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Compressor Mass Flow – Some Thought Experiments
In order to wrap my head around diagnostic issues, it helps me engage in thought experiments where I think of more extreme examples of an issue or situation or consider the ideal to find the “edges” of a concept. Once I find the extreme edges, I can begin to sort down to a more exact conclusion.
So, let's consider compressors and mass flow.
First, don't get overwhelmed by the phrase “mass flow.” I'm not going to start in with confusing words and fancy math. As techs, we rarely need to do advanced calculations anyway; it's more about understanding relationships between factors or IFTTT (if this, then that). If this thing occurs, or I change this, what happens to that?
Mass flow just means how much fluid is moving over a given amount of time. In this case, the “stuff” is refrigerant, and the mass measurement is generally pounds (lbs) in the USA, and the rate could be minutes, seconds, or hours.
Our goal with a compressor is to move as many lbs of refrigerant as quickly as possible with the minimal amount of watts in energy used to move the greatest number of BTUs/hr we can—pretty straightforward, so far.
The typical single-speed, single-stage compressor with no unloading capability runs ESSENTIALLY the same speed and with the same volume in the compression chamber (cylinder, scroll, etc.). That means that a traditional compressor has a fairly constant volume in the compression chamber and rate of compression. I say “fairly” constant because as the compressor moves greater mass or works against greater pressure, the motor will tend to slip more, resulting in a slower rotational speed of the rotor.
So, let's imagine an old single-speed, single-stage reciprocating compressor with no unloading. It's compressing refrigerant with a constant volume in the cylinder that goes from its largest cylinder capacity at the bottom of the downstroke (suction stroke) to its minimum capacity at the top of the upstroke (discharge stroke). This variation in volume in the cylinder as the pistons actively move and down creates the pressure differential between the high side and the low side. This pressure difference allows the refrigerant to move through the circuit.
So, you may think to yourself (as I have in the past):
“If pressure differential is what causes the refrigerant to move, then don't we want a big pressure difference between the compressor discharge and suction so that more refrigerant will move?”
The answer is an absolute NO.
We actually want the minimum pressure differential we can get away with while still accomplishing the task of maintaining an evaporator (or evaporators, in the case of multi-circuit systems) at the desired temperature and (nearly) full of boiling refrigerant.
The reason we want a lower pressure differential has to do with the mass flow rate. If the compressor has a fixed volume in the cylinders and the pistons are pumping away at the same speed, then that part of the equation is fairly fixed. The only way to increase the amount of refrigerant being moved by the compressor is to:
#1 – Increase the density of the refrigerant
#2 – Reduce the amount or re-expansion waste in the cylinder
#3 – Reduce the pressure to overcome in the discharge
The first part of that equation is simple; when suction gas is higher pressure, it is also higher density. When the suction pressure entering the compressor drops, the density also drops. When the density of the refrigerant drops entering the compressor, the compressor moves less refrigerant because there is just less there for it to move.
Think of this as an old PSC blower motor on undersized ductwork. When the static pressure on the return increases, the amount of air being moved decreases because the density of the air is decreased. The blower is still spinning at the same speed (on a PSC). Heck, it may even be spinning faster due to the motor experiencing less resistance, but the airflow decreases. This happens not because the motor is doing anything different; it moves less air mass because the air is less dense entering the blower, and therefore, you are moving less air.
When you drop the suction pressure entering a typical compressor, you drop the mass flow rate because the mass entering the compressor is reduced; a lower mass flow rate means moving fewer lbs of refrigerant, which (by itself) means lower capacity.
Now, let's move to the second part, which is re-expansion. This one applies more to reciprocating compressors where there is a clear compression and expansion stroke vs. a scroll, rotary, or screw, where the compression is essentially a continuous cycle.
Imagine a compressor sitting in a room with no tubing connected, just pumping air. The compressor would be pulling from 14.7 PSIA and discharging into 14.7 PSIA (atmospheric air pressure at sea level). When the piston draws down, it would pull in air and fill up, and then as the piston pushes up, it would start to discharge air out of the cylinder really quickly in the upstroke because the only thing pushing against the discharge is 14.7 PSIA. Therefore, the highest pressure that would build up inside the compressor is slightly more than 14.7 for it to overcome the pressure of the discharge valve and push it out into the air.
If that same compressor pumped into a chamber where the pressure built up to 200 PSIA, what would change?
The compressor would move less air, even if the suction were still left open to atmosphere (and therefore the same air density) because now the discharge valves wouldn't open until the pressure in the cylinder went above 200 PSI, meaning that the effective stroke would be reduced due to the pressure being pushed against (#3 on the list above). It would also need to pull down further to re-expand the gas left behind in the cylinder to below 14.7 PSIA for more air to enter the cylinder again.
In a scroll, rotary, or screw, there aren't valves and cylinders in the same way, but the amount of refrigerant being moved is still impacted by changes in suction density (suction pressure) and the pressure exiting the compressor—in other words, the COMPRESSION RATIO.
Have you ever noticed that the BTU ratings on compressors have dropped over the last 10 years as units become more efficient? Where a 3-ton unit may have previously had a compressor with a 36 in nomenclature for a nominal three tons, you may now find it has closer to 30 or even less.
You may also notice that high-efficiency systems often have larger condensing coils and larger evaporators that bring the head pressure. Therefore, the condensing temperature is closer to the outdoor temp. The evaporators are also running at a higher temperature, bringing up the suction pressure. Manufacturers are increasing how much refrigerant the compressor can move (mass flow rate) by bringing the design head pressure down and the design suction pressure up. They can then afford to downsize the compressor achieving the same capacity with fewer input watts, also known as greater energy efficiency.
Let's give some real-world examples of altering mass flow rate by impacting these factors in the field:
- Dirty condenser coil – Decreases mass flow rate and system capacity because the head pressure and compression ratio go up.
- Low indoor airflow – Decreases mass flow rate because refrigerant density goes down entering the compressor and compression ratio goes up (to a degree). Keep in mind that when there is low airflow or low load, head pressure will also tend to drop as the mass flow rate drops. It is held up by the outdoor temperature as a limitation on how low the condensing temperature will drop, however.
- Overcharge – The impact of overcharge on mass flow rate will vary depending on the metering device and how overcharged the system is. On a TXV/EEV system, it will always result in a lower mass flow because the head pressure will increase. On a fixed orifice, it may result in a slight increase in mass flow initially as suction pressure increases.
- High indoor (evaporator) load – Increases mass flow unless there is some control preventing it from doing so, like a CPR (compressor pressure regulator). Increased heat entering the evaporator will increase the pressure and density of the refrigerant returning to the compressor. That will increase the mass flow rate, system capacity, and head pressure if all else remains the same.
What happens if we change compressor capacity on the fly?
For years in residential and light commercial, we've been used to fairly fixed compressor volume flow rates, but nowadays, we see many different types of multi-stage and variable capacity technologies from a simple dual capacity unloading scroll to a digital scroll all the way to variable- frequency, variable-speed scroll compressors. These compressors have their “rated” capacity, which is the state at which they are tested for bench-marking against other units. They can then reduce their capacity below their rating, and some can every produce a higher capacity than their rating.
In all of these cases, the compressor is altering the amount of refrigerant it is moving by making a change within the compressor itself, resulting in lower mass flow when the compressor stages or ramps down and higher mass flow when it ramps up.
Let's imagine a theoretical 4-ton rated unit with a compressor that can ramp down to 2-tons or ramp up to 5-tons.
What that means in practice is that the compressor is capable of moving an amount of refrigerant consistent with two tons of capacity up to a mass flow that can produce 5-tons of capacity at the same rated conditions.
So, here is what you would see change when that compressor changes mass flow in comparison to rated capacity if everything else remained the same:
Low Stage (2-ton)
High Suction Pressure
Low Head Pressure
High Superheat (potentially)
Low Evaporator Delta T
Poor Dehumidification due to high coil temperature
Low compressor amps
Low Compression Ratio
Low Discharge Temperature
Low Approach (liquid line temperature above outdoor temperature)
High Efficiency (EER/SEER)
High Stage (5-ton)
Low Suction Pressure
High Head Pressure
Low Superheat (potentially)
High Evaporator Delta T
Strong Dehumidification due to lower coil temperature
High compressor amps
High Compression Ratio
High Discharge Temperature
High Approach (liquid line temperature above outdoor temperature)
Low Efficiency (EER/SEER)
Now, think about how a system responds when the compressor isn't pumping properly. It is almost exactly the same as the low stage/low mass flow example listed above, except for the efficiency. When we have lower mass flow than rated, we will see these symptoms whether it is by design or due to a failure.
In practice, these variable capacity systems will often be matched with a variable speed blower and a wide-range TXV or EEV so that the coil temperature and feeding can adjust with the change in mass flow to help mitigate some of the negative effects of staging down.
We can do some interesting things with modern controls and variable mass flow compressors. For example, Bosch-branded condensing units vary the compressor mass flow to set a fixed evaporator temperature, effectively adjusting the capacity to match the load on the evaporator coil. Another is Carrier Greenspeed heat pumps that ramp the compressors up during heat mode to drive up the pressure on both coils to increase the heat produced inside and reduce defrost requirements.