Compressors

The compressor is the heart of the refrigeration cycle, and on a boat it is the most expensive component in the system. This chapter covers scroll vs rotary, the two failure patterns that look like "the compressor is bad" but usually are not, and the #1 marine compressor killer: a saturated suction filter drier.

Why it matters: most "dead compressor" calls are a system starving the compressor. Diagnose the system before you replace the heart.

• Scroll — two interleaved spiral vanes, one fixed and one orbiting. Smoother, quieter, more

efficient, but more expensive and more sensitive to liquid slugging.

• Rotary — an eccentric rotor in a cylindrical chamber. Simpler, more robust, more tolerant of

abuse, but noisier and less efficient.

The type determines the oil it uses, the maximum pressure it can handle, and how it fails. Before you diagnose a compressor, know which type you are looking at.

Short cycling — the compressor starts, runs briefly, shuts off, and starts again. It is almost always a sign the system is not rejecting heat, not that the compressor is failing. The most common marine cause is a restricted raw-water loop: the condenser cannot reject heat, discharge pressure climbs, the high-pressure switch trips, the compressor shuts off, pressure equalizes, and it starts again.

Hard starting — the opposite. The compressor struggles to start, draws excessive current, and trips the overload. Often a low charge, a failed start capacitor, or a mechanically locked rotor.

Both symptoms look like "the compressor is bad." In most cases, the compressor is fine and the system is starving it.

The suction filter drier is a small canister that traps moisture and debris in the refrigerant circuit. Over time it saturates, and when it is full it restricts flow. A restricted drier starves the compressor, causes a low suction pressure, and can lead to liquid slugging — which destroys the compressor.