Understanding the variable speed refrigerant compressor’s performance

[Richard Kollmann]

Originally Posted by malbert73

So I assume my Frigoboat system with Merlin speed control and BD35 compressor achieves this by ramping down compressor speed once box is chilled down ?

Frigoboat 20 years ago gave me one of the first proto type merlin if the one you have has a red and green LED the green flashes a speed flash code. After compressor starts at low speed a single lit green indicating 2000 rpm it will remain lit until thermostat stops and restarts compressor and then the second green code will be lit there are six speed increases until max speed of 3500 rpm is reached. if you watch the green led you will know how the merlan can tell what speed compressor is running at. For those having the Danfoss AEO the only way to tell speeds is by seeing amperage change. I now have a clamp ammeter that reads system current or clamped over thermostat wire speed changes are readable in milliamps of current. Each time thermostat stops compressor it will be restarted at a lower Rpm. Temperature has nothing to do with speeds it is length of compressor running time that triggers speed change. compressor runs the longest when first started, each cycle after that is going to be shorter.

[jt11791]

Originally Posted by Richard Kollmann

Once a system with a variable speed compressor is in operation there are simple ways to check its performance by measuring amperage, compressor heat and compressor cycle off and on running times. A simple test of these small 12 volt compressors is if you can not keep the palm of your hand on top of the compressor dome indicates it is overloaded and high amperage will confirm this.

On my boat I have two BD50s running separate boxes. They both have merlin speed controls and keel coolers. They work well, holding the boxes at the set temp and cycling as expected. My concern is that the compressors fail the palm of the hand test. They both are hot when running, typically 55C to 63C. I just checked the one that is running right now and it is at 55C and is drawing 2.5 amps (merlin shows one flash indicating the lowest speed). The two compressors are mounted together in a small space. Sometime back I added a small fan to improve airflow in that space. I don't know what more I can do to improve the situation, maybe some way to add a heatsink?

Should I worry about the high compressor temp? Or just be happy that the beer is cold and the ice is hard.

[OzeLouie]

Originally Posted by jt11791

On my boat I have two BD50s running separate boxes. They both have merlin speed controls and keel coolers. They work well, holding the boxes at the set temp and cycling as expected. My concern is that the compressors fail the palm of the hand test. They both are hot when running, typically 55C to 63C. I just checked the one that is running right now and it is at 55C and is drawing 2.5 amps (merlin shows one flash indicating the lowest speed). The two compressors are mounted together in a small space. Sometime back I added a small fan to improve airflow in that space. I don't know what more I can do to improve the situation, maybe some way to add a heatsink?

Should I worry about the high compressor temp? Or just be happy that the beer is cold and the ice is hard.

Depends on the ambient temperature, condensing temp etc as the
high side condensing temp can be as much as 20C higher than the ambient. Generally a compressor temp of 50 to 60C is not an issue. Note most comp manufacturers ratings are with a high side rate of 55C or over 200psig And yes, relax and enjoy a coldie or three before this thread goes feral.

Cheers OzeLouir

[Richard Kollmann]

Originally Posted by jt11791

On my boat I have two BD50s running separate boxes. They both have merlin speed controls and keel coolers. They work well, holding the boxes at the set temp and cycling as expected. My concern is that the compressors fail the palm of the hand test. They both are hot when running, typically 55C to 63C. I just checked the one that is running right now and it is at 55C and is drawing 2.5 amps (merlin shows one flash indicating the lowest speed). The two compressors are mounted together in a small space. Sometime back I added a small fan to improve airflow in that space. I don't know what more I can do to improve the situation, maybe some way to add a heatsink?

Should I worry about the high compressor temp? Or just be happy that the beer is cold and the ice is hard.

Jt 11791, I prefer to provide facts in answering questions about popular pleasure boat live aboard refrigeration. These ice box conversion refrigerator companies do not design the units they sell to be compatible with all boater’s different requirements which means design flaws may show up some time later. Hermetically sealed compressors like the danfoss 12/24 volt BD were designed for only air cooled condensing units that rely on a portion of condenser fan air for massive excess compressor heat disposal. Primary compressor cooling is achieved by sufficient refrigerant flow and temperature returning from cold evaporator. Danfoss’s poor condenser cooling protection is determined by the amount of current flowing between Small plus and F terminals of their control module to operate additional compressor cooling ,module cooling and condenser fans. None of these are connected on Frigoboat keel cooler units to over heat module safety devices.

Compressor temperatures that may exceed 55 degrees C (130F) for an extended time should be investigated, Louie’s attempt's of sick humor jokes to sell his refrigeration can again be ignored. I have followed designs of mobile refrigeration especially in boats for forty years. The last time I check all my hard drives for system outages on keel coolers there were over 200 messages related to Frigoboat. These systems must outsell all the other popular brands, I recommend a lot of them but since 2004 and my meeting personally with their design engineer I recommend their inline secondary fan cooled condenser when operation is expected to be used in tropical waters or when the boat refrigeration is used on the hard out of water. I still recommend a separate 40 to 60 Cfm fan blowing air over compressor when seawater temperatures are above 75 degrees F. Any time these compressors run at maximum speed and load danfoss recommends module fan cooling.

Jt11791, Your reported amp draw of 2.5 amps and 55 C indicates a problem You also indicated compressor cycle times were OK, normal. I am no longer able to run my test stands to duplicate your reported conditions but a BD50 with almost no refrigerant in it will draw around 2.2 amps at 2,000 Rpm. Another confusing fact is heat and compressor temperature and how high and low energy consumption was.
I am left with questions: if these are freezer performance figures what are the refrigerator compressor readings?
Can you report the approximate temperatures of refrigerant lines going to each keel coolers and each line returning? I would expect lines going to each keel condenser would be rather warm HOT. Lines returning from the keel condenser have a temperature a little warmer than your body temp, not much more than 115 degrees F except in 90 F seawater.

There have been a number of Frigoboat catastrophic failures of these systems requiring complete system replacement three or four on this forum. As I recall they all operated longer than a year in tropical waters. The first sign of a problem was lack of evaporator ice cover and decisions made to add refrigerant without confirming current amperage draw. Adding refrigerant increased oil contamination .One boater on this forum that I contributed to the oil overheating compressor oil desistor, ended up with a BD50 current draw of 18 amps. The complacent system with a BD80 water cooled normal condenser did not perform as well as the keel cooler. I was shipped the old capillary tube complete for analysis. .

[Bowdrie]

Originally Posted by jt11791

The two compressors are mounted together in a small space. Sometime back I added a small fan to improve airflow in that space. I don't know what more I can do to improve the situation,

Does the airflow have unrestricted entry and exit from the mounting space, or does the "small" fan just recirculate hot air, causing the units to "stew in their own juices"?
Can you rig two fans, one blowing air into the space, one exhausting?

[jt11791]

Thanks guys. My bad for inventing problems where cold beer exists.
Cheers!

[OzeLouie]

Originally Posted by Bowdrie

Does the airflow have unrestricted entry and exit from the mounting space, or does the "small" fan just recirculate hot air, causing the units to "stew in their own juices"?
Can you rig two fans, one blowing air into the space, one exhausting?

Good idea Bowdrie but the low draw reported indicates a bit short of gas (low return gas volume) which as you know restricts compressor cooling. Perhaps a visual as to how much the evaporator plate is or isn't frosting to confirm but if short there may be a gas loss problem.

[Richard Kollmann]

Originally Posted by jt11791

Thanks guys. My bad for inventing problems where cold beer exists.
Cheers!

You are welcome, hopefully the information reviewed of impowered warnings will help prevent the alternative series of troubles experienced by others.
Get back to us when the first signs of high temperature show up in line connector refrigerant Oring seal leakage.

[RaymondR]

When I disassembled my fridge in order to remove the evaporator I discovered that someone had replaced the thermostat with one not fitted with a motor speed control resister and consequently the compressor had been running at it lowest speed. This explained something which had baffled me ever since I had purchased this boat.

My previous experience with these small DC powered fridges was that one could hear a quiet gurgling sound from the liquid refrigerant flowing into and through the evaporator. This one did not and what could be heard was a spitting sound which was obviously caused by slugs of liquid refrigerant followed by vapor into an evaporator plate which was fairly empty of liquid refrigerant because it had been piped in backward. That is it was being filled from the top and most of the liquid refrigerant was being blown out of the plate into the next one. There were three plates piped in series.

My analysis of the circumstances was that because the compressor was running slower than what the system was designed for, insufficient back pressure was being generated by the cap tube to ensure that all the refrigerant was in liquid form when it entered the cap tube and that this resulted in the slugging I could hear.

Being mindful that the flow control systems we use in our DC systems were originally designed for AC systems where the motor speed is locked into the AC frequency which is generally constant this raises an issue in my mind regarding the use of variable speed compressors in a system which is designed for a fairly constant flow of refrigerant through the system. How does the cap tube or TXV orifice, both of which have fixed capacities of fluid flow, handle highly variable flow rates achievable with a variable speed compressor?

[Richard Kollmann]

Originally Posted by RaymondR

When I disassembled my fridge in order to remove the evaporator I discovered that someone had replaced the thermostat with one not fitted with a motor speed control resister and consequently the compressor had been running at it lowest speed. This explained something which had baffled me ever since I had purchased this boat.

My previous experience with these small DC powered fridges was that one could hear a quiet gurgling sound from the liquid refrigerant flowing into and through the evaporator. This one did not and what could be heard was a spitting sound which was obviously caused by slugs of liquid refrigerant followed by vapor into an evaporator plate which was fairly empty of liquid refrigerant because it had been piped in backward. That is it was being filled from the top and most of the liquid refrigerant was being blown out of the plate into the next one. There were three plates piped in series.

My analysis of the circumstances was that because the compressor was running slower than what the system was designed for, insufficient back pressure was being generated by the cap tube to ensure that all the refrigerant was in liquid form when it entered the cap tube and that this resulted in the slugging I could hear.

Being mindful that the flow control systems we use in our DC systems were originally designed for AC systems where the motor speed is locked into the AC frequency which is generally constant this raises an issue in my mind regarding the use of variable speed compressors in a system which is designed for a fairly constant flow of refrigerant through the system. How does the cap tube or TXV orifice, both of which have fixed capacities of fluid flow, handle highly variable flow rates achievable with a variable speed compressor?

There is a good inexpensive solution solution for setting compressor speeds on my website slideshow.
The science behind refrigerant control is simple: All mechanical refrigerant flow control devices produce an audible sound as liquid is expanded to vapor within the evaporator, some call this sound a hiss, others describe it as gurgling. How you want to describe noise is not important. What is important is this phase change is required in confining the saturated liquid vapor to gas only in a single evaporator or a series of evaporators. The term used in confining the majority of heat absorption inside evaporators is superheat control. Capillary tube refrigeration requires relatively stable climate conditions so superheat is confined inside the evaporator.
The problem with mobile and especially refrigeration for ice box conversion ther are two many variables involved, the reality is there will never be two of these locally installed ice box conversion systems with exactly the same refrigerant flow pressures. The biggest problem with mobile refrigerant capillary tube flow controlling volume of liquid refrigerant is going to be unstable.

Now you understand why there is a sound of refrigerant liquid sprayed into mist in evaporator. The pulsing and sometimes vibrating sounds of gas to liquid is not always 100% efficient as liquid refrigerant flows towards the control cap tube; it passes through objects like the filter dryer and line bends causing a liquid expansion gas bubble to form. As the refrigerant containing gas bubbles pass through the long cap tube vpor gaps are created between liquids creating the normal pulsing is heard inside box.

How does the cap tube or TXV orifice, both of which have fixed capacities of fluid flow, handle highly variable flow rates achievable with a variable speed compressor?

Answer: The TXV has greater capacity because its temperature range and flow is far greater than the cap tube. The variable speed of the compressor has little to do with work to be accomplished between the two flow controls as long as the both deliver the volume of refrigerant energy efficiency.
Variable speed efficiency on small 12 volt compressors are not necessary on eutectic plates because these systems mostly require alternative electric energy.