frost buildup detector

Sounds to me like the fan should be the detector since any other device may end up looking at the wrong spot. Monitor the fan blade for rotation. If it's not moving then you have a problem. If ice forms over your sensor, the pulses will stop - also indicating a problem.

You can use a microphone as a pressure transducer that will detect the passing of a blade. You can use a variety of photoelectric devices (a.k.a. photo-eyes). You can use a proximity sensor or a hall effect transducer and little magnets. There are even low pressure air sensors that may work. Keep it simple but build it like its industrial. Ice is mother natures rock breaker.

You may have to do some testing to see how long you have to wait after the fan is turned on before you have a valid signal. Give yourself some room there because that can take longer with age. Thats my excuse anyway, and it works for me!

I'm not sure I understand. If when the frost builds up to the point that the fan jams and this in turn causes cooling to stop...isn't that ultimately doing the same thing as the sensor would do? Unless of course the fan is getting damaged when this happens.

I think there are actually such things as frost buildup sensors. Haven't a clue how they work though. Do a Google search for "frost sensor circuit" and "central heating frost sensor" and see what you can find out.

If that doesn't work out for you or you want to come up with your own brilliant design, the only way I can think of doing it offhand would be to use a photointerrupter. As frost builds up on it the beam should be interrupted. Go to DigiKey's website and do a search for "Optical photointerrupter" to find some examples of what I'm talking about. I was mostly thinking of the slot type, but there are other types that might work better for you. Have fun!

Guest is correct, use a photoeye with a plane of glass between emitter and receiver, or use a mirror if both are contained in the same housing. Simple to set up and meets the electrical requirements.

First guess is that the system refrigerant charge is low. This would cause a lower evaporator pressure resulting in a colder coil which is freezing any condensate. So, check this first.

Suggestion #1. Just use a thermostat designed for the purpose. You set the control temperature, say 33 degrees F, which is the off point, the thermostat won't turn back on until the air temp rises to 41. The temp probe is fastened to the evaporator. This is a readily available commercial refrigeration thermostat.

Suggestion #2. If its an expansion valve system, adjust the valve to keep the evaporator coil just above freezing.

Suggestion #3. If its a cap tube system, add sufficient refrigerant to raise the suction pressure to a value that keeps the evaporator coil just above freezing.

Some commercial refrigeration units for large areas that have to cool to -20F or so, are automatically defrosted several times during the cool down process. They monitor the ice build-up using a "sail switch" located in the air stream. When the ice builds up, the air flow diminishes and the sail drops closing a micro-switch which triggers the defrost cycle. As the ice melts, the air flow increases, the sail raises, the micro-switch opens, the defrost cycle terminates and refrigeration resumes.

You should find that the heat exchanger (cooler coils) will have a temperature differance from the input as against the output.

If the temperature differance is small, then not as much cooling is transfered from the exchanger and the must be a airflow problem, there should be a steady state that has the best effect.

you want to be dropping the ice/frost from the heat exchanger before it grows to stop the fan, not once it gets there.

Another thing could be a differential pressure transducer, placed either side of the exchanger, there should then be a known pressure change from the Hot side to the cool side, once that pressure exceeds a known setpoint, then you have a restriction of airflow.

Another thing you could watch is a drop/raise in the current feed to the motor of the fan, if the current drops, the fan could be cavitating (spinning but not pushing any air through) if the current raises, it could be loading up or beginning to freeze up.

Pressure Transducers from RS-Components

http://australia.rs-online.com/web/search/searchBrowseAction.html?method=retrieveTfg&Ne=4294934505&Ntt=pressure+transducer&Nr=AND(avl%3aau%2csearchDiscon_au%3aN)&Ntk=I18NAll&Ntx=mode%2bmatchallpartial&N=4294856275+4294834841&Nty=1&binCount=376#breadCrumb

Greetings.

The simplest is a refrigerator defrost heater, a defrost thermostat, and a defrost timer. This all uses 110vac and can be wired into circuit to turn off compressor and circulating fan until defrost thermostat warms and stops the heater from working. Magic Chef defrost heater and timer are very dependable. Whirlpool and Frigidaire defrost thermostat are very reliable.

It might be simpler to add simple relay control of a defrost heater. Which is lacking in the original machine?

Good electromechanical control is always a good value for vending. The risk of catastrophic failure can be costly, and without a health control shutdown bit you run the risk of vending perished product.

Adding simple thermostatic heaters to the area around the fan housing would be the way I would go. Your "frost/ice" sensors might detect the presence or lack there of but you run the risk of spoiling/thawing your products and possibly shortening their shelf life, since the sensors tell you whats happening in the compressor but not the cabinet.

To be sure of your rework design some good testing should be done to prove out any design. Good luck!

-mowgli

I had a similar problem on an old walk in box. The KISS solution was simply to utilize a mechanical thermostat with a remote sensor bulb and set the differential to about 5°. The thermostat was wired in series with the primary control thermostat, which was set to about 43°.

It's been a while however I think I ended up sitting the cut out at 32°. This was a very old installation that needed to last about another year until a new facility was finished. I previously tried to increase airflow across the evaporater which helped but didn't cure the problem. The mechanical thermostat set to a 5° Delta T worked like a charm, I never had a freeze up after the installation, and was able to maintain a consistently sub 45° box temperature.

PS. I located the secondary thermostat sensor bulb on lip of the coil, where I typically began to ice up.

This goes toward what Mowgli stated.

The way a typical "frost-less" unit works is that the "defrost" cycle begins after a certain amount of cooling run time. During the defrost cycle, the fan is off and some type of heating is active, either by a separate electric heating strip or reversing coolant action. The result is the ice is melted, the water running off to a condensate drain and not re-entering the air.

You may be able to set up your cycle by simply running a timer while the compressor is on, and starting the defrost cycle after a certain amount of run time has elapsed, without the use of any sensor at all.

It is my understanding that some kind of heating with no air flow is basic to the defrost cycle. The heating being necessary to shorten the defrost cycle and prevent most of the water from re-entering the air.

Hello Lendog: you said,"The way a typical "frost-less" unit works is that the "defrost" cycle begins after a certain amount of cooling run time. During the defrost cycle, the fan is off and some type of heating is active, either by a separate electric heating strip or reversing coolant action" .

You're right most residential refrigerator /freezers do require a heating element to defrost. However commercial refrigerator only units generally either air defrost, that is by shutting off the compressor and allowing the fan to run, or use no defrost cycle at all. It seems the unit in question here was never intended to utilize a air defrost, or heated defrost cycle. I'm sure you could retrofit a heating element to the coil however probably all that is necessary is to shut off the compressor and allow the unit to air defrost.

PS. There is an additional benefit to air defrosting. Heat from the box is transferred into the ice or frost as it melts. The net result is a fairly stable box temperature as an essence you are still cooling while defrosting.

Well, to be honest, this is not my field. I guess what I do not understand about a 'air defrost' method is that where does the heat energy come from to melt the ice if the required or maintained temp is below freezing. There is also the amount of time it could take to defrost if the air is cold. Wouldn't this have the undesired effect of pushing the moisture back into the air only to freeze on the coils again?

I guess there are some basics here that I am just missing, or I'm trying to over-think it.

Hello Lendog:

I probably should've been more specific. I'm assuming since the unit was never equipped with a defrost mechanism it is strictly a refrigerator. Unlike household refrigerators that are combined with a freezer section most refrigerators need not operate at temperatures below 40°. So air defrosting is a viable option. Many larger refrigerators i.e. walk in boxes do not require any defrosting under ordinary circumstances. Correctly designed the coil temperature never drops below freezing. Air defrosting undoubtedly adds some humidity to the box, but not as much as you might think , most of the water from the melting ice goes down the condensate. In fact getting the humidity to low in some refrigerators is problematic i.e. wine coolers low humidity can actually cause the corks to shrink resulting in a poor seal and sometimes some very expensive wine is destroyed.

PS. As far as where the heat comes from, some from the external environment of the machine, and waste heat from the various mechanical components. As I said before during air defrost it cools just like an ice chest, so the temperature remains relatively constant, or least that's been my experience working with some old and cranky machines and retrofitting a air defrost system to them. There are other solutions, however a $25 thermostat that can be installed a hour many times is more financially practical than restoring the machine to perfect operating order.

Actually Yw roadrunner supplied the best answer. Medium temp refrigeration usually requires no defrost cycle. Off cycle provides enough temp rise to thaw a coil that may have dipped below 28 dgrs F for a short period of time and caused minor frosting on the evap. If the coil is iceing up then there are other failures. Dirty coils, resticted cap tube, low refrigerant, and other posibilities. As a refrigeration service person I try to repair equipment, put equipment back in the condition it was before the problem (iceing up) occured. If it worked properly previously it should be repairable without re engineering the controls. That is not solving a problem it is masking it.

Good Luck

Reply of Guest(#2) worth of rate.

Simply monitor the current of the fan with a miniature toroid CT and do it just start of jamming what you want.

Thanks for the answers. This is a very good discussion and I'm getting a lot of good ideas. I think, though, that the reason to defrost needs to be addressed. This unit is a very old Coke vending machine. It was converted from glass bottle to can vending before I was able to acquire it. It never had a defrost cycle; you just wait until you hear the fan clang or the cans are warm, unplug it, wait until it melts off, then plug it in again. Since the electronics are worn out and I'm replacing them with a microcontroller, I want it to automatically (figuratively, not literally) unplug itself until it's defrosted, then restart.

I'm leaning heavily to the optical sensor idea on this one. Another thought, could the ice ground out a 5v lead to trigger the cycle. Just let the ice build until it covers the bare end and trigger the MPU to do its thing?

Is the fan attached to the refrigerant pump?

If not, you can run the fan full time, and switch out the pump, so then yourretaining the air flow thru the condenser coild while the pump is off, that should help drop the ice, then run the drain to the pump housing (electrically insulated of course) and use the heat from the pump to evaporate the runoff)

Another posability is to have a spring loaded "vane" in the air flow, when the air flow drops enough so that the spring overcomes the air pressure, it flips a microswitch.

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