When Mark Palmer, director of engineering for the 1,290-room New Orleans Marriott, was shopping for 54 guestroom floor ice machines last year, he had several criteria on his shopping list.

Machines from Ice-O-Matic feature the harvest assist process.

"Most of those ice machines are more or less designed the same, so we were deciding based on things like [relative] quiet and cube speed recovery, which would be how fast the machine can remake cubes after a huge usage," Palmer said.

With gravity-fed ice technology, ice forms and is routed into a bin or from a dispenser directly into a bucket. Unlike older ice machines, where a guest could scoop ice directly from a storage area at the bottom of the unit, little or no opportunity exists for the guest to reach his or her hand into the area.

Cube speed recovery is controlled and aided by gravity, which is the central component of cube-making and cube delivery for most of today's ice machines.

As a general rule, ice makers work by forming ice on plates inside the unit. Refrigerant is sent through a network of tubes, where it runs through a constant cycle of condensation and expansion. When the refrigerant passes through narrow tubes, condensation forms, while expansion occurs when the mixture travels through larger tubes.

At key points during passage through larger tubes, refrigerant pressure is increased, resulting in a higher temperature for the refrigerant. As the refrigerant passes through coils, outside air is let in, condensing the refrigerant. By means of an evaporation process, the liquid refrigerant is evaporated and turns into a gas. The tubing that runs between the two plates in the ice machine is cooled, and water is run over the cooled evaporator surface, which freezes.

Rimrodt Manitowoc

Next, the ice is harvested. A valve opens that changes the path of the refrigerant. Hot refrigerant gasses are sent directly to the evaporator, which loosens the ice cubes. Because most evaporators are vertically mounted, gravity pushes the cubes into the ice bin.

In some units, such as those made by Ice-O-Matic, gravity gets a mechanical boost. The company calls the process harvest assist.

"We have a little probe that pushes the ice slab off the vertical freezing surface," said Ed Jennings, marketing director for Ice-O-Matic.

He said the harvest assist is effective when the normal 17-minute cycle for new supplies of ice to slide off the evaporator plate lengthens.

"In such cases, the ice-making cycle now takes 21 or 22 minutes, because more of the ice needs to melt," he said.

All modern ice machines continue the ice-making process until the bin control settings sense that enough ice has been produced.

Despite the almost universal use of gravity in the ice-preparation process, there are some subtle differences in how various branded machines work, according to Mike Grossman, director of replenishment sales for Avendra LLC, a procurement company.

In some brands, the whole block defrosts from the plate and by means of gravity, falls into the bin. In other units, individual cubes are produced, pop out and fall into the bin.

"For those machines where the whole block defrosts from a plate, you might see big square chunks of ice if [the unit] is not set properly and does not break the bridge of the ice," Grossman said.

Because most hotel ice machines are completely or mostly gravity-fed, with subtle delivery differences, vendors stress the importance of bin and drain pan size, cube shape and ice cube drop rate.

Drain pan size, cube shape and drop rate are issues hotels should take into consideration when buying ice machines.

"For our hotel floor dispensers, the drain pan needs to be large enough that if ice spills, [and does not wind up in the bucket] it falls into the drain pan," said Mike Rimrodt, product manager for Manitowoc Ice.

Dispensing speed is another important criterion.

"Most hotel ice buckets are significantly larger than the biggest soft drink cups, so if you dispense your ice slow enough to fill an 8-ounce or 10-ounce cup, you are not dispensing fast enough," said Garth Pearson, product manager for Scotsman Ice Systems.

He said that the key to efficient delivery is a combination of dispensing rate, width of the dispensing chute and cube size.

Chute width and dispensing speed usually are preset according to various model specifications, but cube size is configurable when the unit is installed.