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Blowing Smoke.
John explains different refrigeration cold control designs.
 


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Blowing Smoke . . .

        Jose was standing over the griddle frying up a batch of breakfast hash browns when John walked into the kitchen. John studied the topmost maintenance request his clipboard.

            "You have some problems with a couple of refrigerators here, Jose?" John asked.

            Jose flipped the potatoes and nodded affirmative. "The sandwich reach-in is running warm and the diary box is freezing everything."

            Twenty minutes later, John returned. " You should be OK now, Jose. Both coolers had dirty condensing coils. I'll check back later but that should do it."

           Jose frowned, "I think you are blowing smoke, John. One is too warm and one is too cold. But you say a clogged condenser coil causes both conditions?"

            John smiled, "Can I give you a quick lesson?"

            Jose pulled the potatoes to the cool side of the griddle and walked off the cooking line toward John. "Sure, John. I am curious now."

            John pointed to the Norlake double-door reach-in cooler. "That box operates by means of a 'constant-differential cold-control' which is sensing the temperature of the air inside the refrigerator. When you adjust the dial, the compressor start temperature AND the compressor stop temperature BOTH change but always with the same difference between them. That particular refrigerator has a 7 degree differential cold control with a cut-in range from 35 to 44 degrees. That means when the dial is set to its warmest setting, the compressor will start when the inside air temperature is 44 degrees and stop when the inside air temperature is 44 minus 7 or 37 degrees. Over time, the average air temperature would be about 41.5 degrees.

            Jose interrupted, "That's a little warm. Food should be kept below 40."

            John agreed, "We only need to turn the dial down a little bit. OK? If we went too far and set the dial to its lowest setting, the compressor would start with the inside air temperature at about 35 degrees and pull down to 28 degrees. Through the course of the day, the average air temperature would be about 31.5 degrees."

            Jose broke in again, "31 degrees? Now, that's too cold!"

            John continued, "Yes and that brings up an important point. Frost melts at temperatures above 32 degrees. So the evaporator coil in this refrigerator would never defrost with the dial set at its lowest point. The refrigerator would need some sort of defroster to keep operating. Otherwise, the coils would soon plug up with frost and no air would flow."

            Jose declared, "That's like my refrigerator at home."

            John took a breath, "Well, sort of . . . Your refrigerator at home has a freezer section that operates off of a constant-differential cold control maintaining a temperature of around 15 degrees below zero. And, yes. It has a defrost timer that turns on heating elements in the freezer coil a few times per day . Those heaters melt the frost to keep the coils clean."

            "I see", Jose nodded.

            John patted the side of the Norlake cooler with the flat of his hand. "So, in this refrigerator controlled by a 7-degree constant-differential cold-control, the compressor will start and stop to maintain that constant 7 degrees." John grinned. "That's why they call it a constant-differential cold-control."

            "I got it." Jose interjected, "That is simple enough."

            "Now", John resumed the lecture, "Let's consider what happens when the condenser coil gets blocked. Coils commonly get plugged by dust or airborne kitchen grease but frequently a dropped piece of paper or a napkin can get sucked into the coil., When that happens, it becomes more difficult for the refrigerator to remove heat.

            How the refrigerator reacts to condenser blockage depends a lot on how restricted the condenser airflow really is. If the condenser is only partially blocked, then the refrigerator might still be able to maintain temperature but the compressor will need to run a lot longer to do it. A heavier blockage could result in a refrigerator that is only somewhat cool. Finally, if the condenser is totally blocked or if the condenser fan is not working, then the condensing unit will overheat and stop working all together. In that case, the refrigerator will warm up to room temperature. There would be no cooling at all."

            "Now, lets take a look at the milk cooler", John said, pointing to the stainless steel Traulsen Upright Dairy Cooler.

           "That unit uses a 'constant-cut-in cold-control'. The cold control is sensing the temperature of the evaporator coil; NOT the air inside the box. It is a very simple and ingenious design. The cold control always always always starts the compressor when the coil temperature reaches 39 degrees. The dial adjusts the temperature at which the compressor shuts off. Remember, the cold control is sensing the coil temperature, so the shut-off temperature is usually quite low; like maybe 5 degrees or colder. Adjusting the dial actually changes the differential by changing the cut-out set point. But, the temperature at which the compressor is turned on is always always always 39 degrees. That is why it is called a 'constant-cut-in cold-control'."

            "So what?", Jose asked.

            "Well", John replied, "A refrigerator that uses a 'constant-cut-in cold-control' has a guaranteed free defrost cycle between every cool cycle. The evaporator coil must reach 39 degrees before the compressor will be called to come on again. That means that all the ice must be melted off the coil. If there is any ice on the coil, the temperature of the coil cannot rise above 32."

            Jose smiled, "Ohhh, That IS clever..."

            "Yes. In addition, the condensing unit works hard for a long duty-cycle but it also has quite a long off-cycle as well. That is good for the equipment; no short cycles. In all, the simplicity of the design makes for a very durable, trouble-free refrigerator."

            Jose thought for a moment, "Well, That Traulsen is 23 years old and I don't see any reason to replace it."

            John continued, "How the Traulsen refrigerator reacts to condenser blockage is almost the same as the Norlake except for one difference. Again remember, the cold control is sensing the coil temperature, and the shut-off coil temperature can be around 5 degrees or lower. But, the temperature at which the compressor is turned on is 39 degrees. That means this cold control operates with a quite a large differential; As much as 35 to 40 degrees. If the condenser is partially blocked, then the refrigerator might not be able to pull the evaporator coil down to the cut-out temperature. In that instance, the condensing unit will run and run trying to pull down 35 or 40 degrees to the cut-off temperature - which it cannot achieve. . . "

            Jose nodded, ". . . And the food freezes! So, a plugged condenser coil really COULD make a box run too cold as well as too warm."

            A breakfast order was on the wheel as Jose returned to the line. He threw a cut of ham on the griddle, cracked two eggs and pulled an order of hash browns back over the heat. Then he looked at John and smiled.

            "John", He said, "Either what you say is true or you are the best darn smoke-blower ever!"

Doc

2004.07.24







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