Back to basics
Continued from page 67
Have a look at the following load profile from a cooling
system in the UK. They don’t have the higher ambient
temperatures that we have to cope with in South Africa.
I really like the Bitzer solution where the leading
compressor runs on a variable speed drive. So instead
of unloading, the compressor drops its speed to around
50% before it starts to unload. The screw compressor’s
efficiency is best maintained in the 100% to 80% capacity
Load profile of
cooling system in
the UK.
It’s a given that screw compressors are high capacity low foot
print compressor assemblies. However, while the compressors
have a completely variable capacity control system, this is
good because they steplessly match the system’s demand;
they have really poor energy efficiency at low load capacities.
At 20% capacity they could be pulling 50% of the connected
power depending on the operational range. With the
overcapacity installed for the lower ambient/ production
periods this part load performance is significant on the
effective energy consumption.
www.hvacronline.co.za
range. By specifically designing your capacity control
system you can operate at peak efficiencies.
There are electronic systems out there that automatically
optimise the running power and performance to give the
most efficient plant operation. Sure, you need to spend
some money on this, but with the continual increase in
Eskom tariffs can you afford not to look at this solution?
It’s a retrofit without any mechanical plant upgrades other
than a variable speed drive or two depending on the system
involved. RACA
RACA Journal I June 2019
69