Can someone please explain to me the princibles of unloaders on chiller systems

[jbomba]

Hi im just starting to work on large trane chiller systems and would like it if someone could explain what the unloaders do and how they work.For some reason i never had this covered in my bookwork through my apprenticeship.

[mad fridgie]

( Do not know about Trane), but unloading in general.
Firstly understand that it is the mass flow of refrigerant that flow through a system (at certain conditions), determines the net cooling effect, "more flow more cooling" so if your cooling demand is lower than the mass flow produced, you will over cool the product, which is something we do not want, so we either increase the cooling load (which you can not normally do) or we reduce the refrigerant mass flow to meet the cooling demand.
"we unload the compressor" "we change the working swept volume." so with a recip compressor it may have 3 banks of pistons (example only), 2 banks may have unloading heads. The unloading heads basically lifts the valve plate of the block ( so the gas in the piston chamber does not work), so on this example we unload from 100% to 66% to 33% (2 heads unloaded), Scroll unload 100% but use time as a basis of unload %, screw use a slide, you can also use a VSD to change the speed. All change the hourly swept volume and the refrigerant mass flow.

[goshen]

Hi
try these:
http://www.simtronics.com/catalog/spm/spm1300.htm (http://www.simtronics.com/catalog/spm/spm1300.htm)
http://www.blackmer.com/cbpdf/cb039.pdf (http://www.blackmer.com/cbpdf/cb039.pdf)
http://www.turbomachinerymag.com/white papers/Screw compressors.pdf (http://www.turbomachinerymag.com/white papers/Screw compressors.pdf)
http://www.ashrae.gr/PresentationNo2(Centri)version2.pdf (http://www.ashrae.gr/PresentationNo2(Centri)version2.pdf)
good luck and enjoy reading.