Pump Sizing Predicaments
There’s so much involved in water distribution system design, and right from the very start decisions need to be made that can have an impact much further down the line. One of these decisions is pump size. Ian Harman, Marflow Hydronics’ Technical Applications Engineer and resident blogger, takes a look at what’s really involved and why it’s so beneficial to get it right.
It’s seems to me that, as an industry, we are substantially over sizing pumps in variable volume circuits. Over sizing these pumps causes two key problems: firstly it will cause excess energy to be used at the pump and, secondly, it lowers ΔT in the circuit causing plant efficiencies to drop.
If we take the majority of systems, they are based around a single pump (ignoring duty and standby arrangements), sized to fulfil 100% of system load. Traditionally there’s a bias towards ensuring a pump is not undersized, and this means that those involved cater for this bias. For example, a design overhead is added to the duty point by the designer, a safety margin is added to the duty point by the installer, and pumps are often selected so that the duty point is at 75% of the maximum pump output.
In a constant volume system none of these elements cause a particular problem, but if we’re to take a variable volume system then it’s rare for demand to exceed 40-50% of design, and quite often it will be lower than this. Most variable speed pumps can only turn down to around 10% of their maximum speed and at the minimum speed any flow in excess of the demand must be by-passed.
It’s vital to remember that what seems reasonable at design flow will increase proportionally as the demand decreases. For example, 10% system by-pass at design load will become 25% at 40% load. The higher the proportion of by-passes flow in relation to the load then the lower the ΔT will become compared to design.
To improve the situation, two things can be done. Either alternative pump arrangements could be explored, or the circulating pump could be sized with a diversity factor.
Several alternative pumping arrangements could be used. It might be that two pumps are needed, working in parallel, each sized at 50% of design load. A large pump could cover full system loads and a smaller pump to cover lower typical part load duties.
To size the pump with a diversity factor means that the pump is sized to less than calculated design duty. Although this is widely accepted as being suitable for variable volume comfort systems, not many designers are willing to specify this solution because of the lack of guidance on what’s an acceptable level of diversity.
In one of my recent blogs I put together an A to Z of System Design to try and summarise all the essential considerations. Visit the link below to find out more.
