2019 Breeze Jan-Feb

SAFETY

Applying the Basics It was 1989 and the ice makers were not producing the expected amount of ice. The plant had always been a “problem” and brittle. The pressure exerted by the blades as it comes around causes the ice by Don Tragethon

Enter a new operator who was attending RETA chapter meetings. The chapter had studied RETA IR-1 together and were mid-way through RETA IR-2 – Systems. We had just studied the chapter that describes how flooded systems work. One bit of information from the flooded evaporator chapter is a statement that the condition of the refrigerant leaving the surge drum is a saturated vapor. Being new and curious the operator acquired a new battery operated thermometer that had a thermal sensor at the end of a three-foot long wire. The thermometer’s operating range was -40°F to 120°F. The sensor was slipped up under the insulation of the suction pipe leaving the surge drum. The book said the temperature should be -28°F since the vapor was to be saturated. To the operator’s surprise the thermometer read +28°F. The operator had just learned about superheat and saturation. It appeared that condition of the suction gas was 56 degrees superheated (-28° minus +28° = 56°). This is not supposed to be the case according to RETA IR-2. The operator went back to his chapter mates to inquire about this situation. They returned to the book to look for more information about liquid feed to surge drums. The liquid feed to the surge drum came through a thermostatic expansion valve. The book says vessels are not fed by thermostatic expansion valves that depend on superheat for operation. There is supposed to be no superheat at the outlet of a surge drum so a thermo-

to break away from the heat transfer surface and fall down below. The water that does not fuse into ice is captured in a gutter that returns the water to the small reservoir where it is picked up to be sent to the distribution tray again. At least, that is how it is supposed to go. “ ...just because it has ‘always been this way’ does not mean that the system is being operated correctly in the first place. ” This plant had two ice makers that were supposed to produce 84 tons of ice every 24 hours of continuous operation. This plant was producing 54 tons of ice on its best days. The reason for this poor performance was not readily visible. The heat transfer surface on these ice makers is fed liquid by gravity from a surge drum that is integral to each icemaker. Ice was forming the entire length (height) of the cylinder surfaces so it was deemed that things were “ok”. However all the other factors indicated such as evaporating pressure (zero psig) and water tempera- ture was checking “ok” as well. The plant ran this way for two years – it was “as good as it was going to get”. Manage- ment was becoming quite frustrated when the cost of producing ice was calculated. It seemed that it just was the way it was and nothing could be done about the situation.

producing ice. The ice makers were North Star flake ice machines. These machines operate at a very cold tem- perature, -28°F. A North Star ice maker is a rotary machine that is a vertical refrigerated cylinder that has a rotor arm supported by significant bearings centered at the top and bottom of the cylinder. The rotating arm turns at 2 RPM or slower. The arm assembly supports cutting blades that come within .030 inches of the heat exchange surface. These blades are set to be close to, but not come into contact with, the ice building surface. The bottom of the cylinder is open to allow harvested ice to drop down to the storage bin below. Water is pumped from a small reservoir to a water distribution tray that is mounted to the rotating cutter arm assembly. The tray distributes water down over the ice building surface as the cutter arm makes its way around the inside of the ice maker. The water tray covers 270 degrees of the circle seen when looking down from the top of the ice maker. The remaining 90 degrees of the 360 degree circle has no water delivered. The rate of water flowing down from the distribution tray is such that the water cascading down the heat transfer surface fuses into ice and subcools just before the cutter blades reaches the ice. The ice is thicker than the gap between the blades and the ice building surface. The ice is subcooled

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