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Speed - fast is fine, is slow better?

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Compressors, how fast should they run?

We had an inquiry recently from someone asking if it was necessary to have some form of speed control on Secop/Danfoss BD35 and BD50 refrigeration compressors. Well the simple answer is “no, it’s not absolutely necessary”, but without it your system may be working way below its capabilities, and with less efficiency.

You see, the Secop (formerly Danfoss) BD35 and BD50 compressors are capable of being controlled externally to run at various speeds between 2,000 and 3,500 RPM, and the cooling capacity is directly linked to compressor speed; i.e. the lower the compressor speed, the lower the cooling capacity.

So why not simply run every compressor at the highest speed and cover all the bases?

Well that is indeed the case with pumped-water cooled systems, where keeping pump run times to a minimum reduces overall power draw, lessens the possibility of clogged strainers, and maximizes pump life. But for air cooled and Keel Cooled systems there are big advantages to be gained by controlling compressor speed. Here’s why.

The longer and slower a compressor runs, the more efficient it becomes.

Gains in efficiency of around 15% are typical with the most appropriate speed setting compared to running at full speed. What we need to do then is to set the compressor speed to a setting that is high enough to do the job, but as low as possible in order to increase efficiency and reduce overall current draw.

OK, so how do we change the compressor speed?

The speed of these compressors is controlled by adding resistance into the thermostat circuit. With no added resistance, i.e. simply the two wires from the thermostat plugged on the electronic controller, the compressor will run at its lowest speed, 2,000 RPM, and therefore with the minimum cooling capacity. This may not be enough to do the job on medium and larger installations, so a resistor of some form must be added to one of the two thermostat wires in order to increase compressor speed and capacity.

Coastal Climate Control offers three forms of compressor speed control, and these can be added to any Secop or Danfoss BD35 or BD50 compressor in any manufacturer’s system, not just Frigoboat.

  1. SPEED BOARD - This is a small circuit board that simply plugs onto the electronic controller that’s attached to the compressor, and the thermostat wires plug onto it. There is a potentiometer (pot) that is adjustable from 0 to 3,500 RPM by a mini flat-blade screwdriver. The operator must decide and adjust the speed depending on the application, and a period of trial-and-error is inevitable.
  2. MERLIN II - Again, this is a small circuit board that plugs on to the electronic controller, but now the speed is varied automatically dependent on load and run times. The speed will be set, and changed if necessary, on each compressor start-up, and will be seen as a series of flashes on a green LED. There is also a red fault-indicating LED which is an invaluable tool to assist troubleshooting should your compressor stop for any reason other than the thermostat has told it to.
  3. GUARDIAN - Now we’re getting serious! The Guardian is a remotely mounted digital thermostat/thermometer that also features a manual compressor speed adjustment knob plus a red troubleshooting LED. So in one attractive panel you can see and control box temperature, manually control compressor speed, and get an indication of system troubles. Now, here comes the really good bit. If you install a Guardian plus a Merlin II, you will have the choice of either automatically variable compressor speed control, or manual. Set the knob to “Auto”, and the compressor speed will automatically vary under the control of the MERLIN II. Change the setting to “Min”, “Medium”, or “Max”, and the operator has the power to override the MERLIN II. This is a very useful feature, as it can be set to give maximum cooling when stocking the box with stuff from the store, or used simply to twiddle and play during a long, boring ocean crossing.

There is one huge benefit from installing a MERLIN II on a system using the type of evaporator (cooling plate) that is formed into a “bin”, with a freezer compartment inside. With the MERLIN II running the compressor at the lowest capacity possible for the longest time permissible, the freezer compartment turns into a super-freezer, and stuff in there tends to stay rock-hard. Food for thought, although frozen, but prepare for extended defrost times.

So, do you need compressor speed control on your BD35 or BD50 compressor? No, not necessarily, but by not utilizing this feature you will almost certainly not be getting the maximum performance or efficiency from your system.

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Contact Info

Phone: (301) 352-5738
Office | Warehouse:
4831 Tesla Dr., Suite H
Bowie, Maryland 20715
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Coastal Climate Control
Cooling, Monitoring & Solar Solutions


Office | Warehouse:
Coastal Climate Control
4831 Tesla Drive
Suite H
Bowie, Maryland 20715
Phone: (301) 352-5738

Request Information
Click here for directions.