Stall is when a centrifugal compressor reaches a point of maximum flow and minimum head. The wheel can deliver no more gas. (For a description of the centrifugal wheel and head, see “Centrifugal Refrigeration Compressor Surge”.)
Stall is common in aircraft engines, turbo-chargers, and other turbo machinery that operate in an open loop, but much less likely to occur in a closed loop refrigeration machine.
A theoretical wheel curve is displayed bellow, the far-right line labeled “stall”, is at the operating conditions the compressor must reach to induce “stall”.
For a refrigeration machine to reach the conditions at stall on the graph above, there would need to be almost no head across the compressor, and a flow, far exceeding the design.
Refrigeration machines are closed loop systems, take a vacuum cleaner and stick the suction nozzle into the discharge port, wa-la, closed loop system, if you kept the motor cool, the vacuum would run and run and never see a stall condition.
Now, I agree, not a fair comparison, vacuum cleaner and refrigeration machine, but the point is clear, without a mechanism to induce more volume flow into the loop, the flow can’t exceed what the compressor is capable of.
It is true, that a great deal of volume can be generated by the boiling of the refrigerant in the evaporator, and it does, but not more than the machine is designed to handle.
I guess you could have huge evaporator that, at times, gets overloaded, and is piped up to a compressor that runs forward on the curve, with a ginormous condenser that sits in the arctic; I just have never come across such a design or miss-applied machine to witness this perfect storm. However, if you have I would love to hear about it.
Centrifugal refrigeration compressors have an operational condition called “surge”. Surge can be identified by a loud squalling sound (shaft is thrusting against the physical stop or bearing) followed by a whooshing sound (the sudden reversal of flow of refrigerant) in a repeated sequence. Surge will eventually cause extensive damage to the internal components of the machine if not corrected. To help understand the surge condition, it will help to understand compressor “head”.
Centrifugal compressors impart the energy to the refrigerant by spinning bladed wheels at a relatively high speed. The energy imparted to the refrigerant develops a new potential in the gas called head.
The wheel diameter, speed, refrigerant density, and other physical design attributes affect the total head possible from any wheel design. These factors, for the most part, are fixed once put into place, somewhat fixing the surge point to a narrow range of operation.
The energy (or head), the wheel imparts on the refrigerant is responsible for the rise in pressure and temperature of the refrigerant during compression.
Head is measured in foot, the energy used to raise one pound of gas one foot, is one foot of head.
The head a compressor is developing can be found on the pressure-enthalpy diagram. The diagram below shows a compression curve for a R-22 compressor on a pressure-enthalpy diagram
At the compressor inlet, the red line is drawn down to the enthalpy of about 178, and the discharge enthalpy is about 188. The difference between these two numbers is a specific enthalpy of 10. We use 10 Btu/lb of gas multiplied by Joule’s constant, 778.16, to obtain a head of 7781.6 foot.
Varying the temperature of the inlet or discharge will directly affect the amount of head the compressor will produce.
Surge conditions will be in effect when the compressor is asked to exceed the design head. The point it begins is system independent and must be determined by testing actual components in operation, but most would agree, a 50% increase over design head will likely put a centrifugal into surge. This 50% mark is usually enough because the curve a wheel makes when plotting head versus flow, looks like the one below, the wheel is selected based on its pressure and flow requirements, then different wheels are looked at to obtain reasonable efficiency, the best efficiency can be seen close to the top of the curve where the surge break-over point occurs.
That means, if we ask this compressor to generate 11672.4 foot of head, instead of the design head of 7781.6, the compressor will likely surge. That seems like a lot, but that is only increasing the specific enthalpy of compression from 10 Btu/lb to 15 Btu/lb, and you’re in surge territory.
So, how does this happen? Those curved lines are temperature lines, in ten degree increments, the current discharge temperature is 160 F, enthalpy 188, an increase of 5 Btu/lb to 188+5 = 193 is an increase in temperature from 160 F to 175 F would push it to 193, this could easily occur if there was an increase in the heat going into the compressor.
The opposite is true also, a decrease in the compressor inlet temperature by 15 degrees will also push the enthalpy down 5 Btu/lb leading to surge. Conditions such as low load and high condensing pressures will get there very quickly.
The value here, is by charting your compressor operating conditions against design conditions, you can readily see what you need to address to keep the compressor out of surge.
Note on “design conditions”, if the design conditions are not known, you can use conditions where you know the machine runs normally (which is hopefully known) or refer to AHRI standards (on the links page) manufacturers stick pretty close to these standards.
I worked for over thirty years in the HVACR industry. I have designed, installed, serviced, and trouble shot units of various types throughout the years. The posts here are information based on that experience, I hope you find them useful. If you have a different experience, please comment.