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It Pays to Identify the Best Defrost Technique
As far as air cooled evaporators are concerned, defrost is the “inevitable evil.” Frost forms and needs to be melted, or it will severely impact the performance of an evaporator and eventually block air flow.
Unfortunately, hot gas defrost techniques for melting frost are not problem free. Additional compressor energy is required to melt the frost/ice layers formed around the evaporator’s fins and tubes. At least a part of this energy is transferred back to the refrigerated space or heats up the evaporator -- and eventually needs to be removed during the cooling process.
Another drawback: the time used for defrost is not used for cooling. This could be an important factor in food processing plants, where defrost can significantly limit productivity levels. Another important but less obvious consequence: system integrity may be harmed when key components are subject to mechanical stress.
The main source of mechanical stress is the combination of high pressure coming from the condenser side, high discharge temperature, and high pressure differential. When combined, these factors can be quite dangerous and even destructive. Nevertheless, hot gas defrost is being used by more companies, often in CO2 low temperature plants. CO2 control is more complex than with ammonia, due to higher pressure levels and pressure differentials. The complications of hot gas defrost with CO2 may cause customers to avoid this method and look for alternatives, such as electrical or brine defrost.
Yet, hot gas defrost is one of the most efficient ways to melt the frost formed on an evaporator (see: Pearson A., Defrost Options For Carbon Dioxide Systems, 28th Annual IIAR Meeting, March 2006). In fact to reduce energy consumption, performing defrost quickly and efficiently is key to achieving overall energy consumption goals of the refrigeration system. In most cases, it is more cost effective than brine defrost. Several valve and control configurations are examined to find the best technique for optimizing the hot gas defrost process and cutting energy consumption.
Defrost Efficiency Considerations
Optimum hot gas defrost techniques have been the subject of several studies. Critical factors include:
Now let’s look at each factor in terms of valves and controls to determine the optimum hot gas defrost process technique.
- Hot gas defrost pressure. A popular misconception is that higher defrost temperatures are better. In reality, a number of studies indicate that a source of lower pressure and temperature gas also could obtain good results. (See: Stoeker W.F., Energy Considerations in Hot-Gas Defrosting of Industrial Refrigeration Coils, ASHRAE 1983.) The key is to find an optimal pressure/temperature that achieves the highest efficiency. (See: Hoffenbecker N., Hot Gas Defrost Model Development and Validation, International Journal of Refrigeration, January 2005.)
- Hot gas defrost time. In the industrial refrigeration, the defrost period is typically based on a fixed time adjusted during installation startup. The problem with this approach is that time is set on the “safe side” to ensure having a fully clean evaporator. But when the defrost finishes too early, defrost efficiency drops significantly.
- Inefficiency also occurs when the vapor passes through the defrost pressure regulator. The vapor needs to be recompressed, which increases the requirement for the hot gas feed to the evaporator. The amount of vapor passing depends on the type of defrost control in the condensate line. Pressure controlled or liquid level controlled regulators can be used to solve this problem.
- During defrost, less than half the energy is used to melt ice (Stoeker, Hoffenbecker). The rest of the energy goes for heating the space, evaporator, tubing, and the drip pan. Ice is first melted on the coil, then the ice falls into a drain pan where it finally melts completely. What is important here is that the process is sequential; with initially higher demand for defrost in the coil, and only later in the drain pan.
- When the hot gas defrost is started, the initial refrigerant inrush might create a liquid hammer, especially if the evaporator still has some liquid refrigerant that has not been drained. This also occurs if the hot gas supply lines contain pockets of condensed liquid being propelled by the supplied hot gas pressure, causing gas pockets to implode.
Hot Gas Defrost Control Groups
Figure 1 presents a typical industrial refrigeration evaporator using hot gas defrost. Control valves for the evaporator are divided into four groups:
First, the defrost process begins when the liquid supply to the evaporator is shut off. Evaporator fans should still run for some time, while the suction valve remains open to ensure the remaining liquid refrigerant will boil out. Second, the suction valve will be closed, and evaporator fans will be stopped. The hot gas solenoid valve will be opened and the feed of the evaporator with the hot gas starts. Third, when the defrost is finished, the hot gas solenoid valve will be closed, the suction valve will be opened. Finally, the liquid feed is opened again, water droplets on the evaporator fins are allowed to freeze, and only then the evaporator fans will be started again.
- Pumped liquid feed to the evaporator. This valve train typically includes stop valves, filter, a solenoid valve, a hand expansion valve, a check valve, and a final stop valve.
- Hot gas feed line. Traditionally it has a stop valve, a filter, another solenoid valve, and a stop valve.
- Condensate line. Here we either see a pressure controlled valve or a float principle to drain the liquid. Each uses a significantly different defrost principle, as explained later.
- Wet return line. This line needs to have an automatic shut off valve and a stop valve.
Critical considerations in the hot gas defrost process include: avoiding pressure/temperature stresses, minimizing system inefficiency by managing a slow pressure built up in the cooler at the start of defrost, and slowing pressure release from the cooler after the process. Proper selection of the hot gas solenoid and main suction valve is critical when aiming for a safe and efficient defrost process.
Figure 1: Typical configuration for an industrial refrigeration evaporator with hot gas defrost.
Taking into account the efficiency considerations indicated above, let’s review traditional valves configurations. Keep in mind that CO2 defrost is more difficult, and a conservative approach for CO2 evaporators with hot gas defrost should be preferred.
A. Liquid feed line
A liquid feed line has minimal influence on the hot gas defrost process. What is more important is the amount of liquid that is fed to the evaporator. If a PWM (pulse width modulation) strategy is used, the amount of liquid refrigerant in the evaporator will be lower. That should reduce the time needed to get rid of the liquid refrigerant during the pump out cycle prior to the defrost. The amount of ice will be lower as well, because the temperature deviation on the surface in on/off periods is lower (Figure 2).
Figure 2: Pulse width modulation liquid feed
This liquid feed strategy has been successfully used in a number of CO2 systems with pumped recirculation. In ammonia systems, this control method is not widely applied.
B. Hot gas line
The most common way to feed hot gas in an evaporator uses a conventional solenoid valve. Motorized valves and motorized ball valves have also been used for this purpose, especially for CO2 systems. With higher pressures and higher pressure differentials, the risk of liquid hammer in CO2 hot gas defrost systems is higher than with ammonia. Clearly, the downside of motorized valves is more complex to set up, and valve trains with motorized valves are more expensive than traditional ones.
An especially critical factor for ball valves is opening speed, which must be adjusted to a relatively low level. A cost-effective and efficient solution with two-solenoid valves is to size the first for the required hot gas defrost capacity. Then install the second in parallel to the first one with 10-20% of the flow (Figure 3). The smaller solenoid opens first and feeds the evaporator with hot gas for the first few minutes. After that, the second solenoid opens and the main defrost starts. This valve train configuration has proven successful in a number of installations.
Figure 3: Hot gas feed line with double solenoid valve
The benefit of motorized valves in hot gas defrost lines is that they enable intelligent hot gas control. That may include not only slow opening, but slow (or adjusted) closing as well. This is relevant when the defrost is not based on timing, but rather on other parameters, such as surface temperature control.
A final technique for limiting hot gas pressure/defrost temperature and maximizing the defrost efficiency is to install a downstream regulator. It is only necessary to install one such regulator for a group of evaporators connected to the same hot gas line. The sizing of the valve should provide enough hot gas for all evaporators that might be defrosted at the same time.
C. Condensate line
With hot gas defrost of evaporator condensate lines there is a wider variety of regulation devices. Differential pressure regulators are quite common, but upstream pressure regulators as well as float valves are also applied. As previously discussed, float valves are typically the most efficient controls for the hot gas defrost. A combination of float valve in condensate lines with downstream pressure regulators in hot gas lines would be preferable to ensure optimum defrost pressure.
But there is a downside with using float valves. First, the cost could be relatively high. The cost might be partly reduced by installing a float regulator in a common evaporator condense line for several evaporators.
Secondly, for high pressure refrigerants such as CO2, float regulators are difficult to find. In this case alternatives must be explored. One option is adopting steam traps from other industries to manage high pressure. Even though steam traps are gaining popularity, those devices are not yet common in the refrigeration industry. All considerations pertaining to float valves used in condensate lines are applicable to steam traps.
Figure 4: Float valves in condensing lines with multiple evaporators. Only valves in the condensing lines are indicated.
D. Wet return line
Control valves used in wet return lines include solenoid valves and gas powered solenoid valves (both of which need a bypass valves to avoid liquid hammering after defrost), two-step gas powered solenoid valves as well as motorized valves and motorized ball valves. Preferred options are either two-step gas powered solenoid valves or motorized valves. On one hand, they help avoid liquid hammer, either because of the two-step function or because of the slow opening speed. On the other hand, they give minimal pressure drop during the cooling cycle, which is especially critical at low temperatures.
An advantage of two-step gas powered valves is that they require no additional settings. The second stage opens automatically when the pressure difference over the valve drops below a certain value. Motorized valves require speed adjustment, but need no additional hot gas line for power. Motorized valves are especially popular for CO2 systems, as they are easier to obtain for higher pressures. Ball valves with a bypass solenoid valves are frequently used as well. They provide the benefit of low pressure drop during the cooling cycle, but also the possibility of leakage over the stem.
Calculating the Differences in Defrost Efficiency
The efficiency improvements of optimized defrost control compared to conventional techniques in industrial refrigeration cold store have been calculated. The main parameters of a cold store analysis are summarized in Table 1.
Table 1: Main cold store data
In Table 2, the defrost calculation for the system in question was done in two steps. First, the necessary energy for melting the ice and removing the water has been calculated. (For similar calculations, see Pearson.)
The blockage of the air passage between the fins is assumed to be 20%, with fin spacing of 10 mm resulting in one mm ice thickness. Ice thickness is one of the parameters that has a big influence on the defrost efficiency. Higher ice thickness improves the efficiency of defrost, but negatively affects the efficiency during the cooling process. (Determining optimal ice thickness is a subject for separate discussion.)
Table 2: Ice melting energy calculation
The calculation was done for a standard industrial refrigeration evaporator from one of the major manufacturers. Energy losses during defrost are not included in to this calculation. According to Hoffenbecker, the losses during defrost are on the level of 55% or more for lower defrost efficiencies. The losses depend on the defrost time and temperature, as well as on the frost thickness.
The second step assumes that for an optimized defrost process the losses are on the level indicated by Hoffenbecker. That being the case, the defrost analysis would be a comparison with a non-optimized evaporator.
The main results of this comparison are presented in Table 3. (The piping diagram for a standard system was presented in Figure 1 and for an optimized in Figure 5.)
Figure 5: Optimized defrost system configuration
Table 3: Defrost efficiency comparison
The calculations are based on a number of assumptions, and are done for a specific system only. However, they indicate that non-optimal hot gas defrost could not only create equipment problems, but also increase power consumption. Table 4 presents the data summary, as well as an evaluation of the financial impact of unoptimized defrost.
Table 4: Extra costs of unoptimized defrost
It is clear that as more defrosts are needed, choosing the right valve configuration around the evaporator becomes more important. Compared to cold stores where weekly defrost minimally impacts energy, freezing equipment with several defrosts daily increases energy consumption by 5% or more. And that is not considering the worst systems where higher condensing temperatures keep defrost temperatures high.
The calculation above is mainly valid for an ammonia system. In the case of CO2, an additional defrost compressor should be used, which will obviously change the energy balance. As with ammonia systems, a general increase of the condensing pressure on main compressors is not recommended.
Conclusions About the Optimal Technique
This overview examined different control possibilities for hot gas defrost systems. Different options available today were compared and preferable options were identified. The results show that if defrost is performed with the optimum technique, potential system energy savings could reach 5% or more. The comparison was made with common systems used today. In worst case examples (which are not rare), the overall potential savings look even greater. Further testing is expected to confirm these findings in industrial refrigeration applications.