Often high pressure rates are promoted as critical within temperature control, but this is a common misconception. High pressure is actually detrimental to the process.

Here we try to explain the importance of low pressure and high flow rates when achieving constant, stable temperatu-res and good control.

Why is flow important?

In energy transfer the more thermal energy you can pass from the temperature control unit to the process, the quicker the temperature will change. Therefore high flow rates are an important factor in good temperature control.

Circulation pumps generate high flow rates at low pump pressure. Increasing pressure does not speed up the flow of the heat transfer flu-id. Looked at it simply, only so much liquid can travel through a space at any given time, as a result, increasing the pressure only builds up pressure and doesn’t speed up flow. In addition, pressure creates heat which builds up making it harder to achieve low temperatures.

Why is low pressure important?

• Most glass reactors have a maximum pressure of 0.5 Bar anything above this will compromise the reactor.

• Pressure creates heat therefore the heat losses associated with high pressure have a greater impact when trying to achieve low temperatures.

It can be seen that increasing pressure has a rapidly diminishing effect on flow and in the upper part of the curve, and further increasing pres-sure has little to no effect on flow.

Pressure/Flow Relationships & Pump Curves

Many manufacturers promote the high pressure rates that are achie-vable in their systems but the laws of physics tell us that high pressure is actually detrimental to flow rates and thermal transfer as system curves, figures one and two.

System curves

A typical hydraulically sealed system has a pressure/flow relationship as follows:

Increasing pressure does not increase flow

Pres

sure

(Bar

)

Flow (Litre/min)

Figure 2

Catalogue DataMax Flow

Figure 1

Catalogue DataMax Pressure

Flow (Litre/min)

Pres

sure

(Bar

)

Technical NoteHTB02 - Huber Technical Bulletin

Flow and Pressure

Peter Huber Kältemaschinenbau GmbHWerner-von-Siemens-Strasse 1

D-77656 Offenburg/GermanyTelephone +49 7 81 96 03-0, Fax +49 7 81 5 72 11info@huber-online.com, www.huber-online.com high precision thermoregulation

Good heat transfer requires high turbulent flow and low pressure, so powerful pumps are not the answer when trying to improve performance.

Depending on the set up, the flow/pressure curve changes as shown in Figure 3:

• System 1 is a bad set up with typically long hoses and small diameters (High flow restriction)

• System 2 is a reasonable set up, but could be better

• System 3 is a good set up (short hoses with large diameters gene rating minimum flow restriction.)

Figure 4 shows the same 3 set ups with the addition of the pump curve. The points where the system curves cross the pump curve are the operating points of the three different systems.

Figure 5 illustrates the same three representations of system and pump curve (A) with the addition of a second pump curve (B) that represents either a more powerful pump or an added booster pump.

It can be seen that as pressure increases, flow barely increases at all (the curve remains almost vertical). The extra energy injected by the more powerful pump (B) appears as heat in the circulating HTF and can often cancel out any benefit of the minimal increase in flow.

The exception is with the best set up (low flow resistance) were increasing the pressure has a larger effect on flow but again, the extra energy introduced by the stronger pump will (at best) detract from the effects of increased flow.

Conclusion

Using shorter hoses with larger diameters increases efficiencies and performance by keeping flow resistance to a minimum.

Stronger pumps do NOT significantly increase flow rates and the small improvement in flow rate brought about by a stronger pump is cancelled out by the increased energy entering the system as heat from the bigger pump motor.

To demonstrate the performance of the Huber product range over 200 case studies have been created to show the Unistats unique ability to adapt to process requirements.

Visit the website to find out more!

Figure 3

System 1Bad set up, High

resistance, low flow

System 2OK set up, OK

resistance, OK flow

System 3Good set up, low

resistance, good flow

Pres

sure

(Bar

)

Flow (Litre/min)

Figure 5

Curve is almost vertical showing little improvement in flow

Pres

sure

(Bar

)

Flow (Lire/min)

small improvement in flow

A

B

Curve is almost vertical showing little improvement in flow

HTB02 - Huber Technical Bulletin

Figure 4

High pressure, low flow.Poor thermal transfer, high losses,

poor temperature control

Low pressure, high flow.good thermal transfer, minimal

losses, good temperature controlPres

sure

(Bar

)

Flow (Lire/min)