Steam drain / Sludge drying, Slurry drying, Recycling drying

Steams used as heat source of drying process is condensed and changes its state into liquid after releasing its latent heat. At the time of changing state to liquid, it is hot water which have the same temperature as the steam. The hot water is called drain. Among heat energies that steam in gas state has, latent heat is given to the material being dried and the steam becomes liquid as the result. This drain only has energy of sensible heat since it has released its latent heat and it needs to be discharged immediately while it doesn’t have heat energy that steam originally had.

In case that drain accumulates by being not discharged immediately, steam in gas state becomes drain without transferring its heat energy to material which is aimed to be dried originally. Since heat exchanger which heats the materials is filled with drain instead of steam, only heat energy of sensible heat of the drain is use for heating and performance of dryer drastically deteriorates. Also, drain is generated in steam pipes which carry steam since the pipes release heat as well.
Steam trap is used to discharge drain quickly. There are a lot of kind of steam trap and it need to be considered about selection of appropriate types, place of use, and number of uses. Accumulation of drain can be prevented more efficiently by setting smaller traps as many as possible.

Drain discharged from steam trap can be retrieved and re-used instead of being disposed. To recover and reuse drain is called drain water heat recovery. Since drain has sensible heat, fuel cost can be reduced by utilizing the heat and water can be saved as well.
From steam trap, white steam also generated together with drain. This steam is called flash steam. This flash steam is generated when drain which has high temperature under high pressure is released into atmospheric pressure and start boiling.
Drain cannot keep its high temperature since its pressure is decreased by being released to atmospheric pressure, so temperature of the drain drops to 100℃ and the drain start boiling. These drain and flash steam can be recovered and it is distinguished as open type recovery which collect only drain and closed type recovery which collect both drain and flash steam. More running cost can be reduced in closed type recovery since amount of recovered heat is larger though, facility cost will be more compared with open type recovery. Selection of the type is done by considering both initial and running cost.

1）Energy saving

By recovering and utilizing drain within boiler, energy of sensible heat in drain can be utilized. As the result, fuel cost of boiler can be reduced. Also, reduction of used amount of fuel will reduce amount of CO² emission, therefore, it can be said as contribution to the global environment and environmental conservation as well.

2）Water saving

Drain is water which is generated when steam in gas state changed into liquid, so it can be used as industrial water. By using it as supplied water for boiler, amount of water required in boiler can be reduced.

The following is a steam table. Specific enthalpy of sensible heat and latent heat is different in each steam pressure.

Comparison between heat pump type KENKI DRYER and the previous type which uses steam generated by boiler (saturated steam) is the following. Values are not actually proofed values but estimated values.

Heat pump dryers

Even KENKI DRYER uses only saturated steam as its heat source, it is outstanding in safety and hygiene point of view with its unique drying mechanism based on combined use of conductive heat transfer method and heated air method. Since steam is a stable heat source, quality of discharged material after drying is also stable and equable. Maximum allowed steam pressure is 0.7Mpa and adjustment of steam pressure, adjustment of drying temperature in other words, can be easily done. Saturated steam is commonly used in many factories so that it can be said as a familiar and handy heat source. In comparison with drying methods using burner or hot blasts, saturated steam method is an indirect drying applying heat exchange via pipes that steam is passing through, therefore, it hardly burns the materials and is outstanding in safety and hygiene point of view.

Why low-temperature is used for drying? / KENKI DRYER / sludge drying, slurry drying, feed drying

Heat source, Steam