Aug 4, 2015
In this second part of the series on Steam & Condensate systems, We cover condensate behavior and heat transfer, as well as how syphons are used for condensate removal. Finally, we list the most important maintenance checkpoints for air, steam and condensate systems.
Steam is used as the heating medium in a dryer and enters the dryer in a saturated, or near saturated state. As heat is conducted through the dryer shell and into the sheet, the steam is condensed to liquid, giving up its latent heat. The latent heat is the motive heat source for drying paper.
The above figure shows the four main stages of condensate behavior as the dryer rotation speed changes:
The cascading stage offers the best heat transfer because of the very turbulent conditions inside the dryer, but the power surges may cause gear and bearing problems. On high-speed machines, dryer bars are normally used to break up the laminar condensate layer and thus improve heat transfer.
For a given diameter dryer, the speeds at which the above stages occur depend on the amount of condensate in the dryer. As the quantity of condensate in a dryer decreases, rimming will take place at a slower speed. This is important because syphons are more effective in the rimming stage than in the other stages. Most modern machines operate at speeds well above rimming.
Heat is conducted through several layers during the drying process. These include the condensate layer, any scale build-up on the dryer shell inside surface, the cast iron dryer shell, the dirt and/or air gap (if any), and the sheet. All of these layers offer some resistance to heat flow; some more severely than others.
The cast iron dryer shell has a relatively low resistance to heat flow; however, the condensate layer has a high resistance to heat flow. For comparison, a 1" thickness of water (condensate) has approximately 70 times the resistance to heat flow as a 1" thickness of cast iron. Therefore, it is very important that the condensate layer be kept thin so that heat flow will always be at a maximum to dry as much paper as possible.
So how do we keep the condensate layer as thin as possible? Condensate is removed from inside a rotating dryer with a syphon, as shown in the figure to the left. The two main types of syphons are:
All modern syphons require a certain amount of differential pressure, and blow-though steam to properly evacuate condensate and non-condensable gases (air). Blow-through steam is defined as steam that is "blown through" the dryer, without condensing and thus not giving out its latent heat. For a given set of conditions, there is always a certain relationship between the differential pressure and the amount of blow-through steam. The differential pressure vs. blow-through steam characteristics determine the design of the steam and condensate system. Therefore, it is very important that the condensate layer be kept thin so that heat flow will always be at a maximum to dry as much paper as possible.
The following points should be checked yearly. Annual service to the heating and ventilating equipment is usually done during the summer months when very little or no heating of the supply air is required. This ensures that the equipment is in good working condition for the coming heating season.
For more information on maintaining your steam & condensate system in optimum working order, contact your Valmet representative.