WATER HYGIENE AND SAFETY – SPECIAL REPORT
flow from the tap for a significant period of time. In accordance with BS 8558, the cold water must reach a temperature of less than 20˚C within two minutes of running an outlet. If this temperature is not reached within the time specified, then the cold water installation does not meet this criterion within the standard. In addition to the aforementioned considerations, incoming mains temperatures of
Figure 2: A KHS CoolFlow cold water cooler.
above 25˚C are expected to be ever more of an issue in the summer months due to climate change.7
A re-think of current approaches to installation To ensure that hospitals and other healthcare facilities have compliant cold water temperatures in the future, we would argue that some of the installation methods traditionally employed need to be fundamentally reviewed. The measures taken should include consistent thermal decoupling of the cold water pipes from heat sources wherever possible, and steps to reduce or interrupt heat transfer (radiation, conduction, convection) from heat sources to cold water pipes. However, thermal decoupling of the cold drinking water pipes from potential heat sources is not always simple to achieve, particularly with horizontal distribution pipework in temperature-critical ceiling voids. In such installations, if water consumption is too low, heat absorbed from the cold water from the ambient air can no longer be dissipated. This may lead to an increase in the temperature of the cold water to ambient air temperature, and can only be avoided via the implementation of active measures, such as periodic cold water flushing or cold water circulation with cooling.
Influence of external heat loads There is also often insufficient
consideration of to the fact that in addition to the aforementioned internal heat loads, external heat loads can also have a significant impact on the heating of the cold drinking water. In winter, the room air temperatures, which affect the air temperatures in pre-wall installations, shafts, or ceiling voids, are largely constant, and range from 22˚C to 24˚C. External heat loads do not occur in the winter months, as the room air temperatures are usually higher than the outdoor air temperatures. In the summer, the conditions reverse. The outdoor air temperatures are usually higher than the room air temperatures during this period. In non-air-conditioned
38 Health Estate Journal December 2020
buildings, the air temperatures in the installation rooms also approach the prevailing outdoor air temperatures during the summer months. Model calculations show that in winter, when an installation shaft with heating and hot water pipes is fully encased, the average ambient air temperature of 26.2˚C must be expected. On a warm summer day, with heating switched off and room air temperatures of 27˚C, an average
ambient air temperature in the air composite shaft/pretext of 28.2˚C is calculated.
Particular challenges in the summer months
From these initial calculation results it can be inferred, in principle, that the temperature of cold drinking water reaches critical limits in the summer months rather than in the winter months. All the aforementioned passive thermal decoupling measures, which are effective in winter, largely lose their impact in the summer months with high room air temperatures. An unacceptable increase in the temperature of the cold drinking water in winter and in summer above a predetermined temperature (e.g. 25˚C) can therefore only be prevented via implementation of an active process, e.g. by temperature-controlled rinsing or, by cooling.
Active measures for compliant cold water temperature
Comparative simulation calculations show that after intensive manual or automatic water exchange measures, cold water temperatures can rise again to ambient air temperature after a relatively short period. These measures are only ecologically and economically sensible if the cold water can be supplied into the building at low temperatures (<15˚C). During the summer, however, this is often not possible, particularly if the water supply
is from surface water sources. In such circumstances, only active cooling of the cold water in the distribution pipework can ensure compliance with the required temperatures, at any time of the year. Cold water circulation was first successfully implemented in the main distribution lines of cruise ships. In Germany, numerous pilot projects have delivered exceptionally positive results in terms of functionality and cost- effectiveness, while achieving permanent circulated temperature for cold drinking water below 20˚C.
Systems incorporating flow splitters In order for a cold water circulation system to be implemented in conventional plumbing installations, an additional piping system must be set up. This is not necessary in installations that contain flow-splitters, as the loop piping system already available for the forced flow water exchange can be used for the water circulation. Existing KHS systems can often be switched from flushing technology to cold water circulation with little effort (see Fig 1). In contrast to conventional single pipe cold water installations, KHS Flow-Splitter installations enable good temperature control in all parts of the pipe, right up to the connections of the outlets. Calculations, taking into German insulation standards, indicate that that due to the small temperature differences between the ambient air and the cold water, the heat input – and thus also the performance of the required refrigeration unit – is relatively low – at only up to 3 W/m pipe. The KHS Coolflow cold water cooler (Fig 2) from Kemper removes the heat from the heated cold water and dissipates it. The pre-assembled compact unit with integrated pump contains all the required components for the cold water circulation and cooling, is tightly diffusion insulated, and preconfigured. It can be connected to existing cooling systems in buildings, or to a KHS CoolFlow Chiller (Fig 3).
Figure 3: A KHS CoolFlow chiller.
In hot water circulation systems the temperature differences between the water temperature and the ambient temperature are high. A higher circulating flow rate is thus required to maintain the required temperatures. By contrast, the temperature difference between cold water temperature and the ambient temperature is much lower. Volume flows required for temperature control in cold water circulation systems are therefore quite low. For this reason, the regulating valves required for hydraulic balancing have a very low kV value. In addition, the increasing concentration of water constituents must be counteracted by a targeted water exchange during prolonged circulation operation without water withdrawal. Kemper has developed
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