Troubleshooting Steam Heat Exchangers
30 Maret 2011 15:42
When a heat exchanger "stalls," condensate floods the steam space and causes a variety of problems within the exchanger:
Control hunting: As condensate backs up in the exchanger, the heat transfer rate to the process is significantly decreased. The control valve opens wide enough to allow flow into the exchanger. As condensate drains out, the steam space is now greater and the steam pressure increases. The process overheats, the control valve closes down, and the cycle repeats.
Temperature shock: Condensate backed up inside the steam space cools the tubes that carry the process fluid. When this sub-cooled condensate is unexpectedly replaced by hot steam due to poor steam trap operations, the expansion and contraction of the tubes stress the tube joints. Frequently repeating this cycle causes early failure.
Corrosion from:
Flooding - A flooded heat exchanger will allow the oxygen to dissolve, in addition to carbon dioxide and other gases found in the steam. Since the condensate is often sub-cooled due to the time it is in the exchanger, these gases are more quickly dissolved. Together the cool condensate and dissolved gases are exceedingly corrosive and will tend to decrease the effectiveness of the heat exchanger and decrease the heat transfer through the tubes.
Steam collapse - Under very low loads with the steam valve closed, the steam volume collapses to smaller volume condensate, inducing a vacuum. When the vacuum breaker opens, atmospheric air and condensate mix inside the exchanger, increasing the risk of corrosion of the tubes, shells, tube sheet and tube supports.
Freezing - Steam/air coils cannot afford poor condensate drainage, particularly if the coil experiences air below freezing temperature. Condensate backed up inside the coil will freeze, often within seconds, depending on the air temperature. A low temperature detection thermostat is suggested on the coil leaving side to sense freezing conditions.
As earlier explained, the only way to avoid "stall" is to eliminate back pressure on the steam trap. There are a number of options available for designing a system that greatly reduces the risk of "stall." The following are two such options:
o Set up the heat exchanger in a position so that the condensate freely drains by gravity to the condensate return line. In many cases this is not possible on account of existing piping around the area in which the heat exchanger is needed (e.g., the heat exchanger is installed at a level lower than the condensate return tank).
o Use an electric or pressure driven condensate pump package installed below the steam trap to pump condensate back to the boiler.
In actual practice, the first option may not be possible, and so the use of electric or pressure driven pumps to return condensate to the boiler room should be considered.
Technical Tips about Heat Exchanger
30 Maret 2011 15:13
For any heat-exchanger application, a user will benefit from the following suggestions:
* Consider heat exchangers early in system design.
* Avoid being excessively safe in specifying performance criteria.
Asking for more temperature change, higher flow capability, and other just-in-case possibilities can easily double an exchanger’s size and cost. Consider the required effectiveness values; however remember that an exchanger’s size approaches infinity asymptotically as effectiveness approaches 1. So it takes considerably more heat-exchanger area to raise the effectiveness from 0.8 to 0.9 than it does to go from 0.7 to 0.8. High effectiveness—greater than 0.9—can be very costly.
Consider increasing pumping power rather than increasing an exchanger’s size. Higher velocity flow can produce or increase turbulence, which leads to an increased pressure drop and the need for more pumping power. Nevertheless, turbulence also increases the heat-transfer coefficient, in this manner decreasing the required heat-exchanger size. Accepting the increased pressure drop may be a more viable option than increasing size.
Remember that the main criterion is the product of the overall heat-transfer coefficient and the transfer area (U A)—not just the transfer area. For laminar-flow tubes, the total tube length, not the transfer area, is by and large the important factor; so 10 feet of 1/4-inch tubing works as well as 10 feet of 1-inch tubing.
Specify the smallest possible tubing for tube-type heat exchangers, since it gives the maximum thermal performance with the minimum volume. Nevertheless, be aware of the effects of fouling or particulates that may clog small tubes.
Seek turbulent flow to improve heat transfer, although that can be difficult with thick fluids and low flow rates.
Be conscious of fluid thermal conductivity when specifying the cooling or heating fluid. Perhaps amazingly, water frequently works the best
Match the inlet-port size to the piping sizes expected for the rest of the system.
Consider an exchanger’s lifetime and maintenance requirements.
Choose the type and thickness of material that will reduce failure caused by corrosion and erosion. Furthermore consider a system’s ease of mechanical or chemical cleaning in addition to filtration of the fluid streams.
Provide the heat-exchanger vendor with as much information on the total system as possible.
To maximize the performance of a heat exchanger means saving money, especially if the process is built for a long-term project. Here are some ways to improve the performance of a heat exchanger:
1. Heat transfer area
2. Fluid flow velocity
3. Temperature gradient
Maintenance of Heat Exchangers
30 Maret 2011 15:11
Both reactive and proactive approaches to maintenance have advantages and disadvantages. It is up to you to choose which method is best matched to your operation.
Maintaining a plate heat exchanger and its related cost is not just a good idea it is a requirement for any winning business strategy concerned with its sustained manufacturing productivity. It is safe to say that it would be unusual to find someone in this industry that does not consider in having a plate heat exchanger maintenance strategy. The solution is its scope and usefulness.
The challenge comes from knowing when to spend on maintenance efforts with a look toward a satisfactory return on that investment and when maintenance can be successful through lower-cost methods. This is not essentially an either/or proposition: It is more of a good balancing act with a look toward the best methods of maintenance to garner the greatest equipment productivity. It is not just spending money on maintenance that leads to effective productivity; it is spending money sensibly.
Reactive approach to maintaining heat exchangers
30 Maret 2011 14:52
A reactive approach is the most popular method that most manufacturing businesses use to save money short-term; a proactive approach is forward-looking and prepares the business for ultimate equipment failure and solving errors before they can affect production downtime or cash flow.
There are many reasons to schedule regular maintenance. The plate heat exchanger must be allowed to maximize its efficiency as it initially was designed for either heating or cooling at specified pressure drops. Unscheduled production stops are expensive and irritating. As heating will cause the most stress on a plate heat exchanger, it is not essentially true that heating will require more or less maintenance than a cooling system. Moreover, if it is necessary to restore a unit to the original as-built design parameters or redesign based on new criteria, maintenance could be needed.
It does not take a process heating professional to recognize when maintenance is required on a plate heat exchanger. Failures can be noticed easily by anyone through visible leaks from gaskets to the atmosphere or cross-contamination between fluids through cracks in the plates in a regular check of the equipment. Visual checks tell when fouling or plugging occurs on the plate surfaces or when gaskets fail due to temperature excursions or compression set.
Providing maintenance at the point where such noticeable faults take place is a far-too-common practice among industries that use plate heat exchangers. Relying on an orderly check alone for leaks and damage is reactive in nature and can limit the effectiveness of a maintenance strategy. The owner of a plate heat exchanger need not wait for these visible signs of poor condition and failure before scheduling maintenance for a unit.
When pressure drop rises and exceeds original design or when loss of heat transfer happens and the unit does not cool or heat, as designed or specified, maintenance may be required. Monitoring equipment in the form of pressure and temperature gauges used to detect wear and stress on the unit can help to plan maintenance on all affected units. Investment in this type of monitoring equipment may be restricted because of budgetary constraints or not be considered in an effort to cut cost.
Proactive Maintenance of your heat exchanger
30 Maret 2011 14:51
Servicing intervals depend on fluids, pressures, temperatures and the age of the equipment. Monitoring equipment in the form of temperature and pressure gauges helps to see if a unit is operating at or near maximum limits. Failure detection can be associated with sudden temperature and pressure changes. The net effect allows for regular scheduling of overhauling so that time and money are not wasted in unexpected failures and delayed downtimes.
Proactive maintenance is the conception of investing in monitoring equipment such as pressure and temperature gauges, filters and back-flush valves so that errors in unit efficiency can be observed, examined and used to ascertain a schedule of maintenance. Since the methods of providing maintenance to plate heat exchangers are no different in either a reactive or proactive maintenance approach, it is important to remember that it becomes, then, a choice to tackle a current problem as it take place or as an expected part of the scheduled work cycle.
So, while proactive maintenance is the best method of dealing with unit break down, it is not the end-all-and-be-all of maintenance approach. Some surprises will arise, and clearly, reactive maintenance procedures will need to be followed. A mixture of the two systems is the most realistic course, ensuring the least downtime and least expense in repairs and lost production.
How to Provide Maintenance
30 Maret 2011 14:51
Plate heat exchanger maintenance is provided through two distinct methods:
* Clean in place (CIP) on-site maintenance.
* Authorized cleaning facilities off-site maintenance.
The basic difference between these two methods is not how, but where and who performs the maintenance. CIP is performed in the manufacturing plant for quick recovery and continued equipment use, and the person in charge of maintenance generally performs it.
Utilizing an authorized cleaning facility for maintenance requires removing the equipment and shipping it to a facility outside the plant where certified maintenance personnel service the plate heat exchanger.
As a consequence of the advantages of quick turnarounds in a maintenance cycle, CIP is an attractive maintenance method. It is faster, more fitting and easy. It is also a preferred cleaning method when particularly corrosive liquids are being processed.
For all its advantages, one should be careful when making CIP the exclusive method for maintenance of a plate heat exchanger. CIP has its place, but the drawbacks can be costly.
The disadvantages of the authorized cleaning facilities tend to be measured by the up-front cost and rarely for the real advantages achieved.
At an authorized cleaning facility, each plate is logged and visually inspected to determine if it merits reconditioning. The client is informed if any plate is unserviceable and why. The facility crosschecks its database to confirm the plate heat exchanger's original specifications and then conducts a close visual inspection of gaskets and contact points on the remaining plates to determine if there is any erosion or apparent change from the original specification. Then the precise cleaning and treatment process required is determined.
A properly maintained plate heat exchanger can provide many trouble-free years of operation. No piece of equipment is perfect, however every unit will eventually have problems, and if costly downtime and product loss are to be minimized, a maintenance program is one solution. (Bell, 2002).
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