Avoid Unfortunate Surprises

One size does not fit all for freeze protection methods. Varying by climate, they are based on the number of anticipated hours below freezing the chiller will see per year, as well as the minimum extreme ambient temperature the machine will experience. These methods include: draining the evaporator; utilizing evaporator pump control and/or electric heaters; locating the evaporator within the heated building; or adding freeze inhibiting solutions to the chilled water.

DRAINING THE EVAPORATOR

Draining the evaporator is an effective way to prevent freezing, but it is not always as simple as it sounds. Depending on the type of evaporator in the machine and the location of the drain plug, it may be difficult to drain out all of the water. This is problematic as shell- and tube-flooded type and brazed plate-type heat exchangers can be damaged by very small amounts of water left in the evaporator.

Although shell and tube direct expansion type evaporators are more forgiving, they may also damage refrigerant piping if all of the water is not completely drained. Back flushing with glycol after draining eliminates the freeze potential for water trapped in the evaporator. 

Draining the evaporator, however, introduces a new challenge for chillers. When water is removed, there is potential for oxidation since internal components become exposed to the air. This may lead to some fouling of the tubes or plates in the evaporator, so if the evaporator is drained, start-up procedure in the spring should include cleaning any fouling of the equipment – otherwise chiller performance is degraded.

EVAPORATOR PUMP CONTROL

Evaporator pump control is one method of freeze protection that is recommended or required by most major AC chiller manufacturers. If the evaporator entering water temperature approaches freezing conditions and the chiller is not in active cooling mode, the chiller will enable the evaporator pump. The chilled water circulating through pipes in the building will add enough heat to prevent freezing in mild climates. However, the chilled fluid must be allowed to flow through the building, have either direct or over-riding control of the evaporator pump, and maintain power for this strategy to work. This method is most effective when combined with other freeze protection strategies.

ELECTRIC HEATERS

All major AC chiller manufacturers offer the option of electric heaters in the evaporator of the machine for freeze protection. Evaporator heaters are controlled by the chiller and are energized when the chiller senses that there is a potential for freezing in the evaporator. They are not functional during power outages, but electric heaters are often combined with evaporator pump control to help eliminate potential stratification and localized freezing in the evaporator. 

REMOTE MOUNTING

Locating evaporators indoors is a technique that eliminates the risk of freezing in AC chiller applications. The “remote” evaporator ships separate from the chiller and is mounted in a heated mechanical room, while the rest of the chiller is mounted outdoors. All refrigerant piping and control wiring extensions are provided in the field at the direction of the manufacturer, similar to a large split system. This solution not only addresses the freeze protection for the chiller, but also for the chilled water piping.

Remote evaporators may see minor performance losses as a result of the increased refrigerant pressure drop. In addition to ensuring refrigerant piping be performed according to manufacturer’s suggestions to ensure proper performance, ASHRAE Standard 15, “Safety Standard for Refrigeration Systems” should be followed regarding refrigerant management in the mechanical space. The remote evaporator solution is one of the two preferred methods of freeze protection that does not rely on operator involvement and is unlikely to fail in the event of power loss. 

FREEZE INHIBITING ADDITIVE

The addition of glycol to the chilled fluid in a concentration adequate for the application freeze point is another dependable method of freeze protection. Glycol is available as an HVAC chilled fluid additive in two main forms: ethylene glycol and propylene glycol. Both types of glycol must contain suitable inhibitors for thermal stability in order to prevent corrosion in the piping system and equipment.

Ethylene glycol solution is a better choice for operational efficiency. It has lower viscosity and higher thermal conductivity at operating temperatures than a similar concentration of propylene glycol. But ethylene glycol solution can be fatally toxic if ingested, whereas propylene glycol generally has a much lower toxicity.

The freeze protection quality provided by glycol increases with the concentration in the chilled water – to a point. Thermal conductivity decreases and viscosity of the chilled fluid increases with the concentration of glycol in the solution. The minimum concentration of glycol should be used based on the level of freeze protection required for the application 

FREEZE VS BURST PROTECTION

The terms freeze and burst protection are often used when selecting glycol concentrations for HVAC/R systems. Freeze protection is the concentration level required to prevent ice crystals from forming in the solution for the selected temperature range. It is chosen to protect the machine for the operational conditions of the chilled fluid in medium temperature brine applications. Burst protection is the concentration required to protect the equipment from ambient freezing when the equipment is not in operation.

Burst protection concentration levels may let some ice crystals form, but will not allow enough freezing for expansion to occur. Table 1 shows the required concentration levels for both freeze and burst protection of ethylene and propylene glycol solutions.*

Generally, for an extended margin of protection, the glycol concentration should be selected for a temperature in this table that is at least 5F (3C) lower than the expected lowest ambient temperature the equipment will see. The inhibitor levels in most commercially available glycol additives are based on at least 20 per cent concentration in solution.

Inhibitor level adjustment may be necessary when the required glycol concentrations are different. It is important to note that concentrations above 32 per cent for ethylene and 35 per cent for propylene glycol provide no additional protection benefit, but increased concentrations will continue to degrade system performance.

There are many different methods available for protecting AC chillers from freez
ing ambient conditions. It is important to choose the best method that fits each application. Whichever freeze protection solution is selected,  remember that equipment manufacturers rarely warrant failures due to ambient freezing. The solution chosen should be as fail-safe and operator independent as possible to protect the AC chiller investment. <>