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What is Compressor Volumetric Efficiency?
Simply stated, a compressor’s volumetric efficiency (VE) is its ability to pump the most pounds of refrigerant over time. The compressor’s function is right in the name: it compresses vapor refrigerant. After compression, the gas moves to the condenser via the discharge line. From there, the refrigeration cycle continues.
However, some of the refrigerant vapor that enters the compressor may not leave it. The gas that remains re-expands and occupies space within the compression chamber. When this happens, the compressor cannot operate at its full efficiency.
The VE measures that inefficiency by comparing the amount of refrigerant that enters the compressor to the amount that leaves it.
Why should we care about the VE?
We should care about the VE for the same reasons that we care about other types of efficiency.
When the VE is closer to 100%, you can maximize your compressor’s mass flow rate. The compressor can pump more pounds of refrigerant over a given time. In other words, it takes less work for the compressor to make more progress at its job.
You will likely also maximize your compressor’s lifespan if you prioritize its efficiency. This effect isn’t limited to compressors, either. The more efficient a system or part is, the less work it has to do. When a system or part has less work to do, it doesn’t wear out as quickly as it would otherwise.
How do we determine the VE?
Let’s start by defining the variables we compare when we measure VE.
Relatively cool, low-pressure vapor enters the compressor through the suction line. The compressor raises the vapor’s temperature and reduces volume by applying a massive amount of pressure. The hot, high-pressure vapor leaves the compressor through the discharge line.
The compressor’s VE is a ratio of the discharge vapor to the suction vapor. You can express this as a percentage to get an idea of the total efficiency.
What affects a compressor’s VE?
The compression ratio mainly affects the compressor’s VE. The compression ratio deals with the pressures of the evaporator and condenser.
It may seem a bit strange that the compression ratio has little to do with the compressor. However, the evaporator and condenser surround the compressor within the circuit. Those two parts affect the compressor’s function. The compression ratio is the ratio of condenser pressure to evaporator pressure.
The higher the compression ratio, the harder it is for the compressor to perform its job efficiently. When you run against the wind, you waste a lot of your stamina resisting the environmental conditions. A similar principle applies to a compressor with a high compression ratio, where it has a lot to overcome. (You can read more about compression ratio HERE.)
We mostly use reciprocating compressors, which are usually among the more efficient types of compressors. However, their structure makes them prone to developing poor volumetric efficiency.
Why are reciprocating compressors challenging?
The reciprocating compressor relies on a piston to compress the gas. When the piston moves down, it increases the cylinder volume, creating a low-pressure area that draws the refrigerant into the compressor. When the piston moves up, it compresses the gas by reducing the volume in the cylinder.
Pistons have room for clearance, allowing gas to get trapped after most of the gas moves to the condenser. Gas left in the compressor or discharge port will re-expand, and this decreases the VE.
The higher the compression ratio, the less efficient the compressor becomes. More matter is left in the cylinder when the discharge pressure is high. When the suction pressure is low, the piston must drop the pressure further before suction gas can enter.
How can we reduce the compression ratio?
As you know, low evaporator pressure and high condenser pressure negatively impact the compressor’s VE. You’ll want to do whatever you can to keep the evaporator pressure high and the condenser pressure low.
The best thing you can do to keep the evaporator pressure as high as effectively possible to ensure that the pressure doesn’t drop below what is needed to do the job.
There are a few more things you can do to keep the condenser pressure low. Cleanliness is a vital element of condenser maintenance. When you keep your condenser clean, you reduce the likelihood of pressure increase. You’ll also want to keep the condenser cool.
You may also consider checking out alternative compressors. There are opportunities for gas to get left behind and re-expand because of a reciprocating compressor’s piston and valves. Each type of compressor has a unique structure, and ones without valves and pistons will not have the clearance and re-expansion issues of reciprocating compressors.
What is a more efficient compressor type?
The scroll compressor is widely used in the HVAC industry because of its efficiency. A scroll compressor doesn’t require the valves and pistons of a reciprocating compressor. Instead, it has two coils (also known as scrolls): a fixed coil stays in the center while another coil oscillates around the fixed coil. As the moving scroll oscillates, it moves the refrigerant towards the center and compresses the gas before sending it to the condenser.
Scroll compressors do not have the same clearance or valve concerns that reciprocating compressors have. Gas re-expansion is not a concern and will not significantly impact the compressor’s efficiency.
Thanks for the info. But please, I don’t quite understand why for a TXV system, when undercharged, the TXV maintains the superheat by allowing more refrigerant into the evaporator yet the head pressure drops. Since more refrigerant is allowed into the evaporator, shouldn’t the compressor also maintain the head pressure?