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What Is A Heat Exchanger And Why Should I Care?
In the case of the one’s found on your boat’s engines you have sea-water (also called raw-water) flowing through the tubes which cool the coolant (anti-freeze) through the cooling journals of your engine.
And, Why Should I care? They do not last forever even with the best of maintenance. Most have zincs to reduce corrosion. Some do not. If yours has zincs it is extremely important to know your heat exchangers appetite for zincs. It is the maintenance item that will allow you to maximize the useful life of your heat exchanger. Some eat up zincs in just a month to a few, where others may last many months. So it is very important that you keep a ledger to find out just how often yours requires new zincs. Furthermore, if they do not get changed often enough they can break into pieces, and plug up the tubes in the seawater side. Once again, a few dollars in maintenance can save you hundreds if not thousands of dollars in repairs. However, if you have no idea just how long your zincs are lasting in your heat exchanger(s) install new ones, record the date you do this, and then check them every 30 or so days. Keep up this inspection schedule until you see that they are at 75% corroded away. Determine how much time has elapsed from the date you installed them. There… Now you know just when to be changing out your zincs in your heart exchanger(s). What To Look For In A Bad Heat Exchanger
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As its name implies, this type of heat exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids. The set of tubes is called a tube bundle, and may be composed by several types of tubes: plain, longitudinally finned, etc.
These two heater exchangers (above and below) are the sames ones. This one has been wire wheeled, and or sanded. This process of cleaning, though common does not allow for a thorough inspection. |
Unlike in photo #1 the same area that was wire wheeled has now been blasted in our blast cabinet. The result is striking as far as how more efficient it is to bead blast a substrate than to just wire wheel to sand it. In this photo we can clearly see crevice corrosion starting up at the top right edge of the cooler. Furthermore, the red areas are very apparent. These red areas are unhealthy so to speak, as these areas of the bronze are in an advanced state of galvanic corrosion. |
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Removing, and visually inspecting a heat exchanger is only half of the service you can expect to receive from us. All of the related components that are able to be bead blasted, receive blasting in our blast cabinet. Bead blasting provides a 100% clean substrate for visual inspection. To not bead blast these items clean is to chance that a thorough visual inspection is not possible. We bead blast in order to ascertain the level of corrosion your parts may have that can affect the soundness of the part. Attempting to just sand, or wire wheel away paints, and corrosion is not very effective at all. See the two photos.
Additionally, we take your heat exchanger tube bundle, and install it into our pressure tester. See photo. 25 psi of air is pumped into the bundle, and then the entire unit is submerged in clean water. Any signs of leaking will become obvious.
“Why are parts of my heat exchanger turning that red color?” You are seeing Galvanic Corrosion. This is the most common cause of corrosion in the marine environment. Galvanic Corrosion is the interaction of two dissimilar metals in an electrolyte (saltwater). In this situation the least noble becomes the anode and corrodes. In this case you are seeing the depletion of the zinc in your bronze heat exchanger, thus leaving only the copper alloy. This is the red color you are seeing.
This is why it is so important that you keep your zincs in good fashion in your heat exchanger, if they are equipped with them.
Hino Diesel. Heat exchanger end cap clogged with broken impeller blades. Also noted are the mineral deposits fused to the end cap. |
Cummins Diesel: Raw-water pump impeller with a distorted “set” to the blades to due months of non-use. |
Same impeller showing damage to blades. |
Change Out That Seawater Impeller On A Regular Basis.
Why is it that we encourage owners to follow engine manufacturer’s directives, and change out seawater pump impellers on a regularly scheduled basis? Even if… “it has been working fine for years!”
Why? Because It will cost far less to change out an impeller once a year than to pay us for engine damage due to reduced sea water flow, because of blocked cooling journals due to broken and/or fragmented pieces of rubber blocking your cooling system. You can imagine how much time is involved in dismantling an entire raw-water cooling system! And, of course there is the high potential for over-heating damage as well.
What Happens: Rubber impellers last only so long before they start to break down due to exposure to seawater and wear.
Rubber Can Take a “set”: Additionally rubber blades tend to take a “set”. In other words, the impeller blades will acquire a fixed bend to the shape of the housing where the intake and out-take are located. This especially happens to pump impellers where a boat sits for long periods of time with no use.
Cracking And Then Breaking: Cracking typically starts at the base of the blades. An impeller can have cracked blades and still pump. However as the cracks continue to enlarge the pump will lose its ability to pump the proper volume of cooling water. Eventually these blades break off and more often than not become lodged anywhere in the raw water-side of the cooling system.
Wear Due to Use: Boats that do get a lot of use have impellers that through extensive use just plain wear out. These too though they may look fine upon inspection will have a reduced ability to pump the appropriate volume of water.
Silt Wear: A third issue that is not too much concern for those who cruise the Puget Sound, but anyone who cruises in tributaries where there is a measurable amount of silt will have issues of accelerated wear to the tips of the blades as such water is abrasive to the rubber.
How Often Should The Raw-Water Impeller Be Changed Out?
Well… Ask five mechanics, and you may get five different answers. We tend to encourage our clients to start with their engine manufacturer’s recommendation. Most all call for a once a year replacement. We tend to be guided by all manufacturers’ maintenance schedules.
Wear-Plate: Additionally, there is a wear plate at the base of the pump housing that should be inspected when the impeller is changed out. These typically last many years in non-silted water.
To the right are photos with captions of situations we see on a yearly basis.