MAINTAINING HEAT EXCHANGERS Flashcards
Heat Exchangers are used in the following applications:
- MDEs/SSDGs
- Marine Gas Turbines
- Air Conditioning & Refridgeration
AC&R systems used a special type of heat exchanger called a ???
Condenser
What are three of the most common types of heat exchangers used in the CG?
- Shell & tube
- Fin & tube
- Plate type
What is the most common type of heat exchanger used in the CG?
Shell & tube
What applications are shell & tube heat exchangers primarily with?
- Internal Combustion Engines
- Some AC&R systems use them as condensers
It consist of a cylindrical metal shell with tubes inside. One fluid flows through the tubes and the other fluid flows around the outside of the tubes. Heat is transferred from the hot fluid in the tubes, through the tube walls & into the cold fluid in the shell.
Shell and tube heat exchanger
An air-cooled condenser used in AC&R systems.
Fin and tube heat exchanger
Made up of parallel metal plates separated by gaskets.
Plate type heat exchanger
Heat flows from the hot fluid to a plate by convection, through the plate by conduction & to the cold fluid by convection. What type of heat exchanger is this?
Plate type
Heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it.
Convection
Heat transfer by means of molecular agitation within a material without any motion of the material as a whole. If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones.
Conduction
What are the primary applications of a plate type heat exchanger?
Cooling J/W & L/O in internal combustion engines & Marine Gas Turbines. It can also be used to cool L/O in Red Gears…etc.
Maintenance of a heat exchanger encompasses the following:
- Preventiive Maintenance
- Corrective Maintenance
Maintenance is conducted IAW the following:
- Unit’s PMS
- NSTM Chapter 254
- Manufactures Tech Manual for the specific heat exchanger being maintained
Corrective maintenance is conducted conditionally when:
- Fluid is leaking from the heat exchanger
- The primary system is not operating within parameters and the heat exchanger is suspect.
Conditions that indicate corrective maintenance should be completed on a heat exchanger include:
- Temp differentials
- Pressure differentials
- Contamination of one fluid into another
A rise in J/W or L/O temperature
Temperature Differential
An unexplained change in J/W or L/O pressure
Pressure Didderential
Contamination of One Fluid into Another Examples
- Evidence of water in the L/O
- Evidence of L/O in the J/W
- An increase or decrease in J/W level in the expansion tank
- Evidence of L/O in the R/W OVBD Discharge
To determine whether or not a heat exchanger is at fault the following must happen:
- The unit must be secured
- Drained
- Cleaned
- Inspected
Regulations dictate that only _____ certified technicians can recover refridgerants.
EPA
Before draining, cleaning & inspecting a heat exchanger (condenser) that is part of an AC&R system, _____ must recover the refridgerant & evacuate the system.
An EPA certified technician
Maintenance of a heat exchanger begins with WHAT?
- Know what subsystems will be affected
- Make sure all aplicable equipment is tagged-out
- Gather appropriate sized storage containers for holding fluid
- Review MSDS for each fluid being drained
Fluids most likely to require special handling
- Oils
- Jacket water
Fouling, scaling & blockage in the tubes of a Shell & Tube heat exchanger reduce ____. This can lead to damage of internal parts through _____ or _____.
- Heat transfer
- Erosion or Corrosion
Regular cleaning of a heat exchanger keeps it operating properly & should be performed IAW?
The Unit’s PMS; NSTM Chapter 254 & the Manufactures Technical Manual for the specific heat exchanger
Before conducting a thorough cleaning of a heat exchanger, _____________.
conduct a preliminary inspection.
Clean the tube sheet & the inside of the shell using fresh water & either a ___________ or a plastic scraper.
Stiff Nonmetallic Brush
Using fresh water, prepare a degreasing solution of ____________ with 1 percent liquid nonionic detergent.
2 to 3 percent trisodium anhydrous
Heat the solution and circulate ot through the shell for 2 to 3 hours. The solution temperature should be at least 140 F.
Step 4 for “Cleaning a Shell and Tube Heat Exchanger
Follow the degreasing with a thorough fresh water flushing.
Step 5 for “Cleaning a Shell and Tube Heat Exchanger
Name methods for “Removing Soft Fouling in Tubes
- High-Pressure Water jet
- Brushes
- Air or Water Gun
- Degreasing Solution
- Knotted Rope
- Phenolic Disk Scraping Tool
- Rotary Brush
- Back Flushing
Best method for cleaning tubes. Equipment includes a high-pressure pumping unit, high pressure hose, three-way valve & flexible lance with nozzle. The nozzle is designed to direct high-pressure water ahead of it and against the tube wal behind it.
High-Pressure Water Jet method
For small heat exchangers, use a nylon brush on the end of a nonmetallic rod to clean each tube manually.
Brush method
Use an air gun, water gun or lance to clear tubes that are blocked by mud or sand
Air or Water Gun method
This cleaning procedure is similar to the one described for cleaning the shell
Degreasing Solution method
Knot a length of nonmetallic rope & draw it through the tubes to remove ecess marine growth, dirt and soft scale. Size the rope so the when knotted, the knots are slightly larger than the inside diameter of the tube. Refer to NSTM Chapter 254 for additional information regarding this procedure.
Knotted Rope method
A phenolic disk scraping tool is effective for removing accumulated mud, grass and loose scale from tubes.
Phenolic Disk Scraping Tool method
The equipment used in this method includes an electric or air powered drive unit, a flexible shaft in a nonmetallic (nylon) casing, a foot switch and a straight brush with spiraled nylon bristles and plastic covered tip.
Rotary Brush Method
Back flushing will not remove all traces of soft fouling. It can, however, extend the interval between mechanical cleaning of _________.
Heat Exchangers
Is especially benficial to vessels that operate in warm or highly silted waters.
Backflushing
Requires only minor disassembly
Blackflushing
Can be quickly performed by shipboard personnel
Backflushing
Is most effecting on single-pass, straight tube heat exchangers and two pass, U-tube heat exchangers.
Backflushing
Is least effective on heat exchangers with multiple passes
Backflushing
Disconnect the spool pieces from the heat exchanger inlet & outlet
Step 1 for Backflushing a Shell and Tube Heat Exchanger
Attach special flanges constructed for this purpose.
NOTE: Refer to NSTM, Chapter 254 for flange details
Step 2 for Backflushing a Shell and Tube Heat Exchanger
Use standard 2 1/2-inch firehose to connect the outlet flange to a source of firemain water
Step 3 for Backflushing a Shell and Tube Heat Exchanger
Connect another 2 1/2-inch firehose to the inlet flange. Direct this hose overboard or to the bilge.
Step 4 for Backflushing a Shell and Tube Heat Exchanger
Flush heat exchanger with firemain water. Continue flushing until the discharge is free and clear of debris.
Step 5 for Backflushing a Shell and Tube Heat Exchanger
Hard fouling is foreign matter that adheres strongly to the tube and connot be easily removed. This type of fouling includes:
- Barnacles and Sea Scales
- Hard Coatings and Oxide Films
Calcium Carbonate that may deposit in the tube during extended periods of no or low seawater flow.
Barnacles and Sea Scales
May coat the tube during extended periods of intermittent operation in shallow or polluted water
Hard Coatings and Oxide Films
Use one of the following cleaning methods to remove hard fouling.
- High-Pressure Water Jet
- Acid Cleaning
- Nonmetallic Rods
This method, described in the previous section, removes some types of hard fouling. More than one pass of the lance may be required for complete removal.
High-Pressure Water Jet Method
This method removes all types of hard fouling. Due to the potential hazards to material and personnel, only use this method when other cleaning methods are ineffective.
NOTE: Only a naval shipyard or other qualified activity should perform this method.
Acid Cleaning
This method can partially remove hard fouling. Flush the tubes out thoroughly after using this method.
Nonmetallic Rods Method
Backflushing is only required for _______ plate type heat exchangers.
Seawater-Cooled
The procedure is not done according to a maintenance schedule; it is accomplished when the pressure drop through the Plate Type heat exchanger increases by 10 PSI.
Backflushing a Plate Type Heat Exchanger
Gradually close the seawater inlet and outlet valves
Step 1 for Backflushing a Plate Type Heat Exchanger
Gradually open the backflushing inlet and outlet valves
Step 2 for Backflushing a Plate Type Heat Exchanger
Backflush for 15 minutes
Step 3 for Backflushing a Plate Type Heat Exchanger
Gradually close the backflushing valves
Step 4 for Backflushing a Plate Type Heat Exchanger
Gradually open the seawater inlet and outlet valves
Step 5 for Backflushing a Plate Type Heat Exchanger
As with backflushing, disassembling and cleaning a Plate Type Heat Exchanger (mechanical cleaning) is only done when a ________________ indicates that it is necessary.
Decrease in Performance
How many times should you inspect a heat exchanger?
Twice, once before cleaning and once after cleaning
Benefits to inspecting a heat exchanger prior to cleaning.
Inspecting before cleaning may expose internal leaks and their location.
Benefit to inspecting a heat exchanger after thorough cleaning.
Inspection after cleaning accurately determines the condition of the heat exchanger components, such as tubes and plates.
Heat exchanger tubing is subject to a number of problems. Typical problems include the following:
- Corrosion
- Stress Corrosion
- Erosion
- Mechanical Damage
Resulting from current flow between dissimilar metals
Corrosion
Resulting when tubes containing stresses areexposed to a corrosive environment
Stress Corrosion
Resulting from the combination of corrosive agents with mechanical act from suspended sand, foreign bodies or from turbulent flow of the cooling liquid.
Erosion
Resulting from vibration or by foreign bodies in the coolant.
Mechanical Damage
Perform inspection of heat exchangers IAW
- The Unit’s PMS
- NSTM, Chapter 254
- Manufacturer’s Technical Manual
Replace ______ when they are half deteriorated, or if they show no deterioration after having been in service.
Zinc Anodes
Note: Do not use teflon tape on zinc plug threads
Inspection of the Shell and Tube Heat Exchanger includes testing the bundle for leaks. This is accomplished through _________ and should be performed IAW the Unit’s PMS, NSTM; Chapter 254 and the Manufacturer’s Technical Manual.
Hydrostatic Testing
Inspection of Plate Type Heat Exchangers should be performed IAW
- The Unit’s PMS
- NSTM; Chapter 254
- Manufacturer’s Technical Manual
Detecting leaks is a primary goal of inspecting a Plate Type Heat Exchanger. Leaks can be caused by:
- Defective or damaged gaskets
- Defective or damaged plates
- Overpressurization
- Misaligned or reversed plates
- Plate pack under tightening or overtightening
If evidence of leakage is seen on or under the Plate Type Heat Exchanger?
Check that the Plate Pack is tightened to proper dimensions IAW Manufactures Technical Manual or PMS Cards
If results of a hydrostatic test of the tube bundle indicate a leaking tube, plug the ends of the tube. This simply prevents fluid from flowing through the tube. Refer to NSTM?
Chapter 254
No more than ___% of the tubes can be plugged. If this limit is exceeded, the heat exchanger will not function properly.
10%
When replacing a gasket on a Plate Type heat Exchanger, DO NOT use ________.
Hardening Adhesives
When replacing a gasket on a Plate Type heat Exchanger, Place a weighted sheet of plywood on the plate to compress the gasket while the adhesive dries. Allow at least ___ hours of curing time at room temperature for adequate adhesion.
12 Hours
