What is Steam?

Steam is the gas formed when water passes from the liquid to the gaseous state. At the molecular level, this is when H2O molecules manage to break free from the bonds (i.e. hydrogen bonds) keeping them together.

How steam works

In liquid water, H2O molecules are constantly being joined together and separated. As the water molecules are heated, however, the bonds connecting the molecules start breaking more rapidly than they can form. Eventually, when enough heat is supplied, some molecules will break free. These 'free' molecules form the transparent gas we know as steam, or more specifically dry steam.

Dry Steam vs. Wet Steam

In steam-using industries, two commonly referred to types of steam are dry steam (also called ”saturated steam”) and wet steam.

Dry steam applies to steam when all its water molecules remain in the gaseous state. It's a transparent gas.

Wet steam applies to steam when a portion of its water molecules have given up their energy (latent heat) and condense to form tiny water droplets.

Take the example of a kettle boiling water. Water is first heated using an element. As water absorbs more and more heat from the element, its molecules become more agitated and it starts to boil. Once enough energy is absorbed, part of the water vaporizes, which can represent an increase as much as 1600X in molecular volume.

Sometimes a mist can be seen coming out of the spout. This mist is an example of how dry steam, when released into the colder atmosphere, loses some of its energy by transferring it to the ambient air. If enough energy is lost that intermolecular bonds start forming again, tiny airborne droplets can be seen. This mixture of water in the liquid state (tiny droplets) and gaseous state (steam) is called wet steam.

Saturated Steam (Dry)

As indicated by the black line in the above graph, saturated steam occurs at temperatures and pressures where steam (gas) and water (liquid) can coexist. In other words, it occurs when the rate of water vaporization is equal to the rate of condensation.

Advantages of using saturated steam for heating

Saturated steam has many properties that make it an excellent heat source, particularly at temperatures of 100 °C (212°F) and higher. Some of these are:

Property Advantage

Rapid, even heating through latent heat transfer

Improved product quality and productivity

Pressure can control temperature Temperature can be quickly and precisely established

High heat transfer coefficientSmaller required heat transfer surface area, enabling reduced initial equipment outlay

Originates from water Safe, clean, and low-cost

Unsaturated Steam (Wet)

This is the most common form of steam actually experienced by most plants. When steam is generated using a boiler, it usually contains wetness from non-vaporized water molecules that are carried over into the distributed steam. Even the best boilers may discharge steam containing 3% to 5% wetness. As the water approaches the saturation state and begins to vaporize, some water, usually in the form of mist or droplets, is entrained in the rising steam and distributed downstream. This is one of the key reasons why separation is used to dis-entrain condensate from distributed steam.

Superheated Steam

Superheated steam is created by further heating wet or saturated steam beyond the saturated steam point. This yields steam that has a higher temperature and lower density than saturated steam at the same pressure. Superheated steam is mainly used in propulsion/drive applications such as turbines, and is not typically used for heat transfer applications.

Advantages of using superheated steam to drive turbines:

To maintain the dryness of the steam for steam-driven equipment, whose performance is impaired by the presence of condensate

To improve thermal efficiency and work capability, e.g. to achieve larger changes in specific volume from the superheated state to lower pressures, even vacuum.

It is advantageous to both supply and discharge the steam while in the superheated state because condensate will be generated inside steam-driven equipment during normal operation, minimizing the risk of damage from erosion or carbonic acid corrosion. In addition, as the theoretical thermal efficiency of the turbine is calculated from the value of the enthalpy at the turbine inlet and outlet, increasing the degree of superheating as well as the pressure raises the enthalpy at the turbine inlet side, and is thereby effective at improving thermal efficiency.

Disadvantages of using superheated steam for heating:

Property Disadvantage

Low heat transfer coefficientReduced productivity

Larger heat transfer surface area needed

Variable steam temperature even at constant pressure

Superheated steam needs to maintain a high velocity, otherwise the temperature will drop as heat is lost from the system

Sensible heat used to transfer heat

Temperature drops can have a negative impact on product quality

Temperature may be extremely high

Stronger materials of construction may be needed, requiring higher initial equipment outlay

For these reasons and others, saturated steam is preferred over superheated steam as the heating medium in exchangers and other heat transfer equipment. On the other hand, when viewed as a heat source for direct heating as a high temperature gas, it has an advantage over hot air in that it can be used as a heat source for heating under oxygen-free conditions. Research is also being carried out on the use of superheated steam in food processing applications such as cooking and drying.

Supercritical Water

Supercritical water is water in a state that exceeds its critical point: 22.1MPa, 374 °C (3208 psia, 705°F). At the critical point, the latent heat of steam is zero, and its specific volume is exactly the same whether considered liquid or gaseous. In other words, water that is at a higher pressure and temperature than the critical point is in an indistinguishable state that is neither liquid nor gas.

Supercritical water is used to drive turbines in power plants which demand higher efficiency. Research on supercritical water is being performed with an emphasis on its use as a fluid that has the properties of both a liquid and a gas, and in particular on its suitability as a solvent for chemical reactions.

Methods of Estimating Steam ConsumptionHow to calculate steam requirements for flow and non-flow applications. Including warm-up, heat losses and running loads.

The optimum design for a steam system will largely depend on whether the steam consumption rate has been accurately established. This will enable pipe sizes to be calculated, while ancillaries such as control valves and steam traps can be sized to give the best possible results. The steam demand of the plant can be determined using a number of different methods:

Calculation - By analysing the heat output on an item of plant using heat transfer equations, it may be possible to obtain an estimate for the steam consumption. Although heat transfer is not an exact science and there may be many unknown variables, it is possible to utilise previous experimental data from similar applications. The results acquired using this method are usually accurate enough for most purposes.

Measurement - Steam consumption may be determined by direct measurement, using flowmetering equipment. This will provide relatively accurate data on the steam consumption for an existing plant. However, for a plant which is still at the design stage, or is not up and running, this method is of little use.

Thermal rating - The thermal rating (or design rating) is often displayed on the name-plate of an individual item of plant, as provided by the manufacturers. These ratings usually express the anticipated heat output in kW, but the steam consumption required in kg/h will depend on the recommended steam pressure.

A change in any parameter which may alter the anticipated heat output, means that the thermal (design) rating and the connected load (actual steam consumption) will not be the same. The manufacturer's rating is an indication of the ideal capacity of an item and does not necessarily equate to the connected load

Measurement of Steam Consumption

By a steam flowmeterThe use of a steam flowmeter may be used to directly measure the steam usage of an operational item of plant. This may be used to monitor the results of energy saving schemes and to compare the efficiency of one item of plant with another. The steam can then be costed as a raw material at any stage of the production process, so that the cost of individual product lines may be determined.

It is only in comparatively rare cases that a meter cannot measure steam flow. Care should be taken, however, to ensure that the prevailing steam pressure is considered and that no other calibration factor has been overlooked.  

Fig. 2.7.1Typical steam flowmeter installation

By a condensate pumpA less accurate method of estimating the steam consumption is by incorporating a counter into the body of a positive displacement pump used to pump condensate from the process. Each discharge stroke is registered, and an estimate of the capacity of each stroke is used to calculate the amount of steam condensed over a given time period.

Fig. 2.7.2Positive displacement pump with cycle counter

A purpose built electronic pump monitor can be used which enables this to be carried out automatically, converting the pump into a condensate meter. The electronic pump monitor can be read locally or can return digital data to a central monitoring system. If the pump is draining a vented receiver, a small allowance has to be made for flash steam losses.

By collecting the condensateSteam consumption can also be established directly, by measuring the mass of condensate collected in a drum over a period of time. This may provide a more accurate method than using theoretical calculations if the flash steam losses (which are not taken into account) are small, and can work for both non-flow and flow type applications. However, this method cannot be used in direct steam injection applications, humidification or sterilisation processes, where it is not possible to collect the condensate.

Figure 2.7.3 shows a test being carried out on a jacketed pan. In this case an empty oil drum and platform scales are shown, but smaller plant can be tested just as accurately using a bucket and spring balance. This method is quite easy to set up and can be relied upon to give accurate results.

Fig. 2.7.3Equipment for measurement of steam consumption

The drum is first weighed with a sufficient quantity of cold water. Steam is then supplied to the plant, and any condensate is discharged below the water level in the container to condense any flash steam. By noting the increase in weight over time, the mean steam consumption can be determined.

Although this method gives the mean rate of steam consumption, if the weight of condensate is noted at regular intervals during the test, the corresponding steam consumption rates can be calculated. Any obvious peaks will become apparent and can be taken into account when deciding on the capacity of associated equipment. It is important to note that the test is conducted with the condensate discharging into an atmospheric system. If the test is being used to quantify steam consumption on plant that would otherwise have a condensate back pressure, the steam trap capacity must relate to the expected differential pressure.

Care must also be taken to ensure that only condensate produced during the test run is measured. In the case of the boiling pan shown, it would be wise to drain the jacket completely through the drain cock before starting the test. At the end, drain the jacket again and add this condensate to that in the container before weighing.

The test should run for as long as possible in order to reduce the effect of errors of measurement. It is always advisable to run three tests under similar conditions and average the results in order to get a reliable answer. Discard any results that are widely different from the others and, if necessary, run further tests.

If the return system includes a collecting tank and pump, it may be possible to stop the pump for a period and measure condensate volume by carefully dipping the tank before and after a test period. Care must be taken here, particularly if the level change is small or if losses occur due to flash steam.

Steam QualitySteam should be available at the point of use in the correct quantity, at the correct pressure, clean, dry and free from air and other incondensable gases. This tutorial explains why this is necessary, and how steam quality is assured.

Steam should be available at the point of use:

In the correct quantity

At the correct temperature and pressure

Free from air and incondensable gases

Clean

Dry

Correct quantity of steamThe correct quantity of steam must be made available for any heating process to ensure that a sufficient heat flow is provided for heat transfer. 

Similarly, the correct flowrate must also be supplied so that there is no product spoilage or drop in the rate of production. Steam loads must be properly calculated and pipes must be correctly sized to achieve the flowrates required.

Correct pressure and temperature of steamSteam should reach the point of use at the required pressure and provide the desired temperature for each application, or performance will be affected. The correct sizing of pipework and pipeline ancillaries will ensure this is achieved.

However, even if the pressure gauge is correctly displaying the desired pressure, the corresponding saturation temperature may not be available if the steam contains air and/or incondensable gases.