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.