In common with any large residential building, a prison needs copious amounts of hot water. Some estimates put the figure at more than 160 litres of hot water per inmate each day.
Heating, storing and distributing this hot water is energy intensive and, if done inefficiently, can risk the growth of bacteria such as Legionella, which is the root cause of Legionnaires’ Disease.
Saint-Quentin prison, near Lyon, has overcome both of these problems by replacing an antiquated, domestic hot water system with five AquaProtect tap water systems from Alfa Laval.
After a public tender process, a French contractor was awarded the contract to install the new tap water heating system. It chose Alfa Laval’s AquaProtect systems because it is safe, reliable and eliminates the risk of Legionella infection.
Enclosed, warm storage vessels, blind spots in pipework and water systems containing stagnant water provide an ideal environment in which Legionella bacteria can flourish.
The AquaProtect system solves this problem by continuous thermal disinfection of incoming and circulating water. Each of the five AquaProtect units is fitted with a reaction and storage tank.
No additional energy is required to provide thermal disinfection since an integral heat exchanger recovers sufficient energy.
AquaProtect employs two heat exchangers; the first is connected to the heat source and is used to disinfect water at 70°C. The second cools water from 70°C down to the temperature required at the tap (normally 60°C) and pre-heats incoming and circulating water before it enters the first heat exchanger.
Once heated to 70°C, the disinfected water needs to be maintained at this temperature for a given period of time to ensure eradication of any bacteria. Dwell times are generally determined by local legislation.
Saint-Quentin opted for a system with a reaction tank and a storage tank to ensure the correct holding time. The reaction tank has an internal configuration that controls the direction of the flow, thereby uniform temperature dispersion and disinfection.
From the reaction tank, disinfected water flows to a storage tank where it is stored until there is a peak demand period. Disinfected water flows from this tank to the second heat exchanger to be cooled ready for distribution.
A mixing valve mixes the disinfected water from the storage vessel at 70°C and the cooled, disinfected water coming from the second heat exchanger.
