Introduction:
Water is literally a fact of life, and fetching water has always been arduous –but necessary- work. Ancient civilizations emerged and flourished where there was plentiful water for agriculture, and communities grew near rivers, lakes and water wells.
Over the centuries, however, we have moved from rural villages to towns and cities. Urbanisation means that more of us are living and working in high-rise buildings, and humanity is now facing new challenges of this age-old problem: bringing water to where it is needed
Nowadays, water supply systems in tall buildings must be both energy-efficient and reliable.
Purpose:
The purpose of this white paper is to introduce some of the issues of planning and implementing water supplies to high-rise buildings, including the need for high-pressure water boosting pumps.
People are moving closer together: from the villages into the cities. Globally, city populations are growing by 65 million every year with most of this growth occurring in Asia and China. By 2025, more than 2.5 billion people –or half of the world’s total city dwellers- will live in Asian cities.
As people move closer together, buildings need to be taller. The acceleration in the numbers of tall buildings can be seen in the figure below.
Tall buildings require water for tap water and cooling towers to function properly, and providing sufficient water to a tall building is a discipline of its own.
In addition, buildings in general account for 40% of the world’s energy consumption so tall buildings would be a good place to start if we want to save energy globally.
We believe that water boosting is an overlooked area for optimisation and there is considerable difference between the best and the worst booster system designs.
This white paper will consider some of the parameters for designing water supply systems for tall buildings, the components that make up a water boosting system, and ways of ensuring maximum energy efficiency.
Water towers
For centuries, water supply systems relied on crude mechanical lifting devices to raise the level of the water, sometimes supplemented with gently sloping channels and pipes through which the water would flow due to gravity.
With the advent of efficient pumps, it was possible to construct water towers. A water tower is an elevated structure supporting a water tank constructed at a height sufficient to pressurise a water supply system for the distribution of potable water, and to provide emergency storage for fire protection.
Water towers are able to supply water even during power outages, because they rely on hydrostatic pressure produced by elevation of water (due to gravity) to push the water into domestic and industrial water distribution systems. However, they cannot supply the water for longer periods without power, because a pump is typically required to refill the tower.
A water tower also serves as a reservoir to help with water needs during peak usage times.
The water level in the tower typically falls during peak usage hours during the day, and a pump refills it during the night, when demand is low.
This process also keeps the water from freezing in cold weather, since the tower is constantly being drained and replenished.
Break Tanks
Taking the principle of lifting water and storing it in an elevated reservoir one stage further, it is also possible to have a cold water break tank installed in the home. This ensures that water is readily available, and can be used in areas where the mains supply pressure is inadequate.
As we shall see later, break tanks and cisterns can also play an important role in water supply systems for tall buildings.
Booster pumps
Clearly, anyone living high up in a skyscraper is depending on a pumping system to supply the water. Therefore, high-quality pumps are crucial in booster systems. Grundfos produces many different types of pump, one of them being the multi-stage type which again comes in a wide variety of models.
What counts above all though, is the way the core of the pump is designed. Multi-stage pumps for boosters are all in-line multi-stage pumps. That means water pressure is gradually built up when the water passes through the different stages. When the final pressure level is met, the water exits the pump at the same level it entered (hence the name ‘in-line’).
Pressure reduction valves (PRV)
A water supply system in a tall building must treat consumers equally, whether they are on the top or ground floor. This difference in height equates to a difference in hydrostatic pressure. To equalise pressure on all floors, PRVs are often used in multi-story buildings. The pressure is mechanically reduced directly in the string, making it possible to adjust the pressure precisely for each floor. The PRV can either be used individually with one on each floor or in a branch of a raiser supplying 2-3 floors.
The PRV is a rather simple way of controlling the pressure throughout a tall building.
However, there are some disadvantages when using PRVs:
• For each PRV needed in the building layout, the initial cost increases.
• The PRVs also need maintenance and therefore need to be placed at an accessible position.
• Each PRV represents a loss, because energy used to create higher pressure is wasted.
• Risk of pipe damage and flooding if a pressure reduction valve fails and lets high pressure into a lower-grade pipe net.
Key system parameters
The pressure boosting system consists of many parts, all more or less influencing the system size, performance and energy consumption.
However, which ones are most important to focus ones attention on in the design phase? Grundfos has recently carried out a research study investigating the sensitivity of such parameters on the hydraulic side of pressure boosting systems.
This is done by means of an influence coefficient (IC) describing the influence of changes in a given parameter, e.g. pressure loss in riser pipes (Pa/m), on the total hydraulic booster power necessary in a system. As such, the IC value of a given input parameter describes the magnitude of total variability in the output parameter that is caused by the given input parameter. For instance, if the pressure loss in riser pipes constitutes an IC value of 5%, it means that only 5% of any change in pressure loss is ‘felt’ by the booster pumps.
Such knowledge is vital when designing systems, as it enables the designer to focus on parameters that truly influence the result instead of spending time determining and sizing parts of the system that only influence the result marginally.
In the table below, some of the parameters often considered are listed and ranked according to their influence coefficient, when considering the hydraulic booster power.
Load profiles
Different load profiles occur for different buildings as already described in the previous section.
In order to obtain a better overview on how much water is consumed based on use, the table below can serve as guidance.
This figure shows assumptions based on Danish legislation DS 442/1989 Code of Practice for common water-works.
The more one knows about the actual use, the more likely it is that the final annual energy consumption can be as low as possible.
Qyear m3/year: How much water is consumed on a yearly basis per person (e.g. 25 m3/yr/pers in an office building)?
Consumption period, days/year: How many days a year the building is projected to be in operation.
Q (m)day / m3/day : How much water is consumed per person per day (office building: 25/250= 0.1)
fd: Concurrency factor.
ft: Peak flow factor.
Max. flow rate, m3/h
Max. flow rate per hour per unit; e.g. 0.018 m3/h per employee in an office building.
The calculation of water flow etc. will of course vary from country to country, depending on national guidelines and legislation.
Tap pressure
Normally, tap pressure is set to be within the range 1.5 – 5.0 bar,
Due to differences in height or elevation in a tall building, pressure will gradually decrease as the water ‘moves’ upwards.
Example:
When one booster zone covers, e.g., five 4.2m floors, the pressure will as a minimum decrease by 5 x 4.2 = 21 mWc (meters water column)
Firefighting
Fire protection is a highly regulated area. There are different standards around the world that stipulate the design, performance, system approval and maintenance of fire extinguishing systems.
The relation to pressure boosting is rather limited -with one exception. Hose reel systems are often connected to the booster system and, as such, this may affect the sizing of the booster system.
Supply of water for cooling towers
The impact of water consumption by cooling systems (seen from a water boosting perspective) is the sum of the following:
1. Evaporation in the cooling towers.
2. Water used for blow-down: Water consumed for blow-down accounts for the majority of water usage. The actual amount is influenced by the level of water treatment, as the better the water treatment, the less water has to be used for the blow-down operation.
3. Water drift due to wind: Drift is water leaving the tower due to wind passing through the tower. The related loss is uncontrollable and is typically calculated as a percentage of the total recirculating flow. Depending on the tower design, this value can vary greatly, but figures like 0.008 % are used as a rule-of-thumb value for new systems.
4. The evaporated water has to be replaced by tap water, often referred to as make-up water. The amount of make-up water is influenced by several elements, e.g. where is the system placed, wet bulb temperature etc. In Denmark there is a rule of thumb, saying that approximately 1.12 m3/h per MW of cooling capacity. According to an ASHRAE study, the following rule of thumb can be used:
SI: 0.0004 L/s x kW.
Layouts
Booster systems may be designed in several different ways with the elements described above. Which layout to choose depends on many factors and the specific task in question, local legislation and traditions, flexibility requirements or the possibility for future expansions, etc.
No one system layout is ideal for all scenarios.
The advantages and disadvantages of some the most used system layouts are described below.
Single booster systems
A single booster system is perhaps the simplest booster system available. It relies on a single set of pumps supplying pressure boosting from the basement to the point farthest away from the booster system. Such systems may be configured with or without initial break tanks.
Zone-divided booster systems
The building is divided into pressure zones of ten floors or less with a booster supplying each zone from the basement though dedicated risers.
Roof tank systems
Rooftop tank systems are very common, especially in the USA, and the layout uses a fill pump in the basement to service the roof tank by a level switch-operated control. This way the roof tank ensures water pressure as well as water supply.
The solution requires pressure reduction valves on each floor if the building exceeds approximately 15 stories, in order to avoid unwanted high static pressure at the taps. It also requires a booster to provide the top floors with the required pressure, as static pressure there is too low due to insufficient geodetic height at the roof tank.
Series-connected systems with break tanks
Series-connected systems with intermediate break tanks draw on several other systems, utilising centrally placed break tanks to supply both the taps, the tank’s own boosting zone and all the zones above it. With this system, a building is divided into smaller and more manageable pressure zones. Every zone is then served by its own booster set.
Series-connected systems without break tanks
A series-connected system operates on the same principles as the previously mentioned system, but without the intermediate break tanks. This enables an effective usage of power as the water is only pumped to the part of the zone where it is used and not past it.
However, complete control is very important when using this layout. When a consumer draws water on the upper floors, the booster systems must be able to deliver the water from the bottom of the building.
SIZING
Pressure head
The term ‘pressure head’ refers to the height of a column of fluid of specific weight required to give a pressure difference:
H=
where
H = pressure head [m]
ΔP = pressure difference [Pa]
ρ = fluid density [kg/m3]
g = gravitation acceleration [m/s2]
Head Rise
Sizing a booster system is all about determining the necessary pressure increase that the pumps have to deliver in order to transport the fluid from one location with a given initial pressure to another with a required tap pressure, though a closed conduit.
The transportation of water through the pipe system requires the pumps to overcome the pressure head loss of the system. The head loss is a combination of friction loss in straight pipe and friction loss in system components.
In addition to the head losses, the booster set also has to overcome any change in elevation (static height) from system inlet to outlet.
The necessary discharge pressure required of the booster set is calculated as:
hp = htap + hZ + hL – h0
where
hp = head rise by booster pumps [m]
htap = head at tap point (tap pressure) [m]
hZ = head elevation (static height) [m]
hL = head loss in the pipe system [m]
h0 = head at system entrance [m]
Static height
Any elevation change in the transportation system requires an additional head rise equal to the height. This is also called the static height because it does not change and thus determines the minimum capacity of the booster set. The static height is determined as the elevation from the booster’s discharge pipe to the highest tap point. If the building is divided into zones the static height of each zone has to be determined.
Major head loss
When transporting a viscous fluid (water) in a closed conduit (pipe), shear stresses appear in the intersection between solid wall and fluid due to friction, resulting in a velocity profile across the pipe diameter. Near the pipe wall, the fluid only experiences very little movement. However, closer to the centre line, the fluid flows more freely and thus at a higher speed. This friction loss has to be overcome by the booster pumps in order to generate the desired water pressure.
The nature of the flow, either laminar, turbulent or somewhere in between, is characterised by Reynolds number. In the illustration to the left, the velocity profile is shown for fully developed laminar and turbulent flows. The very close area near the pipe wall is called the viscous sub layer. This is where the fluid touches the solid wall, and it is thus of key importance for the friction loss in straight pipe flow when considering turbulent flow.
A rough wall (large contact area with fluid), -e.g. pipes made of cast iron or concrete- will generate large shear stresses and subsequently require more booster power for the same flow than had the pipe wall been made of a smoother material as, for example, galvanised iron or, preferably, plastic.
The pressure loss in straight pipe flow is a function of the average fluid velocity, pipe length, wall roughness, fluid viscosity and fluid density.
This pressure loss – the major head loss — is calculated as
Where
hL major = major head loss [m]
f = friction factor [-]
l = pipe length [m]
D = pipe diameter [m]
v = fluid velocity [m/s]
g = gravitation acceleration [m/s2]
The friction factor is the tricky part of the calculation. It is a complex function of the Reynolds number and the roughness of the pipe that cannot, as yet, be obtained from a theoretical analysis. In practice, an exhaustive set of curve-fitted experimental data is used, often in its graphical form – the so-called Moody chart – or in the form of the Colebrook formula, which requires an iterative solution scheme.
Friction losses
Most pipe systems consist of considerably more than just straight pipes. These additional components (valves, bends, tees, etc.) add to the overall head loss of the system. Such losses are generally termed minor losses and are
calculated as:
hL minor = KL
Where
hL minor = minor head loss [m]
KL = loss coefficient [-]
The loss coefficient (KL) is a function of the Reynolds number and the geometry of the component. It is determined experimentally and may be found in tables. As noticed, the head loss (both major and minor) is a function of the fluid velocity, which means that it will increase for higher flow rates – it is a so-called dynamical pressure head.
Combined, it may be expressed as:
Conclusion:
Providing water for people living and working in a modern city is a prerequisite for the city to function. Pressure boosting is required for a number of different reasons, whether being an extension to an existing building or part of a new high-rise building.
Whatever the reason, there are several parameters to take into consideration when design and sizing water boosting systems. Intelligent selection of the layout type is likely to save a good deal of money on the energy consumed by the building over its lifetime.
