Saturated and Superheated Steam in Paper Drying

Saturated and superheated steam are the two types of steam found in paper mill steam systems. Although they perform different duties, both types are used in the paper drying process. In order to utilize them for maximum efficiency, they must be properly controlled and understood. Let’s start by defining the terms and then we’ll talk about how we make use of them in paper drying.

Saturated Steam

Saturated steam is the gaseous form of water that is heated until it boils by a process called evaporation. Once the water molecules evaporate, they separate from the water and rapidly expand, just as they would in a pot on a stove top. Now imagine you put a lid on that pot thus creating an enclosed container. There’s some open space inside the pot where water isn’t present and the steam begins to fill that space. As the water evaporates, the newly formed steam molecules want to rapidly expand; however, the container doesn’t increase in volume and therefore the steam begins to exert pressure against the water and surfaces of the pot and lid. This is what happens in a pressure cooker as well as an industrial steam boiler. As the pressure rises, so too does the temperature. In fact, one of the defining characteristics of saturated steam is that its temperature and pressure will always match. If one goes up, so does the other. And if one goes down, the other follows suit. They are at all times inextricably linked. We use steam tables to identify the properties of saturated steam as they are reliable and predictable.

Superheated Steam

Superheated steam is steam that is heated beyond its saturation point. Let’s think about that boiling pot of water with the lid and let’s imagine that instead of a stove top to heat it we are using industrial burners. And let’s say we have heated the pot so much that all the water molecules inside the enclosed pot have successfully evaporated into saturated steam. Because the entire container is filled only with steam, and the pressure and temperature are at their peak, does the temperature of this steam cease to rise if we keep heating it with the burner? The answer is no: the steam temperature will continue to rise beyond its saturation point so long as more heat is applied. Superheated steam doesn’t follow the same temperature and pressure matching properties as saturated steam.  Instead, we measure superheated steam by how much higher its temperature is over what the temperature would be at the saturation point of steam at a given pressure. For example, if we use instrumentation and measure pressure in a steam pipe at 350 psig, according to the steam tables, the temperature of saturated steam will always be 435° F.  But if we measure the temperature of the steam inside the pipe at 450° F, we can know that the steam has been superheated. We would call this 15° of superheat at 350 psig. 

Steam in Paper Drying

Here we’ll discuss how the two types of steam are used in paper drying. First we’ll talk about saturated steam which is used for the actual drying of paper. Saturated steam contains a high amount of latent heat, or energy that is transferable from one object to another. Release of latent heat happens when the saturated steam undergoes a phase change, in the case of paper drying, from steam back into liquid. Latent heat is readily available in saturated steam because it exists at the same pressure and temperature from the water in which it was just boiled, so it is always on the edge of a phase change. When coming in contact with a cooler object, the latent heat from the steam is transferred to that object. Once that heat is fully transferred, the temperature drops and can no longer exist as steam at that pressure and so it condenses back into a liquid.  This is what we call condensation. The steam returns to its liquid state, even if only a few degrees cooler than the steam it just came from. In a paper dryer, the cool, wet sheet moving across the outside of the dryer shell is constantly pulling the latent heat energy out of the steam, thereby condensing back into a liquid which then must be removed. More steam then enters, filling in the space left by the evacuated condensate and the process continues. 

Superheated steam is used in the drying process, but only indirectly. One property of superheated steam is that it does not have high latent heat available and so it does not transfer energy nearly as well as saturated steam. Instead, superheated steam is used to deliver large volumes of high-pressure, high-temperature steam to various parts of the paper mill without giving up its energy into the piping and condensing back into a liquid.  Superheated steam functions similar to an insulator, much the same way that air does.  Once the superheated steam is delivered to the drying section, we must manually reduce the temperature of the steam down to saturated steam for paper drying. Often times, once the superheated steam arrives at the drying section of the paper machine, you’ll find a desuperheater that is used to reduce the temperature back into the range of the saturation point where it can be used in the paper dryers to transfer its energy into the sheet.  As a note, we like to control the temperature that enters the dryers from 10 to 15° above saturation temperature. This ensures that the steam doesn’t condense in the piping before it gets to the dryers.

Saturated steam is excellent for heat transfer, but water droplets that form from condensing in the piping can wreak havoc as these droplets impinge on every component they touch; not only the piping but also the rotary joint and syphon hardware as well. This problem is exacerbated where high-velocity steam is found. Once the superheated steam enters, the condensate present in the dryer acts as a natural desuperheater, knocking down the temperature of the superheated steam, bringing it to saturation temperature, and making it ready to transfer its energy to dry the sheet.

Optimizing Steam Systems 

Delivering either saturated steam or the right amount of superheated steam to the dryer requires extensive systems knowledge. Kadant Johnson offers nearly 90 years of process expertise designing and updating steam systems for paper mills while its services team can install and maintain steam systems and dryer section components. Whether you are seeking to increase drying capacity or reduce steam energy consumption, the place to begin is a Dryer Performance Evaluation.