Sizing Discharge Line on Evap Load or Heat of Rejection?

[jrockyroc]

I've never seen a definitive explanation of this, so hopefully someone can help clarify. A lot of pipe sizing tables and tools simply state "System Capacity" for sizing suction, liquid, and discharge lines. For suction and liquid lines, it makes sense to use the full refrigeration (or evaporator) load. That's what I consider "System Capacity". But for discharge lines, shouldn't you use the total heat of rejection (which is about 1.5x higher than the refrig load), the same load used to size condensers? Why don't most tools and tables make that distinction? Am I incorrect in sizing discharge lines on the heat of rejection?

Example table here:
https://icemeister.net/backroom/wp-content/uploads/2015/04/Refrigerant-Piping-Handbook.pdf

[mbc]

Hi
Our total heat rejection in Condenser is :
Maximum heat comes from Compressor(KW) say A+ Compressor power (KW) say B + 15% of (A+B) For safety.
But in suction line we have low pressure and dropping pressure in the lines is effecting to much in our efincy
Also dropping pressure in the discharge line is effect to our efincy but not much as suction lines.
In all sizing pipe we should look for total capacity of our refrigeration load .

[Peter_1]

You start with system capacity, then look in a log ph to the refrigeration effect in the evaporator and how many kJ 1 kg of the used refrigerant can 'absorb', or , can convert to superheated gas.

Then, with the system capacity in mind, you then calculates the needed mass flow through the whole system which stays the same in every component in the system

Line sizing is done starting with the basic: how many kg/sec (sorry, SI units) refrigerant is circulating through the system, in fact through all (!) the different lines.
This mass flow (not volume flow) is the same for suction or discharge lines or liquid line.
The compressor only increases pressure and temperature but the mass flow rate through all the lines and also through the compressor stays the same.
We convert the kg/s to m3/s for the gas line to calculate volume flow and size then the discharge and suction line.