Side-Stream or Full-Flow Filtration
As with any good system design, you must initially state your objectives for the filtration system. Assume any filtration system you install will fulfill these basic requirements:
Now that you know your objectives, the first question to ask is whether the filtration will be a full-flow unit or a side-stream system. Either method will improve the quality of the heat transfer fluid and prevent premature fluid degradation, but which is best for your application?
In weighing these two methods, you first should understand the true meaning of each one. In a full-flow system, 100 percent of the fluid flow is filtered somewhere in the system. Most of us use a full-flow filtration system everyday without even realizing it. The fuel filter in your car is an example of a full-flow filtration system.
By contrast, in a side-stream filtration system, approximately 10 percent to 20 percent of the fluid flow is filtered at any given time. An example of a side-stream, or bypass, system would be the beltways around cities.
Now that you have an understanding of the two options, you next need to determine if these options meet your basic requirements.
In examining a full-flow system and its operation, the automobile fuel system is a good correlation but not exactly reflective of a heating or cooling system. In the automotive system, the fuel tank is our fluid reservoir, which in process applications may or may not be open to the atmosphere. The gas is pulled from the tank by a fuel pump (figure 1), then pushed through the fuel filter and finally into the cylinder. (This is a vague description, as engines have changed over time, and I gave up working on cars once everything went to computer-controlled and fuel-injection systems.) Just like process heating and cooling systems, this is both an open and closed system. It's open whenever you stop for fuel and closed when you are driving.
The next part of the system is the filter. In the automotive example, all of the fuel (100 percent flow) is filtered through this critical unit. This filter must allow for full fluid-system flow, and it must be capable of holding all of the contaminants it has filtered out of the system.
Fluid flow rates in process heating and cooling systems can be a few gallons or hundreds of gallons a minute. Remember that one of the objectives was not to interfere with the operation of the system. In a heating/cooling system with high fluid flow rates, a full-flow filter may have limitations such as:
Because of these restrictions, a full-flow filtration system may not be best for continuously operating thermal fluid heating systems. However, if the heating system is routinely shut down, the filter can be cleaned to avoid any of these issues. Typically, basket strainers and sock-style filtration units are used in this way. One of the best applications of this style of filter is during commissioning of a system to remove fabrication debris, mill scale, weld spotter and slag.
Other issues to acknowledge for a full-flow system are that they tend to be very large in order to accommodate fluid flow rates. Also, without the proper monitoring equipment, a decrease in flow rates may go unnoticed. A decrease in fluid flow rate can affect numerous components of the system. For example, a flow-rate decrease could increase the pressure on the pump discharge and subject the pump to more torque until the pressure relief valve is activated. The fluid might not flow through the heat exchanger correctly, resulting in a decrease of heat transferred. Also, the fluid film temperature could increase in the heating unit beyond design limitations and cause thermal cracking of the fluid. Lastly and most importantly, with a full-flow filtration system, the system must be offline in order to clean the filter.
One possible response to these issues would be to install a valve around the filter (figure 2). This would definitely be a step in the right direction as it would allow for maintaining fluid flow and limit the pressure drop caused by the filter. Better to have unfiltered flow than no flow, right? However, you are still limited to the size of particles that can be filtered from the system. In effect, what you have done is to create a simple bypass filtration unit.
Like the fuel filter example, a beltway around the city is a close comparison to a heat transfer system with a bypass filtration system (table 1). Much like the beltway, the filter bypass yields certain benefits. If you compare the filter to the most direct route through the city, the fluid could take bypass whenever the filter is blocked. Also, if the filter is clogged, the bypass allows full fluid flow. Using bypass filtration, users can filter to levels as fine as 1 micron and not restrict flow. The pressure drop due to the filter is minimal. The heat loss is still a potential problem, but you can address this by placing the filter on the return side.
In some cases, a combination of these two methods is employed (figure 3). In a new system, it may be practical to install a basket strainer to remove the larger particles for startup, then remove the basket and let the side-stream filter remove the coke, sludge and other finer particles created during operation. It may be that a combination approach is best for your application.
Most heating and cooling operations are unique in their design and operation. This analysis is generic to any system. The principles are the same and desired result is always to make the heating system more reliable and efficient.
The selection of the filtration system will depend on many more aspects of the application. For example, is this a new or existing system? In a new system, space can be made to accommodate the filtration system and allow for easy access. An existing system may require the filtration system to fit into available space. Price and budgets are always considered. The lowest price may seem ideal, but at what latent and seldom-seen expense, can it do the job? The highest price unit may filter beyond your necessity. Also, the placement of the filter in the system will affect its operation and performance as well as how maintenance is accomplished.
There are many good resources available to aid in selection, placement and operation of a quality filtration system. One of the first places to turn for assistance would be the fluid manufacturer, who can test your fluid. This will give you a good idea of what is in the system and what needs to be taken out. It can also yield information on performance of other components. PH