SLIPSTREAM FILTER RECOMMENDATION
FOR HEAT TRANSFER FLUID SYSTEM
By: Gerard Bernaldo, BSME Fluid Specialist Engineer & Michael Damiani, Product Manager – MIL Spec Fluids
HTF System Filter Recommendation
For most heat transfer fluid systems, Radco recommends a slipstream filter configuration. Compared to a direct “in-line” filter system, a slipstream unit does not require complete system shutdown and cooling for general maintenance.
Since most heat transfer systems do not have a filter system installed during initial construction, residual inorganics may be present and/or high levels of carbon (or “coke”) may have accumulated over time. The initial start up of the filter unit will require continuous monitoring and frequent changing of filter cartridges/bags, making the “in-line” filter system impractical. The slipstream filter design offers an inexpensive and a maintenance- friendly alternative to scheduled fluid replacement or reprocessing. The installation of a slipstream filter unit for preventative maintenance will also save system wear and tear on system components and greatly extend the operating life of the heat transfer fluid.
Particulates in heat transfer fluids are caused either by polymerization of degradation/oxidation by-products, or are introduced during system construction and consist of inorganic material such as pipe slag, grit, sand, etc.
All heat transfer fluids degrade over time due to thermal stress. These degradation by-products, referred to by most fluid manufacturers as “high boilers” and “low boilers”, are molecular fragments formed when the heat transfer fluid’s molecular bonds are broken. These fragments can polymerize with other fragments, creating extremely large molecules which in turn lead to the formation of high molecular weight carbon particulates. These particulates are commonly referred to “coke” or “sludge”.
High and low boilers, carbon particulates, and inorganics have very little of the heat transfer efficiency of the original heat transfer fluid. The presence of high levels of carbonized and inorganic material can cause mechanical problems in the system such as seal and gasket leakage and/or failure, heater tube fouling (decreasing the amount of BTUs transferred to the fluid), and increased pressure drop (due to coking on pipe surfaces). Carbonization in the fluid also specifically affects the fluid’s heat transfer properties such as viscosity and density, lowering the heat transfer coefficient. Therefore, the elimination and/or reduction of carbon and organics in the heat transfer fluid will lead to longer periods of time between fluid change-outs, higher fluid efficiency, and decreased downtime required for system maintenance.
Filter Piping Design/Placement
A basic slipstream filter design essentially is a side loop to and from the main heat transfer fluid system return line containing the filter unit. The filter unit (or housing) contains either cartridge-type filter elements or a bag-type filter element. The filter is isolated from the return line by two gate valves, one between the return line and the intake side of the filter, and the other between the downstream side of the filter and the main return line.
Located on the main return line, between the filter loop gate valves is a pressure gauge and a throttle valve. A typical filter housing with clean cartridges/bag will create approximately 5 psi system pressure drop. Most “dirty” filter elements create close to 20 psi pressure drop. To maintain consistent fluid flow rates, set the throttle valve to the acceptable level of pressure drop (in this case 20 psi). The filter element should be replaced before the return line pressure gauge reads the maximum acceptable level of pressure, assuring adequate flow through the filter.
The slipstream filter unit should be located between the main circulating pump and the heater. This placement will assure adequate pressure and consistent fluid flow through the filter unit. The bulk fluid temperature will be at the lowest temperature at this location, decreasing cooling time required before the filter elements can be replaced.
Piping to and from the filter unit should be consistent with the intake and discharge of the filter housing. Most filter housings designed for use in liquid phase high temperature (400oF-600oF) heat transfer systems utilize 1 1/2″ or 2″ flanges.
The slipstream unit should be designed to handle from 1% – 5% of the main return line flow. The filter housing and the filter elements should be capable of handling the appropriate system’s maximum bulk fluid temperature, even though actual fluid temperature on the pre-heater, post-heat exchanger will be substantially lower. This will assure adequate filter housing/element safety should a temperature excursion occur. We recommend that initially cartridges or bags of 150-200 micron size should be used, gradually reducing to 10-20 microns as conditions permit.
All heat transfer fluids degrade over time due to thermal stress. These degradation by-products, referred to by most fluid manufacturers as “high boilers” and “low boilers”, are molecular fragments formed when the heat transfer fluid’s molecular bonds are broken. These fragments can polymerize with other fragments, creating extremely large molecules which in turn lead to the formation of high molecular weight carbon particulates. These particulates are commonly referred to “coke” or “sludge”.
High and low boilers, carbon particulates, and inorganics have very little of the heat transfer efficiency of the original heat transfer fluid. The presence of high levels of carbonized and inorganic material can cause mechanical problems in the system such as seal and gasket leakage and/or failure, heater tube fouling (decreasing the amount of BTUs transferred to the fluid), and increased pressure drop (due to coking on pipe surfaces). Carbonization in the fluid also specifically affects the fluid’s heat transfer properties such as viscosity and density, lowering the heat transfer coefficient. Therefore, the elimination and/or reduction of carbon and organics in the heat transfer fluid will lead to longer periods of time between fluid change-outs, higher fluid efficiency, and decreased downtime required for system maintenance.
