ASHRAE Journal - August 2009 - 31

Defrost Approach Applications Advantages Disadvantages Increased safety risks due to hydraulic hammering from condensation-induced shock and vapor-propelled liquid slugs if defrost sequences are not properly managed and proper piping practices not implemented. Extremely high working pressures required for some refrigerants such as CO2. Can lead to increased parasitic energy consumption with improper valve group design and poorly adjusted defrost sequence times. Poor use of high grade primary energy (electricity). High maintenance due to frequent failure of resistance heating elements. Not effective at removing oil accumulation from evaporators. Not relevant in applications where space temperatures are below freezing. Not effective at removing oil accumulation from evaporators. Difficult to apply for “defrost on the fly” during operation for low temperature applications. Not effective at removing oil accumulation from evaporators. Increases plant water use. Additional secondary fluid system and circuiting, which makes the coil larger, heavier, and more costly. Not effective at removing oil accumulation from evaporators. Secondary fluid circuit in the coil can fail (freeze) if the secondary fluid concentration is not properly maintained. Able to achieve effective defrost. Hot-Gas Widely used in most industrial and some commercial refrigeration systems (direct refrigerant). Uses lower grade of energy (waste heat from the refrigeration system). Can be effective at scavenging and returning oil that may have accumulated in an evaporator. Electric Used in some commercial refrigeration systems and in industrial refrigeration systems where CO2 is used as a cascade refrigerant or secondary loop phase change fluid. Used in industrial and commercial refrigeration systems for spaces operating above freezing point (typically >38°F [3.3°C]). Found in some lower-temperature refrigeration systems. This form of defrost can also be integrated into the normal sanitation operations. Decreased risk of damage from events such as hydraulic hammer. Minimizes parasitic load. Avoid extreme refrigerant-side pressure (CO2 refrigerants). Efficient means of defrost. Simple implementation. Inherently safe. Lower capital and maintenance costs. Applies heat directly to the accumulated frost. The defrost process can be integrated into a normal sanitation cycle. Able to achieve fast defrost. Off-Cycle Water Secondary Fluid (Indirect) An alternative to electric defrost in CO2 cascade and secondary phase change systems. Efficient means of defrost. Conceptually simple. Avoids risks of hydraulic hammering on refrigerant-side of coil. Table 1: Advantages and disadvantages of various defrost alternatives. a warm secondary fluid is circulated through the defrost coil to raise the evaporator’s surface temperature and melt accumulated frost. Water can also be used for defrosting evaporators. With water defrost, the refrigerant feed to the coil is interrupted and water is sprayed directly on the external surfaces of the coil to melt the frost. A hot-gas defrost process redirects a portion of the high pressure discharge gas from the outlet of high stage compressors to the evaporator and a heating circuit embedded in its defrost condensate drain pan. As the high pressure gas flows to the unit, it desuperheats and condenses giving up both sensible and latent heat of condensation as it warms the surfaces of the evaporator and the drain pan. The warm evaporator coil causes the accumulated frost to melt and the warm drain pan permits the water to drain out of the unit without refreezing. The liquid refrigerant condensed during the defrost process is returned to a protected lower suction pressure through a re-seating pressure relief regulator. This pressure of the regulator is set in the range of 70 to 90 psig (4.8 to 6.2 bar), which corresponds to a August 2009 refrigerant saturation (condensing) temperature of 47°F to 58°F (8°C to 14°C). For industrial refrigeration systems, hot-gas is the most widely used technique for defrost. Although there are other defrost techniques such as the use of a warm liquid refrigerant, these do not find widespread use in industrial systems so their coverage is not included here. Advantages and disadvantages of the above-mentioned industrial refrigeration system defrost strategies are highlighted in Table 1. Because of its widespread use in industrial refrigeration systems, our focus in this article is on the use of hot-gas for coil defrosting. Let’s first look at the steps involved in a typical defrost sequence. Then, we explore energy considerations associated with the entire cooling and defrost processes. Defrost Sequence of Control Due to its simplicity, a time clock is the most common method used to initiate and terminate the defrosting of individual units. With a time clock, a defrost sequence is initiated a prescribed fixed intervals in time. In attempts to improve the efficiency of ASHRAE Journal 31

ASHRAE Journal - August 2009

Table of Contents for the Digital Edition of ASHRAE Journal - August 2009