10-04-2017, 08:59 PM
ABSTRACT
TOPIC : HELICALLY COILED HEAT EXCHANGERS
Process heat transfer with conventional shell and tube heat exchanger
is familiar to many engineers in many industries.Their use and performance is well-dominated. However, helically coiled heat exchangers, although they have been around for many years, are not well known.
Although various configurations are available, the basic and most common design consists of a series of stacked helically coiled tubes. The tube ends are connected to manifolds, which act as fluid entry and exit locations. The tube bundle is constructed of a number of tubes stacked atop each other, and the entire bundle is placed inside a casing, or shell.
True counter-current flow fully utilizes available LMTD (logarithmic mean temperature difference). Helical geometry permits handling of high temperatures and extreme temperature differentials without high induced stresses or costly expansion joints. High-pressure capability and the ability to fully clean the service-fluid flow area add to the exchanger s advantages.
This concept is a good alternative for ordinary shell-and-tube heat exchangers when it comes to fouling. This is mainly because the curved form of the channels will create turbulence at any point in the flow, even with low velocities. The same curved form causes high shear rates at the walls. These two effects can prevent particles from clinging to the wall. The second reason lies in the fact that the helical heat exchanger is just single-channeled.
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ADVANTAGES
Helically coiled exchangers offer certain advantages. Compact size provides a distinct benefit. Higher film coefficients the rate at which heat is transferred through a wall from one fluid to another and more effective use of available pressure drop result in efficient and less-expensive designs. True counter-current flow fully utilizes available LMTD (logarithmic mean temperature difference). Helical geometry permits handling of high temperatures and extreme temperature differentials without high induced stresses or costly expansion joints. High-pressure capability and the ability to fully clean the service-fluid flow area add to the exchanger s advantages.
This concept is a good alternative for ordinary shell-and-tube heat exchangers when it comes to fouling. This is mainly because the curved form of the channels will create turbulence at any point in the flow, even with low velocities. The same curved form causes high shear rates at the walls. These two effects can prevent particles from clinging to the wall. The second reason lies in the fact that the helical heat exchanger is just single-channeled. In ordinary multiple-channel heat exchangers, when some foulant does manage to stick to the wall of a channel, the flow is restricted in that channel and will divert to less fouled channels. The velocity in the fouled channel will thereby be reduced, causing even more foulant attachment to the walls. In helical heat exchangers, in contrast, there is only one channel, so when some foulant does attach, the flow still has to go through. The velocity will locally increase, as will the shear rate, thereby removing the foulant again.
CONSTRUCTION
Although various configurations are available, the basic and most common design consists of a series of stacked helically coiled tubes. The tube ends are connected to manifolds, which act as fluid entry and exit locations. The tube bundle is constructed of a number of tubes stacked atop each other, and the entire bundle is placed inside a casing, or shell (Fig. 1.)
FLOW PATH
Once positioned in the casing, the assembly forms a helical flow path for both the casing and tubeside fluid. Standard units have manifold and casing gaskets, although complete welded designs are available for high casing-pressure service or hazardous applications. To effectively optimize thermal and hydraulic requirements, the number of tubes (coils) along with their spacing and length may be varied. This allows a design to meet the thermal and hydrodynamic requirements of both the casing and tubeside fluids.
Other available configurations are:
Multiple pass design, which increases tubeside velocity, thereby increasing the heat transfer rate. With this configuration, there is an increase in tubeside pressure drop.
Vaporizer design for liquid vaporization and droplet disengagement (Fig. 2).
Condenser design, which comes in three typical configurations.
Each depends on the process and vessel s discharge connection (Figs. 3a, 3b, 3c).
Weld-seal designs for completely welded units often are specified when cross-contamination must be prevented, or fluids are hazardous or incompatible with gasket materials (Fig. 4). As with any heat exchanger, the flow rate, allowable pressure, physical properties of the fluid, and construction material control final design.
No matter the design, helical heat exchangers offer several distinct advantages to the user. High film coefficients are achieved on both the coil and casing side. The helical flow path imparts higher shear rates and turbulence at a given pressure drop, which can result in film coefficients up to 40% higher than those achieved with many comparable shell and tube units. Departure from laminar floss and fully developed turbulent flow occur at lower Reynolds numbers. The 100% counter-current flow allows full use of the available LMTD and makes temperature cross when the hotside outlet temperature is cooled below the coolside outlet temperature possible without multiple units in series. The flow geometry of a helical unit is such that a temperature cross is managed within a single unit. This is possible because 100% counter-current flow permits closer temperature approaches and improved heat recovery. Cleaning the casing-side flow area is easily managed. The casing can be unbolted and the entire bundle assembly removed for cleaning, inspection or replacement. Fully welded designs (weldseal) though, do not have this feature. The coil arrangement s compactness also provides advantages, because the exchanger requires minimal floor space. The heat exchanger s spring-like coil eliminates thermal expansion and thermal shock problems that often occur during startup or during cryogenic or high-temperature service. High operating pressures are easily handled on the coilside, and without the need for the tube sheet of a shell and tube unit, required thicknesses are minimal, even at high operating pressures. Pressures exceeding 10,000 psi are possible.
The high shear stresses and induced turbulence of helically coiled exchangers reduce the tendency for fouling. This results in longer operating cycles between scheduled cleaning intervals. Additionally, the lower fouling tendency permits the use of less conservative safety margins at the initial design Stage. Conventional designs allow casing-side access for cleaning and inspection. Economical unit selection is possible due to approved film coefficients, full use of available LMTD and minimal required thicknesses.
Economical unit selection is possible due to approved film coefficients, full use of available LMTD and minimal required thicknesses.
FOULING
Fouling of heat transfer surfaces is a major source of uncertainty in the design of heat exchangers processing many fluids, and a particular problem in industries where the fluids being processed are either dirty or reactive.
Fouling is a general term which includes any extraneous deposits of extraneous material that appears on the heat transfer surface during the life of the heat exchanger. Irrespective of cause or nature of the deposit an extra resistance to heat flow occurs due to fouling. In certain cases the deposit can be heavy enough to clog the interfere with fluid flow and increase the pressure drop required to maintain flow rate through the heat exchanger.
The importance of fouling
.
First of all, fouling costs can be separated according to how they are generated. Roughly taken,there are four types of costs:
1) Additional capital costs or costs for special design considerations
Lots of costs in using heat exchangers can be prevented in the R&D departments of a
company. Especially when it comes to fouling. A good design can reduce the effects of
fouling and thereby the operational costs of the heat exchanger. But of course research
and design costs money.
A way to prevent fouling is to choose a bigger heat transferring surface then needed, as
discussed before. The heat exchanger will become bigger and heavier, and thereby also
more expensive.
2) Energy costs
A heat exchanger that suffers from fouling needs additional energy to keep operating at
the same level. This is because the fouling layer decreases the amount of heat transferred
as well as it increases the amount of pressure drop needed to maintain the same
throughput through the smaller cross-section. All this additional energy is pure loss.
3) Maintenance costs
A fouled heat changer has to be cleaned once in while, in order to keep the energy
needed for operation low. This cleaning can be online or offline, mechanical or
chemical, etc. Sometimes it s needed to replace some parts of the heat exchanger, for
instance because of corrosion.
4) Costs of loss production or shutdown costs
When a heat exchanger is cleaned or maintained offline, there is no production. No
production means no income, so this is considered a loss. The effect of this shutdown
depends on the normal plant capacity and the length of the shutdown.
LOSSES DUE TO FOULING
Increased maintenance costs
Over sizing and/or redundant (stand-by)equipment
Special materials and/or design considerations
Added cost of cleaning equipment &chemicals
Hazardous cleaning solution disposal
Reduced service life and added energy costs
Increased costs of environmental regulations
Loss of plant capacity and/or efficiency
Loss of waste heat recovery options
LOSSES DUE TO FOULING
Precipitation / Crystallization - dissolved inorganic salts with inverse solubility characteristics.
Particulate / Sedimentation - suspended solids, insoluble corrosion products, sand, silt.
Chemical Reaction - common in petroleum refining and polymer production.
Corrosion - material reacts with fluid to form corrosion products, which attach to the heat transfer surface to form nucleation sites.
Biological - initially micro-fouling, usually followed by macro-fouling.
Solidification - ice formation, paraffin waxes.
FOULING MITIGATION
One of the major indeterminates in the design as well as operation of heat exchangers is the rate of fouling a select heat exchanger geometry would exhibit over the operation cycles. Gradual deterioration of heat exchanger performance due to the accumulation of fouling film on the heat transfer surface is often accounted for in the form of a fouling resistance, or commonly known as the fouling factor, while
determining the heat transfer surface required for a specific heat duty. More often, the fouling mechanism responsible for the deterioration of heat exchanger performance is flow-velocity dependent. Maldistribution of flow, wakes and eddies caused by poor heat exchanger geometry can have detrimental effect on heat exchanger performance and reliability. Helixchanger heat exchangers have demonstrated significant improvements in the fouling behavior of heat exchangers in operation. In a Helixchanger heat exchanger, the quadrant shaped shellside baffle plates are arranged at an angle to the tube axis creating a helical flow pattern on the shellside. Uniform velocities and near plug flow conditions achieved in a Helixchanger heat exchanger, provide low fouling characteristics, offering longer heat exchanger runlengths between scheduled cleaning of tube bundles. This article demonstrates the Helixchanger heat exchanger option in reducing the velocity-dependent fouling in heat exchangers.
FLOW INDUCED VIBRATION
Excitation mechanisms for flow induced vibrations are
a) Fluid elastic stability
Flow across tubes produces a combination of drag and lift. Vibration occurs when fluid velocity is above a certain critical value.
b) Vortex shedding
Vortex shedding is the principal excitation mechanism for flow-induced vibration in cross flow, producing alternating forces, which occur more frequently if the flow velocity is increased. If vortex frequency and one of the tube frequencies differ by more than 20\%
c) Acoustic vibration
Acoustic vibration only occurs when the shell-side fluid is a vapor or a gas. Two types of frequencies can be associated with acoustic vibration: Acoustic frequency of the heat exchanger and the acoustic wake shedding frequency of the tube bundles
d) Turbulence buffeting
Turbulent flow contains a wide spectrum of frequencies distributed around a central dominant frequency. This frequency increases as the cross flow velocity increases. The forces associated with turbulence buffeting are motion dependent (Fluid Structure Interaction problem)
LIMITATIONS
There are very few limitations for the use of helically coiled heat exchangers. Generally, a pressure limit of 10,000 psig covers the majority of applications. Temperature limits are determined by construction materials, as are the corrosion rates. Surface areas of 1 to 650 sq. ft. are available, and using units in series or parallel may extend this range substantially.
APPLICATIONS
The use of helically coiled exchangers continues to increase. Applications include liquid heating/cooling, steam heaters, vaporizers, cryogenic cooling and vent condensing. Listed below are the details for standard services in which helical exchangers warrant consideration.
Sample Cooling
Analyzer Pre-cooling
Seal Coolers
Condensers
Cryogenic Vaporizers
Compressor Inter- and After-Coolers
General Applications
APPLICATION SPECIFICS
Sample Cooling. Continuous monitoring of process output is necessary to ensure product quality within allowable tolerance. Grabsample cooling (Fig. 5) is needed prior to transport to lab technicians for analysis, so an inexpensive compact unit is needed to efficiently cool sample streams to desirable levels. The helical coil design, with definitive flow path for both sample and cooling water, provides a counter current flow design of high efficiency and predictable close-temperature approach. Typical sampling locations are boiler steam, distillation column overheads, reboiler bottoms, condensate drums, distillation column cut-points and de-aerators. Boiler steam sampling is often at high pressures and coiled tube units are not affected by these operating requirements.
Analyzer Pre-cooling. Many components are processed as liquids at, or near, their boiling points. As the liquid passes through a measuring device there is a loss of pressure, which causes the liquid to flash or boil. Measurement devices lose accuracy when handling two-phase flow. Traditional volumetric or velocity measuring instruments introduce accuracy uncertainties when measuring a two-phase fluid. From the process viewpoint, this is a major problem Typical liquids, such as ammonia, carbon dioxide, sulfur dioxide, freon and ethylene are processed at their boiling point. Pre-cooling the liquid prior to an analyzer may be necessary to ensure that flashing does not occur and measurement accuracy is not compromised. Many plants install a compact helical heat exchanger in their measurement packages for the purpose of pre cooling the saturated liquid prior to measuring. By pre-cooling prior to measuring, the resulting pressure drop across the measuring device does not result in flashing and two-phase flow.
Seal Coolers. Centrifugal pumps require cooling of their mechanical seals to ensure reasonable mechanical seal service life. The pump seal liquid absorbs heat generated at the mechanical seal to shaft contact surface, and this heat must be removed. Compact helical heat exchangers have long been specified for this service, because they can efficiently reject heat absorbed the seal liquid and economically handle the often high operating pressures of a centrifugal pump. Also, the helical coil may be mounted so it operates as a thermosyphon and thereby complies with the stringent requirements of American Petroleum Institute (API) 682.
Similarly, helical coil units are used as seal coolers for liquid vacuum pumps by removing the heat absorbed by the liquid ring a result of heat generated by the pump itself and the heat released by condensing vapors during their compression.
Condensers. Helical coil heat exchangers often are used as condensers within a process loop, such as reflux condensers or as discharge vent condensers at the end of a process. The coil configuration allows for insertion of the tube bundle directly into a distillation tower, storage tank or reactor. This allows vapors flowing up a vessel to contact the cooled tube bundle and condense. The condensate that refluxes directly back into the tower may be directed elsewhere. With the bundle inserted directly into the vessel, not only is there a benefit from direct reflux, but also there is the elimination of overhead piping and support structures.
Vent condensers can control emissions from a process, reactor or storage tank by recovering product and reducing atmospheric emission. Coolants such as liquid nitrogen, chilled methanol, ethylene glycol or brine solutions often are used to provide a cooling media that is sufficiently cool to maximize reclamation of vapors. The coolant allows for greater recovery, of the product with minimal loss to the environment. The coil geometry, with its constant change in flow direction,
forces warm vapor to contact the cold tube wall. This results in vapor condensation and product recovery. The improved condensation efficiency coupled with 100% counter-current flow allows maximum recovery with minimum surface area.
Compressor Inter- and After-Coolers. Multi-stage gas-compression packages require intercoolers between compression stages as well as an after-cooler following final compression. These are rigorous duties for any heat exchanger. The compactness of a helical coil unit minimizes package size and layout. The ability to economically
handle several thousand psi operating pressure at final compression is perhaps a helical coil s best attribute.
General Applications. The applications discussed above are the more common applications for helical coil heat exchangers, but their use in industry is far greater and more diverse. Helical oil units commonly are used as interchangers, preheaters, steam condensers, vacuum system inter- and after-condensers, reboilers, batch heaters and/or coolers and reactor-jacket coolers. Helical coil heat exchangers offer distinct advantages, such as improved thermal efficiency, compactness, easy maintenance and lower installed cost. When an application requires equipment suitable for high operating pressure and/or extreme temperature gradients, a helical coil unit should be considered. The exchangers also are suitable for less demanding applications, such as heat recovery, condensing, boiling and basic heat exchange.