Why don't all modern compressors have this?

ChrisJ

Started working on the second monitor top and have had some minor issues as to be expected.
I suspect either the unloader, or a checkvalve to be leaking so I was exercising the unloader as per the service manual.

When I first unplugged the machine it had been running for 20 minutes or so. It's surprising you can unplug it and plug it right back in and it'll fire up every time even with high head pressure.

The horrid sounding buzz on startup is 10% the relay as the cover is off, and 90% the unloader plunger vibrating in the motor field as it's a known quirk in this model that is even covered in the service manual.

I'm continuously getting more and more amazed at how bullet proof these machines are. Last night while trying to crimp the liquid line to simulate the proper restriction I accidentally crimped it totally shut and the compressor never complained a bit. The only reason I realized something was wrong is because the evaporator stopped boiling after 10 minutes or so because it was empty. This will be next in line getting the damaged float valve removed and a capillary tube installed.

The towels on top were because I was trying to warm the machine up a bit by covering the condenser and compressor.


In 1927-1937 we had unloaders, why don't we anymore?

https://youtu.be/Gc6La-IgF40

jumper

Blow a rupture disc and somebody gets to sell a rebuilt compressor. Why not breakaway clutches in couplings?

bob_46

Chris, I think it boils down to cost. In round figures a monitor cost a little bit more than a model T Ford when new. For >$20,000 mfg. could include unloaders today although we don't need them on domestic refrigerators. GE went beyond the pale when they engineered the monitor, if it hadn't been bullet proof for that kind of money it would have been a disaster for GE and they knew it. Unloaders are common on larger equipment.

Tinman

Many modern day compressors do have unloaders or internal pressure relief valves, no?

njtommy

On a semi-hermetic compresser have unloaders that are either mechanically unloaded by suction pressure or electronically by the control system could be driven off of supply air set point. Depending on application.

Internal pressure relief is a safety so the compresser doesn't blow up. It just dumps the hot gas back into the suction on continuously and basically you can no longer compress the refrigerant. It will just keep pumping and not create a pressure difference.

We also have the capability to run either a 2 stage scroll compressor which is roughly 60% load then the other 40% for second stage.

VRFs systems or modulating scroll compressers are getting more popular too now. They can modulate form say 5,000 btus to 48,000 btus depending on load and humidity conditions.

ChrisJ

Stephen Minnich said:

Many modern day compressors do have unloaders or internal pressure relief valves, no?

If they do I've never heard of one at least not on residential equipment. Anything I've owned will either do a standard delay before restarting, or just trip on overload if you try to restart too early. I am glad to hear bigger units do have them though.

Do systems with TXVs require unloaders by any chance?

Btw, a 1934 Monitor top 7 cubic foot refrigerator was the equivalent to around $3500 today. I'm still concerned about a few issues with this machine, but I'm hoping a capillary tube conversion will solve them.

Tinman

Systems with TXV's require a compressor that utilizes a start capacitor and a potential relay to take the start cap out of the circuit. Most TXV systems don't equalize on the off cycle so extra torque is required to overcome that.

SWEI

njtommy said:

VRFs systems or modulating scroll compressors are getting more popular too now. They can modulate from say 5,000 btus to 48,000 btus depending on load and humidity conditions.

I'm still waiting for these to hit the commercial refrigeration market. Should be interesting...

hvacfreak2

In 1927-1937 we had unloaders, why don't we anymore?

Digital scroll compressors are definitely an " unloading " type. Variable speed compressors are becoming more common as well.

ChrisJ

Digital scroll compressors and variable speed compressors.
Sound awesome but wonder how long term reliability will be?

SWEI

ChrisJ said:

Digital scroll compressors

I have always wondered about these.

variable speed compressors

This really is the future. It can be implemented on larger commercial systems now using off the shelf controls, but as soon as you throw in the added efficiency and turndown that EC motors provide, it gets rather interesting.

njtommy

Yeah I would really like to see how a digital scroll and an EXV would play together in a low temp application.

SWEI

I suspect we're going to see some inverter-driven options soon.

ChrisJ

What exactly is "inverter driven" ?
My guess is it's an ac motor, but you take your 60Hz, rectify it and then use an oscillator to control it's output frequency to whatever you want? I assume you also control output voltage as well?

SWEI

Inverter-driven is a term the Asian mini-split manufacturers chose. Sometimes they call it a DC inverter system. It's really just a VFD, but paired with a permanent magnet AC synchronous motor, which has higher efficiency and much greater turndown than an AC induction motor.

A VFD consists of a rectifier, a big cap for the DC bus, and a 3Ø inverter. The simplest inverters run a Volts per Hz algorithm derived from some basic motor parameters programmed in at setup. There are more sophisticated algorithms for torque control, motion control with optical position feedback, etc. but those are not really relevant to a compressor application.