Make turbulent times your target

A hydronic system relies on many factors to ensure that it works effectively and efficiently. One point that can greatly affect the efficiency of a waterborne system is the state of the water it contains. Achieving a turbulent state is the ultimate goal of those responsible for designing, operating and maintaining any system. This will help to make control and measurement far more accurate, resulting in better efficiencies.

With energy savings from production plant and pumping being high on the agenda, larger Delta ts are allowing greater opportunity to make the best use of these advancements in technology.

However, such increases mean a reduction in the flow rates of hydronic systems. For example transferring from an 82/71°C regime to an 80/60°C will almost halve the flow rate required. While this has the positive effect of helping to reduce pumping costs considerably it also causes a number of other effects that must be addressed in order to ensure they do not inhibit the efficiency of the system.

When drawing a comparison between turbulent flow and laminar flow states there are some notable differences that can impact the way in which the overall hydronic system operates. Whilst laminar flow can be used to good effect in other systems, such as airborne, oil based systems and our own circulatory systems, it can greatly affect the way in which a hydronic, waterborne system operates. For this reason it is absolutely crucial that a turbulent flow state is achieved instead.

Laminar flow

Laminar flow, which can be referred to as a streamline or viscous flow, is a state of non turbulent flow that can occur in pipework or within a boundary where the velocity of the fluid, in conjunction with the pipe or boundary material, becomes low enough to instigate the regime. As a result of low velocity, there is almost no mixing of the fluid between its layers, meaning the fluid particles will move in set streams through the pipework. The effect of this when applied to water is that the efficiency of the hydronic system is significantly reduced.

To ensure such issues do not arise, it is crucial that laminar flow is avoided and steps are taken to instigate turbulent flow. A turbulent flow state will ensure water within a hydronically balanced system moves at its intended velocity and therefore helps the system to perform as efficiently as possible.

Turbulent flow is characterised specifically by random flow, recirculation and localised eddies. Whilst its name suggests that it is a completely random disorder, turbulent flow actually acts in a very controlled and organised manner, resulting in coherent behaviour of the individual water molecules.

Flow regimes such as laminar and turbulent states are usually quantified using the Reynolds number series. Laminar flow would normally be present when Re<2300, moving into intermediary from 2300 to 4000. Intermediary flow is therefore essentially a combination of both laminar and turbulent conditions. The flow will only move into a fully turbulent regime where Re>4000.

Introducing the required turbulent flow state where Re>4000 is relatively simple and it can be achieved in one of two ways. The first requires the pipe size or boundary to be decreased, thereby increasing the flow velocity. In this way, the same volume of water is working its way around the system, yet because it is being forced through a smaller pipe the result is an increase in velocity.

Secondly and as an alternative, turbulent flow can also be achieved by increasing the flow through the pipe already in place to achieve the velocity increase.

Measurement accuracy

When considering the measurement accuracy of a hydronic system, only in a turbulent flow state is Dp q2, thus allowing for accurate flow measurement. On the other hand a laminar flow state occurring in the pipework of a hydronic system will create dramatic inaccuracies. This means that measurement becomes incredibly unpredictable.

In order to overcome some of these inaccuracies in measurement it is possible to utilise measuring valves that make use of the Venturi principle. These valves increase the velocity over the two static measurement points and can provide far more accurate results.

To ensure fully turbulent flow is achieved, it is worth noting the minimum flow permissible in standard 15mm pipe. Taking 15mm Tx Copper pipe containing 70°C water as an example, the minimum flow requirement to maintain a turbulent flow state is 0.016l/s, which corresponds to a velocity of 0.116m/s. Similarly, if 10°C water (without any additives) is used in the same pipe the minimum flow is 0.049l/s in 15mm Tx with a velocity of 0.337m/s. These flow rates are higher than people generally expect. Assuming we are using a Delta t of 20°C then a flow of 0.016l/s would translate to a minimum output of 1.34kw in order to prevent laminar flow.

In terms of heat transfer, laminar flow can impact its effectiveness, due to the fact that the water can become self insular in this state, meaning the ability of the water to transfer heat can suffer significantly. As a result of this, heat transfer is far more effective in a turbulent flow state. For example, in a chilled beam application where water velocity drops enough to instigate laminar flow, the effective heat transfer can drop by as much as 60% reuslting in a less energy efficient system.

Hydronic circuits

In a hydronic circuit, laminar flow can occur due to many contributing factors – fluid temperature, viscocity, velocity, pipe material and pipe size, to name a few. To help avoid such instances, specialist software, such as the TA Select 4 can be used to calculate such issues incredibly accurately. Software such as this will automatically take into account all of the variations of contributing factors that can lead to a system being in this state.

Laminar flow is a less desirable state for water contained in a hydronically balanced system. Instead turbulent flow will deliver more accurate measuring and greater efficiencies of heat transfer. For the building services engineer, by taking such measures to ensure that laminar flow is avoided it is possible to ensure that a hydronic system operates according to the standards by which it was designed, with a turbulent flow state contributing effectively to system efficiency overall.

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