Scale in a hot water boiler

Company: Südbad, Stadtwerke München, Munich, Germany

About the customer:

Stadtwerke München is supplying greater Munich with water, energy and operating several city owned estates. The water they are supplying is coming from the Alps, to produce electricity they are operating their own power plants, supplying hot water for district heating and of course electricity. The area they are supplying is covering the city of Munich itself and the surrounding counties, all together approximately 8 Million people. The actual case is a big  public indoor swimming pool owned by the city of Munich and operated by the customer. MERUS has done for SWM a row of projects in both outdoor and indoor swimming pools.

The Problem:

The actual application in the "Südbad", an indoor swimming pool with sauna, are the hot water boilers. The storage capacity of hot water in the boilers is rather high, in order to meet requirements of peak consumption times. All together there are 4 boilers each with a capacity of 3.000 liters, they have to feed more then 30 showers.

The temperature in the boiler has to be 60°C, to avoid a possible growth of legionella in the water system. Also at days, where there are fewer customers in the pool using the showers. At such days there is only little flow in the boiler. As a result the heat bundle has a high tendency of scaling. The raw water is chemically not treated, due to cost reasons.
Due to little flow in the boiler itself, there is a lot of scale forming at the heat bundle. This scaling is more then it would be in a pipe or a heat exchanger, as the flow across the heat bundle pipes is lesser, then in would be in a pipe or a classical heat exchanger. So the customer has to clean the boiler on regular base. Dismantle the boiler, remove the bundle and clean it chemically and with a lot of manual force.

The Solution:

The 4 pieces 3.000 liter boilers are all in one room, very close to each other, so we have installed only one 3" MERUS Ring at the common feed pipe for the 4 hot water boilers.

The Monitoring:

The monitoring could be done only visual, during shut down at planned service times. The heat bundles have been removed from the boilers, and the result can be seen at the two pictures on this page. There was still some scale found on the heat bundle, but it could be observed the scale was only building at the upper side and the scale is very soft. As can be seen in the right the scale was so soft and loose. There have been big junks of scale, but not sticking to the heat-pipes as before, but loose on top. These junks are coming of during normal operation and fall down, either collected on the lower end of the bundle or at the bottom of the boiler.
Just by putting the bundle on the earth most of the loose scale just fall off. The cleaning was done in short time, just using a normal hose and wash away the scale, no chemicals needed or hard manual cleaning as before.

The Commercials:
By using only one 3" MERUS Ring, the investment has been very minor compared to the savings only during the shut down for cleaning. The savings on energy, having less scale at the heat exchanger bundles, are very difficult to calculate, as no historical data are available. But it is to be assumed, they are significant as well, as one millimeter of scale is decreasing the heat transfer by 14%.