Refrigerant piping line sizing and design field guide

What line sizing decides, and the two demands that fight each other

Refrigerant line sizing is choosing the diameter of each pipe in the system so it carries the refrigerant between the components without costing too much capacity and without losing the oil. That is the whole job, and it pulls in two directions at once. A bigger pipe drops less pressure, which protects capacity. A smaller pipe keeps the refrigerant moving fast enough to sweep the compressor's oil around the loop and back home. You cannot maximize both, so you size to a window that satisfies both, and the window is narrower than people think.

Get it wrong small and the pressure drop eats the capacity: the suction line robs the compressor of suction pressure, the system runs warm, and the power bill climbs for cooling you are not getting. Get it wrong large and the velocity falls below what it takes to carry oil up a riser, the oil pools in the low spots and the evaporator, and the compressor slowly runs itself dry until a bearing fails. Both failures are quiet for months, which is why a system that was piped wrong still cools on the startup day the installer drove away.

This guide is the sizing and design side. The brazing, the nitrogen, the pressure test, the deep vacuum, and weighing in the charge live in the split-system install guide, and reading superheat and subcooling to verify the charge lives in the refrigerant charging guide. Here the question is the one that comes before any of that: what diameter, what slope, what trap, and what lift, so the lines you braze are the right lines.

The three lines and what each one carries

A direct-expansion system has up to three refrigerant lines, and each carries the refrigerant in a different state, which is why each is sized on its own rules. Confuse the conditions in one for another and the sizing logic falls apart.

The suction line, the larger insulated line, carries low-pressure vapor from the evaporator back to the compressor. It is the line where oil return is hardest, because the vapor is light and any oil along for the ride has to be dragged by gas velocity, especially up a vertical riser. The liquid line, the smaller bare line, carries high-pressure subcooled liquid from the condenser to the metering device. It carries no oil-return worry, because the oil moves easily dissolved in liquid, but it has a problem the others do not: if its pressure drops enough, the liquid flashes to vapor before it reaches the valve. The discharge or hot-gas line carries high-pressure, high-temperature vapor from the compressor to the condenser. It is sized like the suction line for oil return, since the gas is again carrying oil, but at much higher density.

On a heat pump the suction and discharge lines swap function when the reversing valve flips, so the line you sized as suction in cooling is the discharge in heating. That is one more reason the manufacturer's lineset tables, which already account for the reversing duty, govern over a generic chart.

How much pressure drop is acceptable in a refrigerant line?

The common design rule sizes the suction and discharge lines for a pressure drop no greater than the equivalent of about a 2°F change in saturation temperature, and the liquid line for a pressure drop on the order of 5 psi, before any vertical lift. Those are the budgets the manufacturer line-sizing tables are built around, and they apply to the whole run, not per fifty feet, so a longer run forces a larger pipe to stay inside the same budget.

The reason the suction and discharge budgets are written in degrees instead of psi is that the cost of a pressure drop is a saturation-temperature shift, and that shift is what steals capacity, not the raw psi. The number of psi that equals 2°F depends on the refrigerant and where on its curve you are. On R-22 roughly 3 psi of suction drop is about 2°F. The same 3 psi on R-410A is closer to 1.2°F, because its pressure-temperature curve is steeper. So a chart built for one refrigerant does not transfer to another, and the move to R-410A and now the A2L refrigerants changed the psi that the same temperature budget allows.

Treat the 2°F and 5 psi figures as the design target, not a law. The equipment manufacturer's lineset tables are the authority, and they already fold in the refrigerant, the equipment, and the rated conditions. Where the manufacturer gives a table, size to the table and let these rules of thumb be the sanity check, not the answer.

What a suction-line pressure drop actually costs

A pressure drop in the suction line shows up as lost capacity and higher power, and the mechanism is worth carrying in your head because it explains why the budget is so tight. The compressor pulls vapor at whatever pressure exists at its suction port. Drop pressure in the line between the evaporator and that port and the compressor sees a lower suction pressure than the evaporator is actually running, which means it pumps less mass of refrigerant per stroke and makes less cooling, while drawing nearly the same power. That is capacity bought and not delivered.

The rough field number people use is that the suction-line drop at the 2°F budget costs a low single-digit percentage of capacity and a similar bump in power for that one line. It sounds small until you add the drop across a dirty coil, a long lift, and an undersized line on the same system, and now you have stacked several of those together and the system is missing a tenth or more of its rated capacity for no reason a gauge at the condenser will reveal.

The discharge line costs you the other way, as higher head pressure and compressor work, but its drop is usually a smaller share of the total because the gas is dense. The liquid line does not cost capacity through ordinary friction. Its pressure drop costs you when it goes far enough to flash the liquid, which is a different and worse failure covered below.

Why does a suction line need a minimum velocity?

A suction line needs a minimum vapor velocity because the compressor's oil rides around the system with the refrigerant, and on the low-pressure vapor side the only thing carrying that oil back to the compressor is the speed of the gas. Drop below the minimum and the oil stops moving up the risers, pools in the evaporator and the low points, and the compressor loses its oil charge a little at a time. The common design minimums are roughly 500 to 700 fpm in horizontal suction and hot-gas lines and roughly 1000 to 1500 fpm in vertical suction and hot-gas risers, measured at the lowest load the system will run, not at full load.

The riser is where this bites, and the lowest-load case is the one that governs, because that is when the gas is slowest. Oil that gets dragged up a riser fine on a hot design day can fall back down the same riser on a mild day when the compressor unloads and the velocity collapses. So you size the riser to keep oil moving at minimum capacity, then check that the same pipe does not blow the pressure-drop budget at full capacity. Those two checks fight, and on a variable-capacity system they fight hard enough that a single pipe size cannot win both, which is what drives the double riser further down.

The numbers above are the widely published ASHRAE-style design minimums, and they vary with the refrigerant, the line orientation, and the source. The equipment manufacturer's lineset tables already build the oil-return velocity into the sizes they list, so when you size to their table for the run and lift you actually have, the velocity is handled. Confirm the minimum the manufacturer used before you second-guess a size in the field.

Can a refrigerant line be too big?

Yes, and an oversized suction line is a more common field failure than an undersized one, because it looks generous and conservative when it is the opposite. Velocity is flow divided by area, so the bigger the pipe, the slower the gas at a given load. Go up a size for safety on a suction riser and you can drop the velocity below the oil-return minimum, especially at part load, and now the oil that should be sweeping up the riser sits in the bottom instead. The compressor starves while the gauges read fine.

This is the trap of sizing a suction line by the stub-out diameter or by going up a size to be safe. Bigger is safer on the liquid line, where there is no oil-return velocity to hold and a larger line only reduces pressure drop. Bigger is a hazard on the suction and discharge risers, where it kills the velocity that returns the oil. The two lines have opposite tolerances for being oversized, and treating them the same is how a careful-looking install ends up with a dry compressor.

When the load swings, as it does on any modulating or staged system, the oversized riser is worst at the bottom of its range. The fix is not a bigger pipe. It is the right pipe for the minimum load, or a double riser if the turndown is wide enough that no single pipe holds velocity at the low end and pressure drop at the high end.

What is a double suction riser?

A double suction riser is two vertical lines piped in parallel with a trap at the bottom, used on variable-capacity systems so the oil keeps coming back at part load without choking the line at full load. It exists to solve the fight between velocity and pressure drop on a tall riser when the load turns down far enough that no single pipe can do both.

The way it works is mechanical and clean. The smaller riser is sized to carry oil at the minimum load the system will run. The larger riser is sized so that, at full load, the two risers together stay inside the pressure-drop budget. At full load the gas goes up both risers and the velocity in each is enough to carry oil. When the system unloads and the total gas flow drops, the velocity through both risers together would fall below the oil-return minimum, so a trap at the base fills with oil and seals off the larger riser. Now all the gas is forced up the small riser alone, where the velocity climbs back above the minimum and the oil keeps moving. When the load returns, the trap clears and both risers carry again.

A double riser is a design feature for tall risers on systems with real turndown, like multi-stage and variable-speed equipment and refrigeration racks. It is not something you add to a residential split. On equipment that calls for it, the manufacturer's piping guide gives the two riser sizes and the trap, and you follow that geometry, because the sizes are matched to the equipment's minimum and maximum flow.

Suction-line traps: the P-trap at the bottom and the inverted trap at the top

Two kinds of trap show up on suction risers, and they do opposite jobs, so name them right before you bend one. A P-trap at the bottom of a riser collects oil and uses it to help lift more oil up the riser. An inverted trap, a loop over the top, keeps oil from draining back down the riser when the system is off or unloaded.

The P-trap at the base is short-term storage. As oil drains down the riser and pools in the trap, it narrows the effective opening, which speeds up the gas passing through and helps it grab the oil and carry it up. On a tall riser the common practice is a trap at the bottom and another every 10 to 15 ft up the riser, so the oil makes the climb in stages instead of one impossible lift. The interval depends on the designer and the equipment, so use the manufacturer's spacing where they give one.

The inverted trap goes at the top, and you need it whenever the compressor or the suction header sits above the evaporator. Without it, oil that made it up the riser drains right back down into the evaporator every time the system cycles off, and the next start has to lift it all again. On a double riser, an inverted trap on the large riser keeps the oil coming up the small riser from spilling back down the large one during part-load operation. The wrong move is a trap in the wrong place, or a trap on a run that should just be sloped: an unnecessary trap is one more low spot for oil and liquid to collect, so trap where the geometry requires it and slope the rest back toward the compressor.

Sizing the liquid line: subcooling, flash gas, and vertical lift

The liquid line has to deliver a solid column of liquid to the metering device with no vapor in it, because any vapor, called flash gas, cuts the valve's capacity and shows up as a starved evaporator and high superheat that no amount of charge fixes. Flash gas forms when the liquid's pressure drops below its saturation pressure for the temperature it is at, so it boils in the line before it ever reaches the valve. Friction in an undersized line, restrictive accessories, and especially vertical lift all spend liquid-line pressure, and subcooling is the bank account that pays for it.

Vertical lift is the one that surprises people. When the liquid has to climb to reach an evaporator above the condenser, the static head of the liquid column itself drops the pressure at the top, on the order of about 0.5 psi per foot of rise for common refrigerants. Lift the liquid 30 ft and you have spent roughly 15 psi just on height, before any friction. The defense is subcooling: a rough rule of thumb is that you need about 5°F of additional subcooling per 30 ft of lift to keep the liquid from flashing at the top, on top of whatever the friction drop costs. Run the liquid line up a tall lift with marginal subcooling and the system flashes on the hot afternoon when subcooling is already at its lowest.

So the liquid line is sized for low pressure drop, which is the one place where bigger is simply safer, and then the design has to guarantee enough subcooling to survive the friction plus the static head of any lift. Where the lift is high, that can mean a liquid subcooler or a hard look at whether the condenser can deliver the subcooling the lift demands. The metering device only meters liquid, and proving you delivered liquid is what the subcooling reading at commissioning is for, covered in the charging guide.

Long line sets and high lift

Every piece of equipment has a maximum total lineset length and a maximum vertical separation between the indoor and outdoor units, and the long-line job is where line sizing stops being routine. Standard residential equipment is commonly rated to around 80 ft of total line length, with vertical separation often capped somewhere in the 25 to 50 ft range, and long-line kits or specific models rated further. The numbers vary widely by manufacturer, model, and refrigerant, so the only figure that counts is the one in the install manual for the exact unit.

Past the standard length, three things change together. The line sizes may step up or down from the standard set to hold the pressure-drop budget and the oil-return velocity over the longer run, and on a long lift the suction riser sizing and traps come into play. The refrigerant charge has to be increased for the added liquid line, using the manufacturer's per-foot adder beyond the included length, which the split-system install guide covers. And the suction insulation may need to be heavier where the line runs through a hot space, because a long line gives up more capacity to ambient.

The direction of the lift matters too. Evaporator above the condenser is the liquid-lift problem, where subcooling has to cover the static head. Evaporator below the condenser is the suction-lift and oil-return problem, where the suction riser has to keep velocity up to carry oil. A long line that also lifts is doing both, and that is exactly the application where the manufacturer requires a TXV, specific line sizes, and sometimes traps. Exceed the rated length or lift and you are off the equipment's listing, with no table that says it will work.

Line material: ACR copper, clean and dry

Refrigerant lines are ACR copper, the air-conditioning-and-refrigeration grade, which ships cleaned, dehydrated, and capped or nitrogen-charged so the inside is free of the oil and moisture that plumbing-grade tube carries. The cleanliness is the point. Anything left inside the line, oil film, moisture, or oxide scale, ends up circulating with the refrigerant and lands in the metering device or the compressor.

ACR copper is sized and sold by actual outside diameter, not by the nominal pipe size that plumbing copper uses, so a 3/8 in ACR line is 3/8 in across, and you order linesets and fittings to the OD the tables call out. Use Type L wall or heavier for the working pressures, and confirm the pressure rating against the refrigerant, because the A2L and high-pressure refrigerants run higher than the old R-22 systems and the tube and fittings have to be rated for it.

Keep the line capped until the moment you braze it, and braze it under flowing nitrogen so you do not make the oxide scale inside that you bought clean copper to avoid. The brazing technique, the nitrogen flow, and the joint prep are in the split-system install and the leak detection and recovery guides, since the same skill builds the line and closes a repair.

The suction line then gets insulated end to end; the liquid line normally does not. The suction line runs cold and below the dew point of the air around it, so bare suction copper sweats, drips inside walls and ceilings, and gives up capacity to every warm space it passes through. The insulation stops the sweat and holds the capacity, and on a long line through a hot attic or a roof the manufacturer may call for a heavier wall, commonly 1/2 in or thicker on long-line runs through high-ambient zones. Slide it on before you braze where you can, because threading split insulation over a finished line and taping the seam never seals as tight.

Outdoors the foam has to survive sun, so the exposed run gets a UV jacket, a coating made for the foam, or a line cover; bare foam chalks and crumbles in a season or two, and the most-baked spot is the few feet at the condenser. Insulating the liquid line is usually unnecessary, the exception being a liquid line through a very hot space where you are protecting subcooling on a marginal lift. Insulate the suction line, protect it from UV, and seal the wall penetration so humid air does not track the cold line into the cavity.

Accessories in the line and the pressure they spend

The liquid line usually carries a filter drier and a sight glass, often a liquid-line solenoid, and at the end the metering device, and every one of those is a restriction that spends part of the pressure-drop budget. Size the line accounting for them, because the equivalent length of the accessories plus the fittings can add up to more drop than the straight pipe on a short run.

The filter drier catches moisture and debris and protects the metering device, and a plugging drier is a classic liquid-line restriction that mimics a low charge: high superheat at the coil while subcooling stays normal or high, because the liquid is there but cannot get through. The sight glass reads the liquid for flash bubbles and, with a moisture indicator, for a wet system, which makes it a direct check that the liquid line is delivering solid liquid. The liquid-line solenoid closes on the off cycle for pump-down control, keeping liquid from migrating to the compressor while it is stopped.

The metering device, a TXV, an EEV, or a fixed orifice, is the deliberate big pressure drop in the system, the boundary between the high side and the low side, and it is sized to the equipment, not to the line. The line sizing job is to deliver solid subcooled liquid to its inlet at the rated pressure. Add up the equivalent lengths of the drier, the glass, the solenoid, and the valves with the fittings, and hold the total inside the budget. The drop you forgot to count is the drop that flashes the line.

Sizing tools: manufacturer tables, ASHRAE, and software

Three sources size refrigerant lines, and they rank in a clear order. The equipment manufacturer's lineset and piping tables come first, because they are built for the exact equipment, refrigerant, and rated conditions, and the equipment is listed to them. When the manufacturer gives a table for your length and lift, that table is the answer and it already accounts for pressure drop and oil-return velocity.

The ASHRAE Refrigeration Handbook is the reference behind the tables, with the line-sizing charts, the pressure-drop budgets, and the oil-return velocity criteria that the manufacturer figures derive from. It is where you go for a field-built system the manufacturer does not table, for a rack or a custom job, and to understand why a size is what it is. The published charts there are indexed to refrigerant, saturation temperature, and capacity, and they assume the same 2°F suction and discharge budget and the velocity minimums discussed above.

Sizing software and the per-manufacturer calculators run the same math faster and handle the equivalent length of the fittings and accessories for you, which is the part that is easy to undercount by hand. Whatever the tool, the inputs decide the answer: the actual capacity at the conditions, the total equivalent length including every elbow and accessory, and the real vertical lift. Garbage length in, wrong pipe out. Measure the routed run and count the fittings before you trust any table or calculator.

VRF, mini-splits, and parallel racks

Larger and multi-circuit systems change the piping problem, and they are where line design gets specialized. VRF and VRV systems run long refrigerant lines to many indoor units off branch fittings or headers, with wide capacity turndown as units cycle on and off, so oil return at minimum load is the controlling design concern and the manufacturer's proprietary pipe sizing, branch selection, and length-and-lift limits are not optional. You size a VRF system to the manufacturer's design software, full stop, because the branch geometry and the refrigerant volume are theirs.

Mini-splits are the small end of the same idea, with the line sizes fixed by the equipment and the only field variables being the length, the lift, and the charge adder. The standard maximums and the per-foot charge are in the install sheet, and the mini-split and VRF sizing by topic follows the maker's tables rather than the generic charts.

Refrigeration racks and parallel compressor systems are the opposite extreme: multiple compressors on a common suction header feeding many fixtures, with huge turndown as compressors stage and fixtures cycle. This is the home ground of the double suction riser, the suction-line trap, and careful header design, because at the bottom of the load range the velocity on a poorly sized riser collapses and the oil never comes back. Supermarket and parallel-rack piping by topic is a design discipline of its own, governed by the rack manufacturer and the ASHRAE refrigeration guidance, and it is not a place for rules of thumb.

Piping the A2L refrigerants

The A2L refrigerants now shipping on new equipment, mainly R-454B and R-32, are the response to the EPA AIM Act HFC phasedown and the technology-transition rule that capped new equipment GWP, and they change line sizing through their pressure-temperature curves and add safety requirements to the piping itself. The sizing math is the same, but the psi that equals the 2°F suction budget differs from R-410A, and a blend like R-454B carries temperature glide, so its saturation behavior is a range, not a point. Use the manufacturer's A2L lineset tables for the specific refrigerant; do not carry an R-410A chart onto an R-454B system.

The A2L class is mildly flammable, lower toxicity with low flame propagation, which the equipment listing handles with charge limits tied to the room volume and, on larger systems, refrigerant detection that shuts down and runs the fan to disperse a leak. The piping is part of that calculation: the total refrigerant in the line set adds to the system charge that the room-size limit is checked against, so a long line that raises the charge can run a small-room install into the charge limit. Confirm the listed charge limit for the space against the equipment plus the line-set charge.

The handling and the codes are still settling, so hedge accordingly. The A2L safety class comes from ASHRAE Standard 34, the install and charge-limit requirements come from the equipment listing and the manufacturer's A2L instructions, and the mechanical and building codes adopted by the jurisdiction govern the rest. Confirm the adopted edition and the current refrigerant rules with the authority having jurisdiction, because the dates and limits keep moving. The refrigerant charging guide covers the A2L safety and glide side in more detail.

From sized pipe to a running system

Sizing the line right is the design half; proving it leak-free and dry and charging it correctly is the install half, and they have to meet for the system to run as designed. The order does not change. Once the lines are sized, routed, sloped, and trapped, you braze under nitrogen, pressure test with dry nitrogen before any vacuum, pull a deep vacuum to about 500 microns and prove it holds with a decay test, then weigh in the charge plus the per-foot line-set adder.

All of that is covered step by step in the split-system install guide, and there is no reason to repeat it here. The point worth making at the seam between design and install is that a perfect evacuation and a perfect charge cannot save a line that was sized wrong. If the suction riser is oversized, the system will pass its vacuum, take its charge, and still lose oil over the following months. If the liquid line is undersized or the lift outruns the subcooling, the system will charge fine and flash on the first hot day. The sizing decision is locked in before the first joint is brazed, and the commissioning numbers only confirm what the design already determined.

So commission against the design intent, not just against a pass. Read superheat and subcooling per the charging guide, and read them knowing what the line sizing was supposed to deliver: solid subcooled liquid to the valve, and a suction line moving fast enough to bring the oil home.

Why is my compressor losing oil?

A compressor losing oil with no external leak is almost always an oil-return problem in the piping, and the line sizing is the first place to look. The symptom is a falling oil level in the sight glass on equipment that has one, oil-related compressor trips or noise, and on a rack the slow drift of oil out of the compressors and into the system. The oil left the compressor with the refrigerant, as it always does, and the piping is not bringing it back.

Run the short list. An oversized suction riser that drops below oil-return velocity at part load is the most common design cause, and it gets worse the more the system unloads. A missing or wrong trap lets oil drain back down a riser every off cycle. A suction line sloped the wrong way, away from the compressor, pools oil in the low spot. A double riser that was needed and not installed leaves the velocity too low at minimum load. And a system running far below its design load, more than the piping was sized for, can starve oil return even on correctly sized pipe.

The other failure the piping causes is the high-pressure-drop kind, where the line is undersized and the symptom is lost capacity rather than lost oil: high suction-line drop showing as a system that cannot make its rated cooling, with the suction pressure at the compressor noticeably below the evaporator. Measure pressure at both ends of a suspect line under load to find it. Oil logging and excessive pressure drop are the two ways bad line sizing shows up months after the install, and both point back to the diameter and the geometry, not the charge.

What to document

A line-sizing decision that lives only in the installer's head cannot be checked when the system loses oil or capacity two years on. Record the design so the next person can see what the pipe was meant to do. The record is also what proves the install matched the manufacturer's tables, which is what the warranty and the inspector care about.

Capture each line by name with its size and type, the total equivalent length including the fittings and accessories, the vertical lift and its direction, the design capacity the sizing was run at, the velocity or the table the size came from, any trap or double riser and where it sits, the suction insulation, and the charge adder for the line length. If you sized off the manufacturer's table, note the table and the conditions; if you sized a field-built run off ASHRAE charts, note the budget and velocity you held to.

Common mistakes

• Oversizing the suction line so velocity falls below the oil-return minimum and the compressor starves at part load.
• Sizing a line by the equipment stub-out diameter instead of by the actual length and lift.
• Leaving out a needed suction-riser trap, or putting a trap on a run that should just slope to the compressor.
• Skipping the double suction riser on a wide-turndown system, so oil does not return at minimum load.
• Running the liquid line up a high lift with too little subcooling, so it flashes at the top on a hot day.
• Undersizing a line so the pressure drop runs past the 2°F or 5 psi budget and the system loses rated capacity.
• Counting only straight pipe and ignoring the equivalent length of the elbows, drier, sight glass, and solenoid.
• Leaving the suction line uninsulated or its outdoor foam unprotected from UV, losing capacity and sweating in walls.
• Carrying an R-410A line-sizing chart onto an R-454B or R-32 system instead of the refrigerant's own table.
• Exceeding the manufacturer's rated total length or vertical separation with no table that covers it.

Field checklist

Standards and references

The equipment manufacturer governs the line sizing. Their lineset and piping tables set the line sizes, the maximum total length, the maximum vertical separation, the required traps and double risers, the per-foot charge adder, and where a long line forces a TXV or heavier insulation. Where a general rule of thumb and the manufacturer's table disagree, the table wins, because the equipment is listed and warranted to it.

The ASHRAE Refrigeration Handbook is the design reference behind those tables, with the line-sizing charts, the pressure-drop budgets, and the oil-return velocity criteria, and it is what you reach for on a field-built or rack system the manufacturer does not table. The pressure-drop budgets commonly cited, about 2°F equivalent saturation temperature on the suction and discharge lines and on the order of 5 psi on the liquid line, and the oil-return velocity minimums of roughly 500 to 700 fpm horizontal and 1000 to 1500 fpm vertical, come from that body of guidance and vary with the refrigerant and conditions.

ASHRAE Standard 34 sets the refrigerant safety classification, including the A2L class for the lower-GWP refrigerants, ASHRAE Standard 15 covers refrigeration system safety, including charge limits for occupied spaces, and UL 60335-2-40 is the appliance safety standard that drives the A2L leak detection and mitigation built into the equipment listings. The mechanical and building codes adopted by the jurisdiction cover the installed piping, the charge limits, and leak mitigation. Refrigerant handling and the ban on venting fall under EPA Section 608, covered in the charging and the leak guides. Cite the body that owns the point, follow the manufacturer for the size, and confirm the adopted code edition and the current refrigerant rules with the authority having jurisdiction before relying on a specific figure.

Units, terms, and conversions

Line sizing carries its own vocabulary, and the same line reads differently across a manufacturer table, an ASHRAE chart, and a metric spec.

Refrigerant copper is ACR tube, sized by actual outside diameter in inches, not the nominal sizing plumbing copper uses. Velocity is in feet per minute in field tables and meters per second in metric sources, where 1000 fpm is about 5.1 m/s. Pressure drop is in psi, but the suction and discharge budgets are written as an equivalent saturation-temperature drop in degrees Fahrenheit, because that is what costs capacity. Vertical lift static head runs about 0.5 psi per foot for common refrigerants. Subcooling and superheat are in degrees Fahrenheit in the field, degrees Celsius in metric literature.

Equivalent length

Straight pipe length plus the added pressure-drop length of every elbow, valve, and accessory

Oil-return velocity

The minimum vapor speed that drags oil back to the compressor; the controlling number on suction and hot-gas risers at minimum load

Double suction riser

Two parallel risers with a base trap, so oil returns at part load through the small riser and pressure drop stays in budget at full load

P-trap / inverted trap

A base trap that helps lift oil up a riser; an inverted top trap that stops oil draining back when the header is above the evaporator

Flash gas

Liquid that boils to vapor in the liquid line when pressure drops below its saturation point, starving the metering device

Static head

The pressure a vertical liquid column loses to height, about 0.5 psi per foot of lift for common refrigerants

ACR copper

Air-conditioning-and-refrigeration tube, cleaned, dehydrated, and capped, sized by actual outside diameter

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