Marine Products

To quickly and evenly remove heat from our proprietary true eutectic solutions, Glacier Bay has developed a radically different type of evaporator called "Spider Coil". This patented coil  (US Patent #5,507,340) uses multiple parallel runs of 1/4" OD stainless steel tubing rather than a shorter single run of large diameter  tubing as is typically used. 

The patented Spider Coil^tm evaporators, Trimline^tm Hybrid Plates and TSS True Eutectic^tm Solutions are just three of the many innovations unique to Glacier Bay refrigeration systems.

 

To better understand the benefits of the Spider Coil, lets start by stating the obvious - refrigeration is heat exchange.  It then logically follows that better heat exchange makes for a better refrigeration system.  Therefore, it is understandably the goal of every refrigeration system engineer to get the best heat exchange possible within the limitations (space, cost, etc.) of their design.  When it comes to designing the most important of heat exchangers - the evaporator, engineers have three basic options:

1.  "Roll-Bond" - This is one of the oldest and lowest cost choices.  It consist of two flat sheets of aluminum "bonded" together with small passages routed between them through with the refrigerant can flow and evaporate.  Roll-Bond evaporators have the advantage of being lightweight and taking up little space in the icebox.  Unfortunately, they also provide very little surface area for heat exchange and are, therefore, generally limited to use in small, low-load systems.  Small, low-cost and non-self defrosting home refrigerators often use this type of evaporator.

2.  "Holding Plates" - The term Holding Plate refers to the practice of enclosing a coil of tubing (evaporator) in a tank of liquid phase-change solution.  Because it is much easier to remove heat from either a liquid or a solid than it is from air,  surrounding the evaporator with this solution (typically one or a blend of  anti-freeze compounds) helps to speed the exchange of heat and improve efficiency.  In the process the liquid solutions freezes and continues to absorb heat from the surrounding air even when the compressor is not running.  This "holdover" reduces the frequency of the start/stop cycling of the compressor which further enhances energy efficiency.  The principle disadvantages of holding plates are their physical size, weight, cost and the limited internal space for evaporator coil.

3.  Aluminum-Finned Copper Coils - These are the same basic design as the coils commonly used on air conditioners.  They are typically hung from the ceiling of the box and use a fan to force air over the fins and circulate air throughout the cooled space.  Their power and ability to circulate large volumes of air  make them the evaporator of choice large, high load coolers such as commercial walk-ins.  Also, most self-defrosting home refrigerators use a variation of this type of coil.  Unfortunately, the high energy efficiency one might normally expect is largely offset by the need to frequently defrost the coil (thereby injecting heat into the box) and the energy required to drive the air circulation fan.

Ultimately, in most land-based refrigeration systems some factor other than a need for maximum energy efficiency that drives the final choice of evaporator design.  Only in marine refrigeration does energy efficiency take on such preeminent importance. 

Why a new design?

In well-designed marine iceboxes the superior insulation and relatively small size make efficient use of a finned coil evaporator impossible.  The heat required to drive the fan and defrost the coil can double or even triple the natural heat load.  This fact alone restricts the practical choice for marine evaporators to either the roll-bond or holding plate type.  Given the requirement for high energy efficiency, the historical trend for high-end equipment had been to use holding plates.

However, as mentioned above, holding plates also have their own disadvantages.  It is well known to manufacturers of holding plate refrigeration systems that the difficulty of  properly bending and conforming at sufficient tubing length into the inside of the holding plate dramatically reduces the overall efficiency of many marine holding plate system installations. In an effort to correct the problem most manufacturers have, at one time or another, experimented with the use of "finned" type coils. Unfortunately testing quickly showed that, while the addition of fins works well when the coil is surrounded by air, it doesn't do a thing when the coil is encased in a liquid. In these "liquid to liquid" heat transfer situations (as is found in holding plate applications), both the inner and outer surface areas must be increased simultaneously to improve heat transfer.

What was needed was a way to use the superior heat exchange property of phase-change solutions while dramatically increase the amount of both inner and outer surface area of the submerged evaporator tubing.

Introducing the Spider Coil^tm and Trimline^tm Hybrid Plate.

Increasing the inner and outer surface area of the evaporator cannot be accomplished by simply lengthening the tubing.  As the tube length increases so does the pressure drop.  Excessive pressure drop in the evaporator can be even more harmful the insufficient surface area.  Also, there is the not insignificant matter of how to bend up all that tubing to make it fit in a holding plate of reasonable size.

The answer is to break the single length of large diameter tubing into multiple parallel runs of small diameter tubing.  The small tubing can be bend in tighter radius turns to improve the density within the plate.  Also, flow velocity and pressure drop can be perfectly optimized by adjusting the number of tubes in parallel. 

This approach has several important benefits.  First, it provides an average of six times more heat exchange surface area than either a conventional holding plate (refrigerant to liquid)  evaporator coil or a "roll-bond" (refrigerant to air) type direct expansion evaporator . This dramatically increased surface area allows faster heat transfer resulting in improved energy efficiency and shorter system run times.  Secondly, the refrigerant can be routed so as to cross-feed from each side of the plate rather than simply flowing from one end to the other as occurs with a simple tube evaporator.

The ability to pack so much inner and outer evaporator surface area into such a small space means that the thickness of the holding plate can be reduced without sacrificing evaporator performance.  This is the basis for our Trimline^tm Hybrid Plate design concept.  The thin plates give all the performance and efficiency advantages of traditional Holding Plates but take up about the same space as a roll-bond evaporator.  With Trimline^tm, you finally get the best of both.

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