Seawater Wake-Up Call

Author:
Publish date:
Social count:
0
071512004A

Plumbing materials are key to A/C system health. 

As I squeezed my way into a narrow gap in the lazarette I heard an ominous crack shortly after, and sensed drops of extremely cold water landing on the back of my neck and head. I had an inkling of what had happened even before extricating myself from the labyrinth of machinery, plumbing, and equipment.

Air conditioning systems aboard most boats rely on raw or sea water to cool pressurized, hot refrigerant after it leaves the system’s compressor. Via a heat exchanger or condenser, the sea water absorbs heat from the refrigerant—heat that has been created as the gas is compressed within the unit’s compressor—and in the process, the refrigerant is both cooled and condensed into a liquid. The cool, high-pressure liquid then enters the evaporator, passing through a small orifice.

As it does so, the pressure drops, turns back into a gas or evaporates, and, in the process, is able to absorb heat from the cabin, (heat that later passes on to the sea water flowing through the condenser). The refrigerant then returns to the compressor as a cool, low-pressure gas, where the cycle begins anew. The process works in very much the same way for refrigerators and freezers. The hot air that blows on your feet when you stand in front of many refrigerators is heat that is being removed from the condenser.

For household air conditioners the same process occurs with one exception—instead of sea water, air is used to absorb heat from the refrigerant as it passes through a finned radiator-like condenser. It only makes sense, therefore, that aboard a boat the most readily available cooling medium, sea water, should be used to carry out this process. The benefit to this approach is efficiency. Sea water absorbs and carries away with it heat from the air conditioning condensers, using a heat exchanger that is a fraction of the size of an equivalent air-cooled model, and no excess hot air is produced aboard the boat.

‘ACTIVE FLOODING’ RISK

The quid pro quo for using sea water for onboard air conditioning and refrigeration systems is the risk of failure and subsequent flooding. Raw water must be brought aboard though seacocks, strainers, and hoses as it travels to a pump. After it leaves the pump it travels to the metallic air conditioning condenser, then through more hoses and to an overboard, typically above the waterline discharge fitting. For larger systems raw water manifolds are often involved, enabling a single pump to supply multiple air conditioning units, as well as collecting water from multiple units for a single point of discharge.

Unlike most other raw water plumbing systems aboard, where water within hoses and pipes is “pressurized” as a function of how far below the waterline it is located, what’s known as head pressure, much of the plumbing associated with air conditioning systems is under pressure. That is, once it leaves the air conditioning raw water pump, it’s under pressure. As a result, a leak is no longer a function of head pressure—it’s being pumped into the boat, what I refer to as active rather than passive flooding. Failures of air conditioning raw water plumbing have led to downflooding and sinking, while vessels were dockside. Their integrity, therefore, is of paramount importance.

ROBUST AND RELIABLE

Plumbing used in air conditioning raw water systems must be designed and installed to ensure the greatest durability and reliability. Both material selection and the manner in which systems are installed must be carefully thought out.

Among the most common faults involves the use of metallic plumbing that is unsuited for raw water applications, including brass and stainless steel. Not long ago a client contacted me and asked about installing a bilge drying system to remove incidental water accumulation. I asked, “Where is the water coming from?” He answered, “I’m not sure, but don’t all boats have water in the bilge?

In fact, while some bilge water accumulation can be considered normal, from a stuffing box or minor deck leak for instance, for the most part bilges should remain dry. In this case a mechanic was asked to find the source of the water, which turned out to be a seriously corroded brass (which was thought to be bronze by the professional yard that installed it only 10 months earlier) pipe nipple that conveyed air conditioning raw water overboard. Whenever the air conditioning system ran, which at the boat’s Florida location was nearly continuous, water shot sprinkler-like from the porous pipe nipple.

Stainless steel, while highly corrosion resistant when afforded a ready supply of oxygen, can be a liability when used with raw water, particularly if sea water remains inside the pipe when the system shuts down. In this scenario, the water is quickly deprived of oxygen. As it evaporates the salt concentration rises, setting up an ideal environment for corrosion.

PVC ISSUES

Yet another material that is ill suited for air conditioning raw water applications is PVC. While essentially corrosion proof because of its potentially brittle nature, PVC is less than ideal for raw water scenarios. I have a collection of fractured PVC components I’ve collected over the years that serves as a reminder to avoid this material in raw water applications.

If it is used—preferably only as part of a system supplied by an air conditioning manufacturer, installer, or boatbuilder (for which they take responsibility)—the CPVC variety should be chosen for its greater ductility and crush resistance. Plumbing must also be well supported. While this holds true for all raw water plumbing, it’s especially true of CPVC. It should not support the weight of hoses or a large manifold.

There is a contingent of materials that are well suited for air conditioning raw water systems. These include bronze (with a zinc content of less than 10 percent), fiberglass, glass-reinforced nylon, and SAE J2006-rated marine wet exhaust hose. Provided they possess the necessary tensile strength and modulus of elasticity, non-metallic components are very desirable in that the otherwise ever-present threat of corrosion is eliminated.

As for the system that gave me an impromptu saltwater shower, it incorporated a tenuous PVC manifold that broke when my coverall pocket snagged on a fitting, driving home the need for robustness where these and all raw water plumbing components are concerned. Simply put, if you have to worry about its failure as a result of being leaned, sat, or stepped upon, it’s probably not strong enough.

Related