The two most popular words in the HVAC&R industry today are energy savings.
These two words attract attention because reducing energy usually equates to a drop in business operating costs and that’s a goal everyone wants to achieve. This is especially true if applied to refrigeration systems where there has been a raft of new technologies specifically created to increase energy efficiency. This month CCN examines these technologies and the very real potential of evaporator defrost technology.
There are plenty of technologies in use today that have been proven and accepted by the mainstream HVAC&R industry. These include compressors optimised for refrigerants, inverter-driven compressors, EC fans on condensers/evaporators, electronic expansion valves and intelligent control systems.
Interestingly, evaporator defrosting technology has bucked this trend with the majority of installations using low efficiency electric defrosting.
Bitzer Australia managing director, Ruediger Rudischhauser, says this is remarkable because the savings potential of electric defrosting power consumption far outweighs other technologies.
“Depending on what defrosting technology is used, between five per cent and 65 per cent of daily power consumption required within an evaporator can be from defrosting,” he says.(1)
So what is evaporator defrosting and why is it important?
Within the refrigeration cycle the evaporator sits within the cold room. It absorbs heat from the room via its cold surface generated through the refrigeration gas cycle.
Air from the cool room or freezer room continuously passes over the cold evaporator surface, as the air cooled moisture is removed. Within freezer rooms this results in frost/ice formation over the evaporator surface. Routinely through the day, this frost must be removed to maintain system operation and system efficiency.
The most common forms of evaporator defrosting available today include air, electric, water, hot gas and warm glycol. Each of these functions differently.
Comparing technologies
To facilitate a reasonable comparison of each system, here are the pros and cons. The power consumption comparisons are based on an industry available Buffalo Trident *BBL323 evaporator. Such an evaporator provides 34.2kW of cooling on R404A at 6kTD in a -18°C freezer room.(2)
Air defrost – Air defrost can be performed two ways: room air defrosting and also the less common penthouse design.
Standard air defrosting is achieved when the fans are left running and the refrigeration gas cycle is stopped i.e., the cooling to the evaporator is stopped.
However, this is only possible in cool rooms where the air temperature is above 0°C. The above 0°C passes over the frosted evaporator, the warmer air melting the frost until all frost is removed. This is shown in figure 2.
Penthouse design air defrosting is achieved in a similar way, but with penthouse evaporators mounted outside the cool room or freezer room. During defrost air guides are moved so that the air passing over the evaporator is from outside ambient air, not cool air from the room.
It is only suitable for environments where ambient temperature are above 0°C all year. This can be seen in figure 3.
One of the benefits of standard air defrost is that it is simple to implement and cost effective as defrost is achieved only using fan power.
Penthouse air defrosting has the same benefits and it can be used to defrost freezer rooms. The downside is that air defrost can only be used on above 0°C rooms.
Also, penthouse air defrosting requires special room construction and is more costly compared to other methods. It also requires motor-actuated air guides. If they fail, it causes warm air to be pumped rapidly into a freezer room, resulting in stock loss. This is why penthouse defrosting is often deemed difficult to implement.
The power required during defrost is 3.12kW.(3)
Electric defrost – Within the evaporator, electric resistance heater elements are installed. These get very hot during the defrosting cycle and can reach temperatures over 200°C. They radiate heat to the locations they are fitted and over time conducts/spreads to melt the frost from the whole evaporator. See figure 4.
This is a proven technology that is very effective. It is easy to install, and simple to implement and maintain.
On the downside, there is high power consumption. If heaters fail, sections of the evaporator become covered in frost, triggering alarms and requiring replacement.
The high temperatures cause evaporation of water and moisture can build on freezer room ceilings, resulting in icy floors and wet product.
The power required during defrost is 19kW.
Water defrost – To do this, fresh water either at room or a warmed temperature is poured over the evaporator inner fins.
This melts frost and then the water drains out of the drip tray. See figure 5
It is simple to implement and only requires a water source, control device and water pump. The water also cleans the evaporator surface.
It does require significant water usage, and there is the risk of water leaks and pouring water into the room.
Often water can splash from the unit (avoidable with good design) and there are higher installation costs compared to electric and air defrosting.
The power required during defrost is 1kW. (4)
Hot gas defrost – This can be performed in two ways. Firstly, hot discharge gas from the compressor can be diverted from the condenser and passed through the evaporator.
In this way a portion of the refrigeration gas is used for defrosting and not cooling. This requires that the system has multiple evaporators as the system must provide heating and cooling at the same time.
Alternatively, a system can be designed to operate reverse cycle so the evaporator becomes the condenser for a small period of time. See figure 6.
This is a long-standing, proven technology and it is more efficient than electric defrost.
However, it requires intelligent logic and quality installation otherwise it can cause damage to compressors. It still requires a compressor to operate which uses power.
Finally, implementation is typically more difficult than all other methods, although implementation costs are moderate.
The power required during defrost is 11kW to 22kW. If the defrost heat is generated by reverse cycle operation, it equates to 22kW .(5)
If the heat is pulled as a side load the exact power consumption is often debated, but it’s clear to say a portion of the compressor’s power is required to generate this heat and 11kW (50 per cent) is a reasonable assumption.
Warm glycol defrost – During normal operation, a tank of glycol is warmed to 50°C using the system's discharge gas and standard heat recovery.
The evaporator is specially circuited with separate glycol tubes and drain pan glycol tubes.
During defrost a separate circulation pump is activated and the warm glycol from the tank is pumped through the evaporator.
In parallel systems heat can continuously be added to the tank and a relatively small tank can be used.
In standalone systems the tank is sized so the thermal mass is sufficient to perform one defrost. See figure 7.
The glycol is warmed from 100 per cent waste heat.
On the upside, its installation and implementation, while new to the industry, is relatively simple.
It only requires a small pump and low power consumption, defrost occurs at 50°C and doesn’t cause steam around the evaporator, and there is reduced risk of frost formation within the room.
On the downside, it is a new technology and installation costs are higher than other types of defrosting.
The power required during defrost is 2.2kW.
(Note: Requires an additional 0.15kW heat recovery pump, based on a close position of the compressor system and glycol tank).
Based on all of this information, Bitzer’s Rudischhauser says it is clear that there isn’t one form of defrosting to suit all installations.
“It really does depend on investment costs, ease of installation, operating costs and other factors,” he says. “Clearly water and warm glycol defrosting offer the lowest power consumptions.
“Between these two technologies, warm glycol defrosting has some clear advantages including reduced risk within the system and the best quality outcome in terms of room moisture.”
As power consumption continues to come under scrutiny, Rudischhauser says it will be interesting to see whether defrosting strategies continue to be overlooked with commercial refrigeration installations in lieu of simple electric defrosting.
“It is more likely these technologies will have a resurgence and warm glycol defrost will find its place prominently among them,” he says.
References:
(1) Range of power consumptions considered across the Buffalo Trident evaporator equipment. Evaporator power consumption is considered to be fan plus defrosting power but not including the compressor system.
(2) Data based on Buffalo Trident BBM product brochure.
(3) A small amount of actuator power is required to move guide vanes. This has been ignored.
(4) Pump size/power based on a close coupled system and evaporator mounted at 6m height.
(5) Based on selection from Bitzer software v5.3.2 Reciprocating compressors, R404A, 6G-30.2Y at -24sst, 45sct.
(6) Based on two hours defrosting per day, 730 hours per year. For hot gas defrost, comparison based on the lower estimated power consumption