Tag: compression ratio

Basic Compressor Functions

The job of the compressor is to circulate refrigerant through the system by means of vapor compression, similar to the way your heart moves blood through your circulatory system.

Refrigerant circulation is measured in lbs/min or lbs/hour; called mass flow rate. The mass flow rate changes depending on the density of the refrigerant and the compression ratio.

The denser (higher the pressure) the refrigerant is coming back from the evaporator the greater the mass flow rate and the lower the suction pressure the lower the mass flow rate.

The ability of the compressor to move refrigerant efficiently is often measured in volumetric efficiency. This is a measure of how much refrigerant enters the suction line vs. how much leaves the outlet of the compressor in the discharge line. The difference between the two is loss or waste to re-expansion of the gas in the compressor cylinder (in a reciprocating compressor).

The greater the compression ratio (absolute head pressure divided by absolute suction) the lower the mass flow rate will also be and lower the volumetric efficiency will be . In other words, low suction with high head pressure are the worst case scenario for mass flow rate and volumetric effeciency when the compressor is working as it should.

Proper lbs/min or lbs/ hour of refrigerant circulation is vital to the capacity of the evaporator, condenser and metering device as well as the cooling of th compressor if it is refrigerant cooled.

The Compressor size (pumping ability) controls the system’s lbs/min or lbs/hour mass flow rate.

Compressor pumping action also performs two other functions.

  1. It maintains the evaporator pressure: when the compressor runs, it lowers evaporator pressure. This sets evaporator pressure, operating TD, and BTUH capacity.

2. It increases condenser pressure: when a compressor runs, it pumps heat into the condenser, this causes condensing temp and TD to go up until heat can flow out of condenser as fast as it enters.

As evaporator heat load and temp increase, compressor heat output increases and drives condenser TD even higher to increase condenser heat rejection.

Compressor response to changing Evaporator heat loads

Here is a way of thinking about load and how it impacts mass flow rate, compression ratio and volumetric efficiency.

Higher heat loads produce vapor faster than compressor can remove it from the evaporator. When this occurs the evaporator pressure and temperature go up with the increased heat load.

The compressor’s flow in lbs/min or lbs/hr increases as the suction pressure increases and compressor draws more amps due to pumping more refrigerant.

Lower evaporator heat loads produce vapor slower than compressor is removing it from the evaporator. Evaporator pressure and temperature go down with the reduced heat load. Compressor’s flow in lbs/min or lbs/hour goes down. The Compressor draws fewer amps due to pumping less refrigerant.

Compressor’s Volumetric Efficiency

The goal is to keep the Volumetric Efficiency as high as possible. With a higher VE, a compressor produces more lbs/min or lbs/hour of refrigerant flow
Systems operating conditions, evaporating and condensing pressures, directly affects compressor pumping ability VE Ratio of Condenser pressure to evaporator pressure is called compression ratio. To calculate compression ratio, convert pressures to absolute values (add 14.7 to existing pressure) then divide condenser pressure by evaporator pressure

Volumetric Efficiency Charts

VE (Volumetric Efficiency) Charts show the effect of compression ratio on Volumetric Efficiency: As CR goes up, VE goes down. As CR goes down, VE goes up. Our goal is to keep volumetric efficiency of the compressor as high as possible for capacity, energy usage and compressor longevity.

Factors that determine system CR

System compression ratio is based on a few factors, primarily desired space temp and temperature of the cooling medium. Corresponding evaporator and condenser pressure establish the compression ratio the compressor must work against. Refer to the compression ratio chart for each compressor as a guide.

Keeping Volumetric Efficiency Up

In order to improve VE, you must keep the compression ratio low. You can do this by keeping condenser pressure low, maintaining clean condenser and supply it with a cool condensing medium (proper temperature and flow of air or water across the condenser coil or condenser HX). You must also keep the evaporator pressure up, don’t run the evaporator pressure any lower than needed to do the job. Lower compression ratio allows the compressor to pump more lbs/min or lbs/hour through the system. Higher compression ratios reduce the compressor’s ability to maintain the desired mass flow rate.

Compressor Approved Application Range (operating range) 

Hermetic and semi-hermetic compressors are designed for specific evaporator temperature ranges. The range of evaporating temps varies by manufacturer and model and you will need to do some reading to be sure you have it right. Evaporator temperatures above maximum approved temperature results in motor overload; drawing excess amps and overheating. An evaporator temperature below the minimum approved application temperature will result in poor motor cooling due to a low lbs_hour flow rate.

Compressor Data Sheets

Data sheets show compressor performance in its approved application range. Data may be shown in a table or as performance curves, these tables or curves will show : capacity  mass flow rate, power and current. This can be used for design, proper commissioning and system diagnosis. Just keep in mind that the compressor when working properly is still at the mercy of system conditions, it is up to us to set it up for success.

Compressor Amp Ratings

Compressor amps change as the evaporator and condenser temperatures change. Under load conditions, the compressor could draw more than rated load amps and not necessarily be in any danger of motor overload. As long as the motor amperage drawn is well below trip amperage. Most compressors will run at less than rated load amps during normal conditions but may run high under heavy evaporator load. All of this can be found by looking carefully at the compressor charts or curves.

– Louie Molenda

 

 

 

In HVAC/R we are in the business of moving BTUs of heat and we move these BTUs on the back of pounds of refrigerant. The more pounds we move the more BTUs we move.

In a single stage HVAC/R compressor, the compression chamber maintains the same volume no matter the compression ratio. What changes is the # of pounds of refrigerant being moved with every stroke(reciprocating), oscillation (scroll), or rotation (screw, rotary) of the compressor. If the compressor is functioning properly the higher the compression ratio the fewer pounds of refrigerant is being moved and the lower the compression ratio the more pounds are moved.

In A/C and refrigeration the compression ratio is simply the absolute discharge pressure leaving the compressor divided by the absolute suction pressure entering the compressor.

Absolute pressure is just gauge pressure + atmospheric pressure. In general, we would just add the atmospheric pressure at sea level (14.7 psi) to both the suction and discharge pressure and then divide the discharge pressure by the suction. For example, a common compression ratio on an R22 system might look like-

240 PSIG Discharge + 14.7 PSIA = 254.7
75 PSIG Suction + 14.7 = 89.7 PSIA
254.7 PSIA Discharge ÷ 89.7 PSIA Suction = 2.84:1 Compression Ratio

The compression ratio will change as the evaporator load and the condensing temperature change but in general, under near design conditions, you will see the following compression ratios on properly functioning equipment depending on the efficiency and conditions of the exact system.

In air conditioning applications compression ratios of 2.3:1 to 3.5:1 are common with ratios below 3:1 and above 2:1 as the standard for modern high-efficiency Air conditioning equipment.

In a 404a medium temp refrigeration (cooler) 3.0:1 – 5.5:1  is a common ratio range

In a typical 404a 0°F to -10°F freezer application 6.0:1 – 13.0:1 is a common ratio range

As equipment gets more and more efficient, manufacturers are designing systems to have lower and lower compression ratios by using larger coils and smaller compressors.

Why does the compression ratio number matter? 

When the compressor itself is functioning properly the lower the compression ratio the more efficient and cool the compressor will operate, so the goal of the manufacturer’s engineer, system designer, service technician and installer should be to maintain the lowest possible compression ratio while still moving the necessary pounds of refrigerant to accomplish the delivered BTU capacity required.

The compression ratio can also be used as a diagnostic tool to analyze whether or not the compressor is providing the proper compression. Very low compression ratios coupled with low amperage and low capacity are often an indication of mechanical compressor issues.

Compression ratio higher than designed = Compressor overheating, oil breakdown, high power consumption, low capacity 

Compression ratio lower than designed = Can be an indication of mechanical failure and poor compression

Understanding compression is critical to understanding the refrigeration process. Don’t be tempted to skip past this because it is a really important concept.

Look at the pressure enthalpy diagram above. Top to bottom (vertical) is the refrigerant pressure scale, high pressure is higher on the chart. Horizontal (left to right) is the heat content scale, the further right the more heat contained in the refrigerant (heat, not necessarily temperature).

Start at point #2 on the chart at the bottom right. This is where the suction gas enters the compressor. As it is compressed it goes to point #3 which is up because it is being compressed (increased in pressure) and toward the right because of the heat of compression (heat energy added in the compression process itself) as well as the heat added when the refrigerant cooled the compressor motor windings.

Once the refrigerant enters the discharge line at point #3 it travels into the condenser and is desuperheated (sensible heat removed). This discharge superheat is equal to the suction superheat + the heat of compression + the heat removed from the motor windings. Once all of the discharge superheat (sensible heat) is removed in the first part of the condenser coil it hits point #4 and begins to condense.

Point #4 is a critical part of the compression ratio equation because the compressor is forced to produce a pressure high enough that the condensing temperature will be above the temperature of the air the condenser is rejecting its heat to. In other words, in a typical straight cool, air cooled air conditioning system the condensing temperature must be higher than the outdoor temperature for the heat to move out of the refrigerant and into the air going over the condenser.

If the outdoor air temperature is high or if the condenser coils are dirty, blades are improperly set or the condenser coils are undersized point #2 (condensing temperature) will be higher on the chart and therefore will put more heat strain on the compressor and will result in lower compressor efficiency and capacity.

As the refrigerant is changed from a liquid vapor mix to fully liquid in the condenser it travels from right back left between points #4 and #5 as heat is removed from the refrigerant into the outside air (on an air cooled system). Once it gets to #5 is is fully liquid and at point #6 it is subcooled below saturation but ABOVE outdoor ambient air temperature. The metering device then creates a pressure drop that is displayed between points #6 and #7. The further the drop, the colder the evaporator coil will be. The design coil temperature is dictated by the requirements of the space being cooled as well as the load on the coil but the LOWER the pressure and temperature of the evaporator the less dense the vapor will be at point #2 when it re-enters the compressor and the higher the compression ratio will need to be to pump it back up to point #3 and #4,

This shows us that the greater the vertical distance between points #2 and #4 the higher the compression ratio, which means that both low suction pressure and/or high head pressure result in higher compression ratios, poor compressor cooling, lower efficiency and lower capacity.

In some cases, there isn’t much that can be done about high compression ratios. When a customer sets their A/C down to 69°F(20.55°C) on a 100°(37.77°C) day they will simply have high compression ratios. When a low temp freezer is functioning on on a very hot day it will run high compression ratios.

But in many cases, you can reduce compression ratios by –

  • Keeping set temperatures at or above design temperatures for the equipment. Don’t be tempted to set that -10°F freezer to -20°F or use that cooler as a freezer
  • Keep condenser coils clean and unrestricted
  • Maintain proper evaporator airflow
  • Install condensers in shaded and well-ventilated areas

Keep an eye on your compression ratios and you may be able to save a compressor from an untimely death.

— Bryan

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