By Anders Nielsen, Application Manager, CBS HVAC Solutions, Grundfos
How new pumps can be a tool to increased system efficiency
Traditionally, we use pumps in hydronic systems to deliver the needed flow and pressure; that’s it, nothing more. Pumps including frequency drive have been around for years, but do we utilize their full potential? The author is confident we do not!! One area where we see challenges in district heating (DH) systems is when we want to introduce low temperature DH. Let’s look at a small and simple example:
In this case, we increase the flow from 491 m3/h to 688 m3/h. This leads to a (688/491)2 = 1.96 factor increase in pressure resistance. If we assume the distance from production to final residential consumer is 4,000m originally, we have a design pressure loss of 150 Pa/m, which leads to a total original pressure loss of 4,000*150 = 600,000 Pa = 600 kPa equal to 6.0 Bar. If we then increase the flow to the mentioned 688 m3/h, the pressure losses increase to 1.96*6.0 = 11.8 Bar. In other words, the pressure per meter will be: 150*1.96= 294 Pa/m.
Here, we have a traditional system with design flow temperature at 90°C and a return temperature at 55°C giving a delta T of 35°C.
If we decide to add this needed additional pressure at the same point as before, the pressure profile will look like this:
In this case, we set the DH plant to deliver a maximum capacity at 20 MW leading to a needed flow of: 20 * 860/35= 491,4 m3/h.
Now, if we want to operate the system by another temperature set like 65°C/40°C, the consequence would be a delta T drop from 35°C to 25°C as distribution of heat is expressed by:
Ф = Q* ∆ t, Power is equal to flow multiplied by delta T.
This leads to the following new flow: 20 * 860/25 = 688 m3/h; in other words, we need a 40 % increase in the flow demand to deliver the same heat capacity. Based on affinity laws, we know the relation between flow and pressure is so that if we want to double the flow, the pressure resistance increases by a factor 4. See below example, where the pressure increases from factor 1 to factor 4 (even though the flow only doubles):
Well, it might be possible to add the pressure, but in this case the maximum pressure grade for the pipes is only 10 bar, so as the needed pressure will be 11.8 bar, this maximum is clearly exceeded. The 10 bar can be illustrated as above red dotted line.
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