Frost detection on evaporators

Our refrigerated storage area has eight evaporators which are defrosted by closing the liquid line solenoid and allowing the fans to pass air over the fins. There is no hot gas defrost. The temperature is between 2C and 4C (35.6F and 39.2F). Most of the time, having the defrost logic running on a sequencer works fine, but not when it's hot and humid...

Three of the evaporators are in the 'loading dock" end of the area. When it's humid, some outside air enters around dock seals when trailers are loaded. This sometimes causes some (or all) of the evaporators in the area to ice up faster than normal. I've already tried reducing the amount of time between defrosts by monitoring the enthalpy of the outside air and increasing defrost frequency accordingly. This strategy keeps the evaporators free of ice, but it also causes the temperature to increase because there's more time when only two of the three evaporators are actually cooling. Also, if the enthalpy is high but the doors aren't open, there is little need to defost more frequently.

My strategy is to detect frost and trigger defrosting accordingly.

I'm hoping that someone can recommend a method to detect frost. I've been considering a pair of photoelectric send-receive sensors mounted so the beam breaks if a band of ice develops. I've also considered a diffuse-close sensor mounted parallel to air flow behind the evaporator coil to detect frost. Monitoring the current flowing to the fan motors may also work, but I think the ice will have built up too much before a change in current draw will be noticed.

Does anyone have experience with either of the above plans, or can anyone recommend a more efficient method of defrost control? Any help would be appreciated.

Two methods I have seen: laser distance sensor pointed at the surface to detect buildup OR an RTD or thermocouple mounted a few mm away from the coil surface on the inlet side. Once the inlet air temp sensor shows below 0, you know its covered in frost.

I know squat about refrigeration systems so I'm just throwing this out there as a possible simple solution:

How about a piece of reflective tape affixed to the coil, sensed by a polarized retro PE? When the ice builds up on the reflector the returned light will lose its polarized quality and the PE will sense that.

.02

Google

www.apogeeinstruments.com/content/SF-110manual.pdf

Suction line pressure transducer. Once frost begins to build suction line pressure should fall off.

Would the fin assembly gain appreciable weight as frost accumulates? Cauld that be detected and used to trigger a defrost cycle?

Does the frost buildup inhibit the air flow? Maybe you could detect it by measuring air flow through the coils?

I've only seen filters frost up on rare occasions in my applications, but it can be a major problem when it happens.

I've never needed to do this, but I think if you use a conductive level probe (http://www.omega.com/pptst/LVCN_LVCF_LVCR_LVCP.html) installed with the probe tips a fraction of an inch (or a fraction of a mm) from the coil surface they should short out when the frost forms. It should be cheap to try, anyway.

All great ideas. Thanks, guys. I'm leaning towards the laser distance probe right now. It's easier to set up than a send/receive pair and I can mount it much farther away that a diffuse-close photoelectric sensor. Coincidentally, I just used a Banner laser distance probe to position a head connected to a pneumatic actuator - worked beyond my expectations.

what type of expansion valve is used, if mechanical you are out of luck.
you can try to keep the evaporating pressure high, which gives less frost, or even no frost. If the temperature drop over the cooler is getting low, you will need to defrost, most systems just work with timers.