Why - Suction Pressure Optimisation?

In this article, we're going to discuss what Suction Pressure Optimisation (also known as floating suction pressure) is, why we can "optimise" it, and why we should be optimising it.

What is Suction Pressure

In previous articles we've discussed evaporating temperatures and Saturated Suction Temperatures (SST), which is the saturated temperature at the inlet to the compressor. As we know, temperature has a relationship to pressure, and an SST is the saturated temperature equivalent of suction pressure.

R404A with an SST -10°C corresponds to a suction pressure of 3.29 bar(g).

Optimising suction pressure

When we hear "Suction Pressure Optimisation" it can just as reasonably be read "SST Optimisation" or "Indirect Evaporating Temperature optimisation" - which then enables us to use our rules of thumb regarding increasing or decreasing saturated temperatures for CoP.

There will be various methods of optimisation, but a centralised controller will poll various evaporator controllers for if they are satisfied based on a variety of conditions.

If all or most evaporator controllers report they are satisfied, the centralised refrigeration controller will remotely increase the suction pressure setpoint of the refrigeration system, usually by a fixed increment. This polling process will then repeat on a fixed poll frequency and the process continues. If some fixtures start to become unsatisfied, the revised suction pressure setpoint might be held, or decreased depending on the control philosophy and commission. Effectively one will be looking to "squeeze the pips" of the evaporator controllers, so they are on the verge of being unsatisfied.

The principle is quite simple; however, the devil can be in the details. One, for example, should bear in mind the maximum opening pressure of the expansion values as commissioned, before increasing the suction pressure ceiling. You tend to get phone calls.

Why can we optimise suction pressure?

If you've asked that question, it's a good one. And there are two answers. to this.

Diversity of Load

When people talk about diversity in refrigeration, it is a label that refrigeration systems do not operate at full load for 100% of the time. Why is this the case?

Take a 2.5 metre upright refrigerated display cabinet, which has been rated to ISO-23953 3M1 performance. Taking it out of the environmental test chamber, and place it in a supermarket, it will (very likely) do less duty. One of the obvious reasons for this, is that a UK supermarket does not have an internal condition of 25°C and 60% relative humidity for 24 hours of the day, unless something is very wrong.

The evaporator is still sized for this environmental condition, however. Reduce the related environmental load, we have an excess of heat transfer area for a given temperature difference. This excess could be exploited by decreasing the temperature difference of the evaporator, by increasing our suction pressure setpoint.

Ever-variable mass flows.

Previously when we've gone through our TEWIs etc, you will have seen that CoP, in line with our rules of thumb, goes up when our condensing temperature goes down. This is for a variety of reasons, but the most important reason is that at lower condensing temperatures, you get much further down the enthalpy axis.

In simple terms, a kilogram of refrigerant does more kW of refrigeration in winter, than it does in summer and our refrigeration mass flow reduces (in addition to it reducing due to diversity).

What does this mean fundamentally? Two things.

• Our evaporator performance increases with lower condensing temperatures.
• Compressors do less work and consume less energy.

This increase in performance gives us another increase in excess in evaporator heat transfer area (and condenser heat transfer area) to exploit.

Why should we optimise suction pressure?

Our path towards achieving net zero will, in part be winning the game of inches

Any Given Sunday - because just give me an excuse.

At 4%-12% or greater reductions in energy due to correctly regulated suction pressure - optimising suction pressure is one of those inches.

Excluding increasing evaporating pressures - we should be doing it anyway...

A variable mass flow rate of gas also corresponds to a variable volume flow rate. Volume flow rates affect velocity, and velocity affects pressure drops. The lower the velocity, the lower pressure drop. Typically, this is an unalloyed good, providing that there is sufficient velocity for oil return.

If we have 100 kW of evaporators sized for -8°C, at nominal (full load, max ambient) suction pressure drop of 2 K in equivalent saturated pressure, and an SST fixed setpoint of -10°C (-5°C - 2 K =-10°C). This gives us a nominal volume flow rate which our pipe work is sized on.

But as we might not reach Climate Class 3 in our Supermarket, we might have night blinds and such like, and we're condensing at 20°C in winter, our refrigerant volumes might be significantly less. Taking the above cycle, and using Coolpack...

If our refrigerant velocity goes down, our suction pressure drop goes down, in this example, we have the equivalent of less than 1 K. With a fixed suction pressure set point what happens to the evaporating temperature?

The evaporating temperature reduces, and in this example would be below -9°C. Ignoring how else the evaporator is controlling, this reduces our "Apparatus Dew Point", and increases our latent duty (i.e. dehumidification) load increases for no useful purpose. That increased ice formation then increases our defrost durations and associated energy uses from where they otherwise would be.

That's another inch that most systems are paying the price on, every single day. Slightly less of an issue some systems, and slightly more of an issue with some refrigerants.

Conclusion

We've just discussed what suction pressure optimisation is, how it should work in principle (albeit the exact mechanism might differ between manufacturers), and we even got a PH Diagram into the mix (useful for everything!).

Things to remember.

The SST at the pack directly affects the power input of the system and indirectly the duty of the system. The evaporating temperature directly affects the duty and indirectly power input of the system. We need to consider both. Optimising either characteristic needs thought and consideration through the design to commissioning, from operation to decommissioning stages; but the savings in terms of energy, cost, and carbon are more than worth it.