Thermal Storage - Ice-On-Pipe

Ice on pipe thermal storage designs produce ice by pumping very cold liquid refrigerant (usually HCFC-22 in commercial applications or ammonia in industrial refrigeration applications) through on array of pipes immersed in a tank of water.

Technically, ice-on-pipe thermal storage is process refrigeration. System components - i.e., the compressor and condenser(s), high and low-pressure receivers, refrigerant pumps, evaporators and ice tanks - are individually selected for the application, but must perform together in a reliable refrigeration system. The designer has wide latitude in component selection and may customize the system with a variety of accessories. Of course, this means the designer must assume total responsibility for system performance and reliability!

Unlike direct expansion systems, which rely on additional heat transfer surface area to separate refrigerant vapor from liquid refrigerant, ice-on-pipe systems use a low-pressure receiver and a method called liquid overfeed to accomplish this.

A liquid overfeed system works like this. Chilled water and/or ice is produced by pumping cold liquid refrigerant to a chiller evaporator or an ice tank at a rate 1.3 to 1.6 times faster than it can be evaporated there. What results is a "two-phase" solution of refrigerant liquid and vapor that is returned to the low-pressure receiver. This 30-60% higher refrigerant flow is why the system gets the name liquid overfeed.

The refrigerant returned to the low-pressure receiver is quite saturated, so no additional evaporator heat transfer surface is lost in the task of performing superheat. Refrigerant liquid that does not "boil off" in the evaporator is sent back for a second pass.

Notice that the open or "atmospheric" design of an ice-on-pipe system dictates the use of a heat exchanger to separate ice water from the building cooling water loop. (The cooling loop is normally a closed system).

Here, on the high-pressure side of the system, the cold refrigerant vapor that collects at the top of the low-pressure receiver is drawn off by the compressor. From the compressor, the pressurized (and now hot) vapor is sent to the condenser, where cooling tower water circulating through the shell causes the refrigerant to condense. The liquid refrigerant, still under high pressure, leaves the condenser and passes to a high-pressure receiver where it is stored for later use. Refrigerant flow from the high-pressure receiver is regulated by a refrigerant metering device to assure that a minimum liquid level is maintained in the low-pressure receiver.

The ice produced by an ice-on-pipe system forms on the exterior surfaces of an "ice coil." This coil is actually a series of steel pipes immersed in a tank of water. Cold refrigerant (usually HCFC-22) is then pumped through these pipes to freeze the water that surrounds them.

The bubbles around the steel pipes agitate the water in the tank - sometimes by injecting air at the bottom - which is important. The rising air bubbles promote dense, even ice formation during the freezing cycle and uniform melting when the tank is discharged.

As suggested earlier, the low-pressure receiver plays a critical role in liquid overfeed ice-on-pipe systems: it separates the two-phase refrigerant solution returning from the ice coil (or chiller evaporator) into liquid and vapor. Gravity induces this separation, causing the liquid refrigerant and oil to settle to the bottom of the receiver while pure refrigerant vapor collects at the top. As the compressor draws this vapor from the receiver, the liquid level falls. To assure that there is always sufficient liquid in this vessel, a liquid level control adds refrigerant from the high-pressure receiver as needed.

While only hinted at here, liquid overfeed systems require a separate oil return/recovery system. This is because the preferred compressor type - helical rotary/screw - expels significant amounts of oil into the discharge line. Entrained in the refrigerant, the oil makes its way through the condenser and high pressure receiver, eventually ending up in the low-pressure receiver. There, the oil collects at the bottom of the tank (along with the liquid refrigerant) and cannot return to the compressor through the suction line. A separate oil recovery system is needed to capture, distill and return the oil to the compressor. This must be carefully addressed in the system design.

The complexity of the liquid overfeed ice-on-pipe system translates into significant fixed costs that are independent of the quantity of ice produced and stored. The refrigerant and oil inventory control systems, refrigerant pumps and other system accessories plus the field labor required to install them constitute a sizable investment.

Depending on the system's size, the tank can be either premanufactured to include both the ice coil and tank, or field-assembled by installing the ice coil in a field-erected concrete tank. While this makes the per-ton-hour cost of the tank attractive, it only partially offsets the combined cost of field labor and accessories, even when the lower compressor cost is considered. The high costs of engineering and installing liquid overfeed ice-on-pipe systems typically limit their use to larger applications.

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