Why Suction Lines Must Be Sized To Manufacturers G

1 Why Suction Lines must be Sized to Manufacturer s GuidelinesTech to Tech Column May 08 Make daily deposits to your box of knowledge, soon it will have manyreference cards. --Randal S. RipleyPreciousmetals can be defined as high-value, low-volume, scarce metals ofthe Gold, Silver and Platinum groupsand have value as currency or jewelry. Thesevaluable metalsthat arecharacterized by their superiorresistance to corrosion andoxidation offeran alterative form of investingand areoften considered a hedge an HVACRS ervice technicianand employed by a supply house, I wouldlike to offer our industries versions of precious metals,Sheet metal & metals may nothave any value as jewelry yet butthey sure cost a lot ofcurrency and if their price continues to rise at what seems like a bi-weekly basiswejust may see them as jewelryor dangling from apiercingor two onour.

2 This is not going to be a column on precious metals but it is a column on apractice that is going to become more prevalent with the price of copper continuingupwardand the ever increasing distance between the unitsdue tocustomerswantingthem placed where they will be hidden, the undersized Suction line . I am receiving more calls than normalfrom techniciansthatquery me withquestions like, I know theinstallation & Operationmanual requires a1 1/8 suctionlinefor this runbut it will work with a 7/8 ,right? Onlytypes K (heavy wall) and L (medium wall) copper should be used forrefrigeration and air conditioningpipingmaking the1 1/8 Suction linea bear towork with but I think thesignificant price difference betweenthe 7/8 and 1 1/8 linesets(around $75)and the necessity of an additional line set needing to be purchasedwhen the run is over 50 is a big factor here as reply to this is always What do you mean by work?

3 If you mean thesystem will operateata lower SEER& capacityratings, be susceptible to prematurecompressor burn out and have potential oil return problems,yes it will work;but,ifyou want the system to operateat its peak efficiency and performance, then youmust use the manufacturer srecommended line amsure all of you have heard the phrase, the compressor is the heart ofthe system , so it is not a stretch to say the line set isthe main arteries and thatthe refrigerantand oil arethe life blood .Refrigeration/air conditioning line setsmust be sizedbased on three things:Velocity, Pressure drop and Refrigerant charge. In addition,theymust bepitched&trappedproperlyaccordingtoappl ication,compressor above or below theevaporator,toassure maximum refrigerant flow and oil under-sizing (which is a restriction) or other type of restriction in thesevital conduitsthat affects the flow of thisrefrigerant/oil vapor mixturecan cause amultitude of problems from diminished capacity, refrigerant system blockages due tooil concentration evenoutright small amount of oil must pass through the compressor cylinders to lubricatethem and is alwaysentrained in the refrigerant.

4 Oil and liquid refrigerant are highlymiscible (readily mix together),however, oil and refrigerantvapordo notreadily mixand oil willonly properly flow through the systemif the velocityand massof therefrigerant vapor is great enough to push the oil minimum velocities of 700 FPM in horizontal Suction Lines and 1500 FPM in verticalsuction Lines have been used for years as Suction line sizing designstandardsaccording say an average Suction line velocity of 1200 FPM will Suction Lines should be pitched downward in the direction of flowwith at least a inch in 10 pitch to aid in oil vertical suctionrisers (the run of piping wherethe condenser is higherthan the evaporator, that causes the upward flow of refrigerant vapor)

5 Of more thanthree feet in height should have a P trap at the base of the riser to move the oil upthe long vertical Suction risers, additional traps should be installed every15 -20 feet to aid in lifting the oil up the tends to crawl up the inner surface of the pipe on vertical risersand thegreater the tubing size,the higher the velocity necessary in the center of the tubingto keep this oil that the oil doesn t drop out along the way back to thecompressor, Suction Lines must be kept small enough in diameter to keep refrigerantvelocities flip side of this isin our attempt to keep thesuctionline small enough tokeep refrigerant velocity up, we can make the Suction line toosmall, causingresistance to flow(expressed as pressure drop).

6 Pressure drop reducescapacity andefficiency because it decreases the Suction pressureand reduces the density of therefrigerant vapor returning to the compressor causingless oil to be pushed back tothe compressor,a highercompression ratio and more energy to be used by denser the refrigerant vaporentering the compressor cylinder, the morevolume the compressor can pump. This will lower the compression ratio and increasethe efficiency of the refrigerant vaporalsocools the windings of the hermetic compressoranda lack of vapor will cause overheating and slowly melt the shellac/varnish likewindings insulation, eventually causing a burnout, turn to turn shorts or a a trap works:All vapor carrying Lines should have oil traps installed to maintain proper can prevent large slugsof oilfrom forming by trapping asmallamount of oiland returning it to the compressor at a gradual avoid largequantities of oil from pooling in the trap,it should be constructed with a minimumdepth and the horizontal portion as short as traps are constantly being filled and emptied at a gradual rate, protectingthe collectsin the trap.

7 It increasesthevelocity oftherefrigerant gasby causing a small restriction in the the refrigerant vaportravels throughthisslight restriction of the pipeitpicks up small surface droplets ofoil from the trap and carriesthem to either the next trap in line or the oil traps are necessary in some applications, a line set should never beallowed to become anoil allow theline set to dip or be pitchedin thewrong direction,allowingoilto pool in these closing notes:The brands of units I am familiar with have lineset sizing charts for up to 74 in the installation & operation manuals, and I can assist you with sizing even longerlengths if follow the manufacturer s Installation Instructions forthe recommended line set more oncompression ratios & volumetric efficiencysee the April 07 Techto Tech Columns on Compressorin the tech column archiveson the technical page ofthe As always, you can reach me by phone at 603-889-0100.