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In the January-February 2009 issue of Cold Chain, we discussed the introduction of “Why Scroll in Commercial Refrigeration applications? We’re continuing with the feature in this issue.
Scroll with vapour injection in low temperature
The principle of injecting vapour with the use of a subcooler effectively gives two stages of compression with one compressor. This gives an efficiency gain, and also a further significant increase the capacity obtained from a given displacement. With vapour injection it only requires 60cc scroll displacement to provide the same capacity as the 300cc piston displacement in the example above. The vapour injection concept has been proven in back-to-back field trials where a five scroll pack was monitored against an eight conventional scroll pack over a period of more than one year. These back-to-back tests proved that a reduction in displacement of almost 50% is possible with vapour injection in a low temperature application with an increase in efficiency of 11%. This is one fifth of the displacement that would have been necessary with semi-hermetic piston compressors, and takes further the size and weight benefits.
The figures below show the scroll optimized design for vapour injection.
Flat Capacity Curve
As well as the significant size benefit, the scroll capacity varies much less with changing conditions. The refrigerant volume flow rate of a piston compressor reduces as the pressure difference increases due to the clearance volume effect. In Figure 1 the desired capacity is provided in all cases at the design point, 32°C ambient. At lower condensing temperatures the piston capacity rises sharply. Over-capacity at practically all normal operating conditions leads to frequent cycling, repeated thermal stresses and potentially reduced leak free equipment life.
The volumetric efficiency of the scroll does not change very much as operating conditions change. It remains consistently high. In other words a scroll compressor pumps very nearly the same volume flow rate of refrigerant from the evaporator at all conditions. There is still an over-capacity at lower condensing conditions, but much less. The capacity enhancement due to vapour injection is greater at the design point than at lower ambient and so the capacity curve is even flatter. The result is that the vapour injection scroll cycles on/off even less than the conventional scroll. It is clear from Figure 1 that with vapour injection the
scroll sized for 3.5kW load at 30°C ambient will have a capacity less than 4kW at lower ambient conditions, and so will tend to run almost continuously. The capacity of other compressors will become very high – up to 6kW in the case of a piston and this will tend to result in frequent switching on/off.
Another noticeable effect is that scroll requires less power at start up when the back pressure is high – again due to the flat capacity curve characteristic. Many engineers unfamiliar with scroll wonder why the power values shown in the tabulated data show no or little increase in power at high evaporating conditions. It’s simply because the scroll pumps less. Motor size and starting amperage tend to be lower, and there is no need for start assistance such as part wind motors.
Reliability
Probably the best known statistic relating to scrolls is the small number of moving parts, but this is only part of the story. The relative motion of
the compression components is very small because the scroll moves in an orbit governed by a small orbit radius. The movement at the tip of the spiral is identical to the movement at the centre. Surface speeds are lower than in piston compressors although the rotational speed is higher. This small relative movement allows the scrolls to come into contact with each other both at the tips and at the flanks to seal the compression pockets. However these pockets only remain sealed during normal operation. Under fault conditions, such as liquid or oil flood back, which is probably the most common fault, the excess pressure causes the scrolls to separate both axially and radially, thus quickly relieving the pressure. This is managed by the unique axial and radial Copeland Complianceâ„¢ mechanism illustrated in Figure 2. Liquid floodback is probably the most common cause of failure in reciprocating machines where the suction valves are especially vulnerable. In the small liquid filled space at the top of a cylinder extremely high pressures are generated, and these valves are the first to suffer. The refrigeration scroll has no suction valves.
The resistance to this and other conditions such as flooded start, loss of oil, loss of charge, overheat and mis-wiring are proven by rigorous life tests. The standard procedures of Emerson Climate Technologies for
Copeland® brand products ensure that each scroll design passes tests in which statistically representative sample quantities are subject to repeated fault conditions, usually beyond the boundaries of operation allowed for users.
Efficiency
Most refrigeration applications require evaporating temperatures in the region of –10°C for chilled food or –
35°C for frozen food. The most common refrigerant is R404A. With air cooled systems the design point could typically be 45°C condensing. When speaking of efficiency, any comparison should be made at actual running conditions. Anyone interested in cooling efficiency will be lowering the condensing temperature at low ambient conditions, and by definition, the ambient temperature is almost always below the design point. Studies of ambient profiles show that in northern Europe, a very high percentage of operating hours is spent at condensing temperatures in the region of 30°C. Plotted in Figure 3 are efficiency characteristics for medium temperature (MT) scroll and piston compressors. The isentropic efficiency is pressure ratio dependent. A band is shown for pistons because many types are available and the Discus type is a market leader in efficiency. Scroll has a quite rapidly diminishing efficiency at higher pressure ratios due to the built-in volume ratio. This is the reason why scrolls are dedicated to various application conditions.
At the design point (e.g. -10/45°C = pressure ratio 4.7) a piston type may have a better efficiency and hence COP than scroll but for most of the time running conditions will be at significantly lower condensing temperatures. At typical MT operating conditions of -10°C, with condensing 25 - 30°C the pressure ratio is close to 3. At these conditions the MT scroll has superior COP. The scroll set has been carefully designed and optimized to give the best running cost result for the user. Condensing conditions as low as 10°C are possible with scroll for applications that minimize energy consumption with floating condensing pressure control.
Turning now to the low temperature (LT) conditions we have a similar story. At LT conditions the pressure ratio will be typically 8 – 9. At these conditions the standard liquid injected scroll may have a lower efficiency than a Discus semi-hermetic type. Vapour injection can give 15% efficiency enhancement when compared to standard liquid injection. This places the scroll on a level with the best Discus as shown in Figure 4. Effective insulation is needed to minimize heat pick up in the liquid lines since this could slightly reduce evaporator capacity and hence overall efficiency.
A summary of COPs corresponding to these efficiencies at design points and at typical running conditions is given in Table 1. Figure 5 shows the typical running condition comparison illustrating the scroll capability of matching and even exceeding the performance of the best semi-hermetic types.
To support this conclusion field trials have been made with a back-to-back comparison on the same site over a 12 month period. Here two low temperature packs on separate circuits serving similar loads demonstrated that
10% lower power consumption can be achieved with vapour injected scrolls when compared to the standard type. This result was obtained
with an installation having upwards of 60m of pipe run between the compressors and the coolers.
The scroll tip and flank contact pressure is very carefully controlled by the compliance mechanism. A very small bedding-in process occurs during the initial running period resulting in a slight performance increase. Any further wear at the contact points during the lifetime of the compressor is taken up automatically and this ensures that that there is no deterioration in performance.
With pistons, rings inevitably wear causing blow-by losses to increase with corresponding decrease in capacity and efficiency.
A key requirement for running cost and minimization of environmental impact is efficiency, and scroll offers a sustainable best efficiency solution.
In the next issue of Cold Chain, we’ll continue
with part 3 of this topic. â—Š
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South African Institute of Refrigeration &
Air Conditioning
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