Introduction to Marine Engineering

When a liquid boils, it generates a vapor, that is, some or all of the liquid changes its physical state from liquid to gas (or vapor). As long as the vapor is in contact with the liquid from which it is being generated, it remains at the same temperature as the boiling liquid. In this condition, the liquid and its vapor are said to be in equilibrium contact with each other.

The temperature at which a boiling liquid and its vapor may exist in equilibrium contact depends on the pressure under which the process takes place. As the pressure increases, the boiling temperature increases. As the pressure decreases, the boiling temperature decreases. For each liquid, therefore, the boiling point is determined by the pressure.

When a liquid is boiling and generating vapor, the liquid is called a saturated liquid, and the vapor is called a saturated vapor. The temperature at which a liquid boils under a given pressure is called the saturation temperature, and the corresponding pressure is called the saturation pressure. For each pressure there is a corresponding saturation temperature, and for each temperature there is a corresponding saturation pressure. A few saturation pressures and temperatures for water are listed below.

Atmospheric pressure is 14.7 psia at sea level and lesser at higher altitudes. Trying to cook potatoes at the top of a high mountain takes a great deal longer than it would at sea level. Why is this? As noted before, temperature and pressure are indications of internal energy. Since it is not possible to raise the temperature of the boiling water above the saturation temperature for that pressure, the amount of internal energy available for cooking the potatoes is much less at high altitudes than it is at sea level. By the same line of reasoning, you should be able to figure out why the potatoes cook faster in a pressure cooker than in an open kettle.

A peculiar thing happens to water and steam at an absolute pressure of 3206.2 psia and the corresponding saturation temperature at 705.40 °F. At this point, which is called the critical point, the vapor and the liquid are indistinguishable. No change of state occurs when the pressure is increased above this point or when heat is added. At the critical point, we can no longer refer to “water” or “steam,” since we cannot tell the water and steam apart. Instead, the substance is merely called a “fluid” or a “working substance.” Boilers designed to operate at pressures and temperatures above the critical point are called supercritical boilers. Supercritical boilers are not used, at present, in propulsion plants of ships; however, some boilers of this type are used in stationary steam power plants.

If we generate steam by boiling water in an open pan at atmospheric pressure, the water and the steam which is in immediate contact with the water will remain at 212 °F until all the water has been evaporated. If we fit an absolutely tight cover to the pan, so that no steam can escape while we continue to add heat, both the pressure and the temperature inside the vessel will rise. The steam and water will both increase in temperature and pressure, and each fluid will be at the same temperature and pressure as the other.

In operation, a boiler is neither an open vessel nor a closed vessel. Instead, it is a vessel designed with restricted openings which allow steam to escape at a uniform rate while feedwater is being brought in at a uniform rate. After boiler operating pressure has been reached, therefore, the process of steam generation in a boiler takes place at constant pressure and constant temperature, if we disregard any fluctuations that may be caused by changes in steam demands.

Although it is impossible to raise the temperature of the steam in the steam drum above the temperature of the water from which it is being generated, we can raise the temperature of the steam if we first remove the steam from contact with the water inside the steam drum and then add heat. Steam which has been heated above its saturation temperature for any given pressure is called superheated steam, and the vessel in which the saturated steam is superheated is called a superheater.The amount by which the temperature of the superheated steam exceeds the temperature of saturated steam at the same pressure is known as the degree of superheat. For example, if saturated steam at a pressure of 620 psia and the corresponding saturation temperature of 490 °F is superheated to 790 °F, the degree of superheat is 300 °F.

All ship’s propulsion boilers are equipped with superheaters. The primary advantage is that superheating the steam provides a greater temperature differential between the boiler and the condenser, thus allowing more heat to be converted to work at the turbines. Another advantage is that superheated steam is dry and therefore causes relatively little corrosion or erosion of machinery and piping. Also, superheated steam does not conduct heat as rapidly (and therefore does not lose heat as rapidly) as saturated steam. The increased efficiency which results from the use of superheated steam reduces the amount of fuel oil required to generate each pound of steam, and so reduces the space and weight requirements for the boilers.

It should be noted, however, that most auxiliary machinery is designed to operate on saturated steam. Reciprocating machinery in particular requires saturated steam for the lubrication of internal moving parts of the steam end. Ship’s boilers, therefore, are designed to produce both saturated steam and superheated steam.