced sampling professionals to work together in developing a sampling solution that best suits your needs and to offer further guidance in successfully implementing that solution.
