You may have noticed that in 5-ton and under equipment 3/8 liquid lines are generally the norm. We went to a job recently where the system had a 1/2″ liquid line and it got me thinking about the ramifications of going larger or smaller on the liquid line.

Liquid Line Basics

The liquid line should be full of liquid with additional subcooling to prevent flashing due to pressure drop from the length, rise, fittings and filter/drier. Because liquid refrigerant is much denser than vapor the liquid line contains a relatively large amount of refrigerant compared to the much larger vapor line.

Even small changes in liquid line size can have a big impact on refrigerant velocity in the liquid line as well as the amount of charge contained in it. This is why we see a big variation in suction line size but very little change in liquid line in residential applications.

Pressure drop in the liquid line is only a concern when it results in flash gas or when it results in an unacceptably low pressure drop across the metering device. Flashing occurs when the refrigerant pressure drops to the point that all of the design subcooling is “used up” and the refrigerant in the liquid line begins to boil off. Up until that point, the pressure drop will result in negligible temperature drop.

From the Lennox Design and Fabrication Guide

Liquid Line Sizing Factors

We need to size the liquid line with the following factors in mind

  • Keep the velocity low enough to prevent noise
  • Minimize pressure drop to prevent flashing
  • Do not oversized the liquid line to prevent excess refrigerant charge

At first, it may seem like bigger would be better on the liquid line but that isn’t the case. An oversized liquid line can lead to a lot more refrigerant charge which will result in a greater likelihood of off-cycle refrigerant migration and flooded starts in addition to the cost associated with more charge for no good reason.

Lennox Liquid Line Chart

Our goal should be to use the smallest liquid line size that will still reliably provide a full line of liquid to the metering device under all load conditions that the system will be reasonably operated under. Luckily for us, we don’t need to guess as the manufacturers provide us guidelines for liquid line sizing.

Carrier Liquid Line Sizing Chart

Vertical Pressure Drop / Gain 

In general, on an R410a system, we don’t want more than about a 35PSI pressure drop in the liquid line otherwise we run the risk of flashing. When the condenser is LOWER than the evaporator the liquid line pressure loss is about 0.5 PSI per foot of vertical rise which limits the rise to around 60′ for R410a systems by the time you consider the other pressure drops.

If the condenser is ABOVE the evaporator then the pressure actually increases the longer the vertical separation allowing the liquid line to be downsized in some cases.

Each manufacturer has their own piping guide or has the details in the install instructions or the product data. In most cases 3/8″ liquid line is a safe bet but just like the suction line there is some wiggle room depending on the system and the specific application.

Here are some great guides

Carrier Guide

Lennox Guide

Johnson Controls / York Guide

— Bryan

 

In residential most techs and installers size the suction (vapor) and liquid lines to the stubs on the equipment and in larger built-up systems it is rarely the responsibility of the technician to size the piping.

But what happens if we show up to a job and the lines cannot be (reasonably) replaced and the size is different? or what if the stub on the evaporator coil is a different size than the condenser?

The Factors in Suction Line Sizing

  1. Pressure Drop – Any pressure drop in the suction line due to size, length or fittings results in lower refrigerant density and higher compression ratio at the compressor without a corresponding decrease in evaporator temperature. In other words, you get all the bad stuff like compressor overheating and poor capacity without a lower evaporator operating temperature. This is why pressure drops in the suction line are generally recommended to be less than 3PSI on R22 and 5PSI on R410 but the lower the better.
  2. Oil Carry – When mineral oil was used oil carry was a bigger deal than it is now but manufacturers still design the systems with a minimum suction refrigerant velocity which means that bigger isn’t better. The York / Johnson controls guide says the following

Minimum (suction) velocity of 1000 fpm for vertical lines and 800
fpm for horizontal lines guarantee proper oil return

Equivalent Length

Before we look at an example of a suction line selection chart we must first square on this topic of “equivalent” length. You need to calculate the actual line length and then add-in the equivalent length of fittings (very similar to duct design). By just a quick glance at the chart above you can see the advantage of using long-radius 90’s.

Manufacturers Chart

Now it’s time to look at your manufacturer’s literature to see if a particular suction line size is acceptable. The one above applies to standard Carrier R-410a single-stage heat pumps and comes from the system product data.

Take a look at the 5-ton system at the bottom of the chart. You may have thought that a 3/4″ line set could NEVER be used on a 5-ton but this chart clearly shows that on line sets with equivalent lengths of under 50′ the % of capacity loss will only be 1%. While this isn’t ideal it may be the only economical option in some cases. On the other hand, a 5/8″ suction line on a 3.5 ton is “off the chart” meaning it isn’t acceptable under any conditions.

Either way, our goal is to keep suction line pressure drop to a minimum while maintaining the minimum suction velocity and this is the sweet spot manufacturers are keeping you in with these charts.

 

 

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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