Basic Air Conditioning Piping Recommendations – Suction Lines – Part 1

In my last post, I mentioned orientation of the equipment and that it can affect how we size a line set.  Before I discuss the various orientations, it is important to understand a few things about suction and liquid lines.

First, let’s consider suction lines. To me, suction line sizing is probably the most critical part of sizing any refrigerant line set. Suction pressure loss and suction line velocity are two of the most important things to get right when sizing a suction line.

Let’s look at suction line pressure loss first.  Suction pressure loss reduces  the system capacity by 1% for R-22 and 0.6% for R-410A for each psi loss in the lines. This can be a serious problem if suction lines are not sized properly and there is too high of a pressure loss.  As a good achievable guideline, suction pressure loss should not be allowed to exceed 3 psi for R-22 or 5 psi for R-410A.  If we look at actual losses, the question comes up, “can the capacity sizing of this unit withstand a 9 or 10 % CAPACITY loss because the line has a lot of restriction (pressure drop)?” This can be a yes or no answer based on you initial sizing of the equipment.  In some applications the YES answer needs to be used because a larger pressure loss is acceptable to assure the more critical component of suction line sizing –proper oil return.

Part of all air conditioning systems line sets is the presence of refrigeration oil required for proper and continuous lubrication of the compressor bearings.  All air conditioners circulate oil throughout the system due to the miscibility of refrigerant oils. It is not unusual for a given system to circulate as much as 15% of the original compressor oil charge out in the system. This can become a problem if not recognized and managed.

So, how do we assure proper oil return to the compressor?  Oil return is a function of refrigerant gas velocity in the suction line. There are two parameters to look at in regards to velocity:

  1. Minimum Velocity of at least 1000 fpm for vertical risers.
  2. Minimum Velocity of at least 800 fpm (I prefer 1000 fpm) on horizontal pipe runs.

Internal refrigerant velocity of at least 1000 feet per minute (fpm) is required to carry oil up a suction riser. Of course, this is only a factor when the condenser/compressor is ABOVE the evaporator/ indoor unit and the oil must overcome gravity to return to the compressor.  Greater refrigerant velocities are obtained by decreasing the size of the suction line.   This is usually accomplished by decreasing the size of the suction  riser  but then using larger tubing for the horizontal suction portion of the suction line.  The suction riser needs to be sized to maintain the 1000 fpm to get the oil up  the pipe and back to the compressor while larger tubing is used to minimize the pressure drop and help maintain capacity.

These two factors — suction line pressure loss and suction line velocity –are the most important parts of sizing suction lines for any air conditioning system — either residential of commercial.

In my next post, We’ll continue on suction lines, since, as I said at the start, are the most critical part of any line sizing.  We’ll get into some specifics and we’ll talk about oil management more and some accessories like accumulators and traps.  But even when we get into these areas, the two basic application “rules” apply —  friction loss and velocity — to create a properly operating air conditioning system.

About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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3 Responses to Basic Air Conditioning Piping Recommendations – Suction Lines – Part 1

  1. Bruce Porter says:

    It must also be noted that the pressure loss effect of any percentage above the maximum allowed will result in a loss of refrigerant flow to the compressor. As such, the loss will always show up under the highest load condition on the hottest day of the year. That being said the culprit that will show up is superheat. This kills compressors. Usually speaking when the compressor has less refrigerant to deal with things are usually good however when the load continues to be high the discharge superheat goes up exponentially. Generally speaking discharge temperatures of a round 245 are within range however you must remember the temperature in the compressor is usually 30 to 50 degrees higher approaching the breakdown temperature of the oil. The really burning question is can i get away with it? Will the undersizing of the suction line cause enough havoc in the system to create a compressor failure? In my experience when you hold the value in the suction line from 5 to 9 pounds 5 being the recommended maximum, that only gives you 4 more pounds of loss that you have a window for until you cause the superheat to get way out of hand. The refrigerant itself contains superheat in it as a component. When you start adding superheat and cannot reject it due to restrictions in the suction line, death to compressor will be the result. Remember that most all failures are truly mechanical in nature first then electrical will be the fail mode.after the oil has been broken down or burned or lubricity has been compromised due to high temperatures.

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