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Fitting
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Equivalent Pipe Length
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90 degree elbow
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30 times pipe diameter
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45 degree elbow
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20 times pipe diameter
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90 degree swept bend
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4-8 times pipe diameter
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Tee (Water flowing straight through)
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16 times pipe diameter
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Tee (Water flowing through side branch)
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60 times pipe diameter
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Gate valve (Fully open)
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9 times pipe diameter
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Losses through typical fittings expressed in equivalent pipe lengths
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Sometimes the manufacturer’s maximum recommended flow rate through a piece of equipment such as a heat exchanger, heat pump or UV clarifier is lower than the desired flow rate for the system as a whole and a bypass has to be fitted to divert some of the flow around it. With very little effort, friction loss can be used to make the bypass more controllable and therefore more effective. These diagrams show how this may be achieved.
Although ball valves are ideal for use in applications where the valve is either fully open or fully closed, they are not the most suitable valve in bypass circuits where control over water flow is concerned. In bypass circuits a gate valve is preferable to a ball valve since a typical 1˝ inch gate valve takes seven turns of the wheel to go from fully open to fully closed whereas a ball valve goes from open to closed in just a quarter of a turn. Since a seven turn gate valve takes a full turn of the wheel to make, approximately, a 15% change in the flow through it, these valves are much easier to set to give a precise water flow and are very easy to tweak to slightly readjust the flow if need be.
Gate valves that have been set to give a particular flow are also unlikely to cause a change in that flow due to them being knocked. On the other hand, the handle of a ball valve can easily be knocked and, with only a quarter turn from open to closed, any slight alteration in the position of the handle will cause a significant change in the flow through them. For this reason there is no listing in the table for ball valves and gate valves are used in all the diagrams. Despite that disadvantage ball valves have similar losses to gate valves and can be used instead if particularly preferred.
From the table above it can be seen that the friction loss for water flowing straight through a tee is equivalent to the loss when water has to flow through a length of pipe equal to 16 times the pipe diameter of the tee. This may seem surprising but this loss is due to the turbulence caused by the water having to flow past the open side branch.
The loss when water flows into or out of the side branch is equal to the friction loss if water flows through a length of pipe equal to 60 times the pipe diameter of the tee. The losses through a 90° elbow is 30 times the pipe diameter and the loss caused through an open gate valve is equal to 9 times the pipe diameter.
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Figure 1. A commonly seen but inefficient way to plumb in a bypass around equipment such as a heater
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As shown in figures 1 and 2, a typical bypass that has been plumbed so as to use the least number of fittings requires two tees, two 90° elbows and one valve but there are two ways in which they can be arranged. Figure 1 is the most common way in which the bypass is usually plumbed. Unfortunately this gives the least control over the amount of water flow that can be bypassed.
Consider the losses through the two branches, the flow through the heater (or UV) and the flow through the bypass around it. The losses through a heater (or UV) will be very small so, in order to simplify the maths, it will be ignored in these examples.
Looking at the bypass method in figure 1 first. For 1˝ inch pipe, with a typical inside diameter of 1.7 inches, the friction losses through the heater branch will be equivalent to a pipe length of 32 inches which is caused by the two tees that have losses of 16 inches each. So the total loss through the heater branch is equal to 54.4 inches of pipe (32 x 1.7).
The friction loss through the bypass branch is much greater. There are two tees with water flowing through the side branches, two 90° elbows and a gate valve. With the valve open, the losses given by the table taking each fitting in order of flow are; are 60 + 30 + 9 + 30 + 60 inches which totals 189. So the total friction loss is equivalent to 321.3 inches of extra pipework (189 x 1.7 inches).
Where there are two routes for water to flow through, the majority of the water will travel through the path of least resistance and, with the friction losses just calculated, the flow of water through each of the two paths is simple to determine. With the bypass valve fully closed, obviously 0% of the water can flow through the bypass but surprisingly, even with the valve fully open, only 17% (54.4 ÷ 321.3 x 100 = 16.93%) of the flow can flow through it. This means that the flow through the bypass can only be controlled over the range of 0% to 17% of the total flow! With only that amount able to be bypassed around the heater, the flow through the heater can only be varied from 100% down to 83%! Clearly a commonly found plumbing arrangement as shown in figure 1 is not very efficient at bypassing equipment such as the heater as shown or other equipment including UV units.
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Figure 2. A more efficient way to plumb in a bypass around equipment
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The bypass method shown in figure 2 may seem to be a counter intuitive way to plumb in a bypass but is much more efficient as can be shown by considering the friction losses through the two paths.
With the bypass valve fully open, the friction losses through that branch are the total of the losses through two tees and an open valve i.e. 16 + 9 + 16 multiplied by 1.7 which is equal to the loss caused by 69.7 inches of pipe work.
The losses through the heater branch is the total of two tees (with water flowing through the side branch) plus two 90° elbows. These, in order of flow, are 60 + 30 + 30 + 60 = 180 which, when multiplied by 1.7 is equal to 306 inches of pipe.
With the bypass valve in figure 2 fully closed, obviously 0% of the total flow can go through the bypass so 100% of the flow must pass through the heater. With the valve fully open, the flow through the heater is reduced to less than 23% (69.7 ÷ 306 x 100 = 22.78%) of the total flow which means that the flow through the heater can be controlled from 100% down to less than 23% of the total flow. Although the arrangement in figure 2 may seem unusual, it gives a much greater control over the flow of water through heaters that have to have the flow through them reduced by a bypass. Sparing you the mathematics, 90° elbows are preferable in this case, swept bends would be of no advantage since, depending on the manufacturer, they would actually reduce the control range of the bypass to 100% down to about 31%.
As mentioned before, the same plumbing arrangement may be used to bypass other equipment, such as UV units, without modification.
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Figure 3. For heat pumps, the bypass arrangement is essentially the same as in figure 2 but there is an additional valve that is used to provide the optimum back pressure through the unit
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Heat pumps are heating devices that often need a bypass to control the flow of water through them. They were originally designed as heaters for swimming pools etc. which use much higher power pumps than are used on the average koi pond so a typical heat pump could take the full flow from the much lower power pumps that are usually used on a pond. However, a rule of thumb is that they function best when the rise in water temperature through them is about 1°C to 2°C.
As always, the manufacturer’s data should be checked for the optimum temperature rise through the unit since this rise will affect the pressure of the refrigerant. The full description of why the refrigerant pressure is important to the performance of a heat pump is beyond the scope of an article on bypass methods. However, in short, the manufacturer will have designed the compressor and expansion valve in the refrigerant circuit to provide the maximum transfer of heat into the water flowing through it at a specific refrigerant pressure.
The bypass plumbing arrangement for a heat pump is essentially the same as the bypass shown in figure 2 but there is an additional valve immediately after the unit. This is called the flow control valve and is there so as to adjust the back pressure which will often be necessary in order to trigger the flow control switch that must operate in order to “prove” to the heat pump that there is sufficient flow through it for it to safely begin its heating cycle.
Individual manufacturers may have slightly different procedures for adjusting these two valves but, in principle, the flow valve should be adjusted so as to trigger the unit to come on and then the bypass valve should be adjusted to give the optimum water flow or refrigerant pressure. The flow valve may then have to be readjusted if the heat pump switches off and this may be followed by a small tweak of the the bypass valve to restore the optimum water flow. This may sound complicated when described in words but the procedure is simple in practise - adjust the flow valve, adjust the bypass valve followed by tweaking the flow valve and then the bypass valve for optimum performance as described in the user manual.
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