Okay — hopefully I’ve made my point that the 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. After my last post, I received this comment from Bruce Porter – Senior Manager of Field Service for UPG – Johnson Controls and it really shows how important suction line sizing is to a system:
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 too much 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 around 245 degrees are within range. You must however, remember the temperature in the compressor is usually 30 to 50 degrees higher approaching the breakdown temperature of the oil. The real burning question is — can I get away with it? Will the under-sizing 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 the compressor will be the result. Remember that almost 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.
So how do we size suction lines to accomplish this? Suction Line Selection Chart: manufactures have put together line sizing charts based on industry acceptable practices. Most of these charts are the same from manufacturer to manufacturer. The chart below shows the line size that can be selected for each tonnage of unit. This chart is for R-410A refrigerant application (there are others for R-22). The chart is based on TOTAL EQUIVALENT LINE LENGTH. You need to keep in mind that the number of elbows and other components (solenoids, check valves, accumulators, etc) will increase the equivalent length. In the far right hand column is the velocity. NOTE that the velocities are mostly above 1000 fpm which is one of the requirements. Across the top is the equivalent length (the maximum actual line length for most systems is 200 feet). The numbers in each column under each equivalent feet is the PERCENTAGE OF CAPACITY LOSS based on suction line pressure loss or friction. (Remember, we prefer to see less than 5% loss on a system with R-410A refrigerant).
This chart is a rule of thumb for sizing and the key is to keep the percentage loss as low as possible and still maintain over 1000 fpm in the entire line set. In most cases, using the larger line choice gives you the best scenario for the application as long as you can maintain enough velocity to get the oil back to the compressor. Sometime you need to use a smaller line to get the velocity and then lose capacity due to pressure loss in the lines. As long as your system sizing can stand the loss of capacity (up to a maximum of 9% loss), it is always better to sacrifice capacity in order to maintain proper suction line velocity. (You need to remember Bruce’s comment above — the maximum loss should not exceed 9 psi to control superheat.)
This, again, is a rule of thumb chart and should only be used for checking possible line sizing problems on service calls or estimating a job. Nothing can replace doing an actual calculation for proper line sizing. As you can see right away, the very smallest tonnage units really start losing capacity quickly as the line lengths get longer. Those little “pumps” are just not designed for that type of “load”. In fact, if you look at most manufacturer’s installation guides and look at the limitations — 75 to 100 actual feet (not equivalent) is usually the longest recommend line size for most small tonnage units.
One thing to keep in mind is making sure you know if the system is single-stage or multi-stage. When looking at the suction line for a system with either a 2-stage scroll compressor (residential — full capacity and 66% capacity) or when a single refrigerant system uses compressor staging for capacity reduction (commercial — usually 50/50), ALWAYS select the pipe size that maintains the velocity at minimum capacity — in other words, if you have a 3 ton residential 2-stage scroll — at reduced capacity (66%) the system is now a 2 TON system. So the suction line needs to be sized to maintain the minimum 1000 fpm at 2 tons rather than 3 tons. You may have to compromise on capacity, depending on length, but the oil return is still more critical.
So, as a reminder —Suction Line velocities and pressures:
- Vertical Riser — 1000 fpm (Etched in Stone)
- Horizontal runs or Vertical fall — 800 fpm (to be safe use 1000 for both)
- Suction Line maximum velocity is 3000 fpm
- Suction Line recommended pressure drop maximum 5 psi.
- Suction Line maximum pressure drop 9 psi.
A couple of things to discuss before I finish up on suction lines. In the past, systems had mineral oil or alkylbenzene oil. Mineral oil and alkylbenzene oil were only partially miscible (capable of being mixed in all proportions) with the refrigerants so they moved through the system by peristalsis (velocity pulled the oils through the system). When the system shut off, these oils would drop down and migrate to a low point. So on older systems, traps needed to be installed at the bottom of vertical risers to help get oil back to the compressor. With R-410A systems and P.O.E. oil being totally miscible, the oils moves very freely through the system. As a result — are you sitting down because this will go against everything you have ever been taught or were told about traps — traps are not required for R-410A systems. YES — you read that correctly. Traps are not required if the piping is properly sized. Traps will only add pressure drop to the system, further reducing capacity.
No need to use Suction P Traps on R410a systems if your velocity is above minimums.
Hopefully, this has given you the basics of refrigerant piping of suction lines . There are computer programs out there that work very well in doing this but, even when you use those, they are only as good as the information you provide it. If you give it incorrect lengths, elevations differences, elbow/fitting count and anything else it may ask for, you will not get a correct line sizing.
One last take away — it is alway best to try to keep the line length to as short as possible. This will give you maximum capacity and eliminates a lot of issues that can occur with longer line length.
In my next post, I’ll start on LIQUID LINE sizing. These are equally as critical as suction line sizing. Improper liquid lines will create noise, use more refrigerant than necessary, can create problems with metering devices, and a few other things. So we’ll go through those to help you understand proper liquid line sizing.
[PS: – I will be on vacation for the next 10 days so the next post on liquid line sizing will not be until the first week of July. Even I need some time off :>)]