By Phil Boudreau
Subcooling is simply the process of reducing the temperature of the refrigerant below its saturation temperature. Natural subcooling takes place as the cool ambient temperature or cool fluid temperature enters the condenser and removes additional heat from the already condensed liquid.
Mechanical subcooling is a concept that has been applied for so many years that it is now used quite extensively. Supermarkets are a great example of this because they require both medium and low temperature refrigeration, and the medium temperature system can provide refrigeration more efficiently than the low temperature system.
Let’s look at the effects of subcooling a low temperature or LT system using a medium temperature or MT system. Due to the lower compression ratio, MT units generally provide refrigeration more efficiently than LT units. If we compare the values in Figure 1, we can see two systems that use the same compressors operate at significantly different capacity, power and EER levels.
The MT system has a capacity that is 249 per cent greater than the LT system. Since the power increase is only 80.8 per cent. The resulting increase in EER is 93.3 per cent The main reasons for these differences in performance is the compression ratio that the compressors operate at along with the specific volume of the refrigerant entering the compressors.
Of course, the specific volume entering the LT compressors is greater than the MT compressors. The specific volume of the refrigerant entering the compressors in these two scenarios is 2.32 ft3/lb and 0.94 ft3/lb, respectively. So, in this case, the specific volume of the vapour entering the LT compressors is approximately 148 per cent greater.
Now let us look at how we can use the same MT circuit (see Figure 2 on the page 34) by adding another compressor to subcool the liquid feeding the LT expansion valves, and compare the performance of this added subcooling compressor capacity to the additional performance that we gain in the LT circuit. Afterwards, we will summarize the results.
By adding the semi-hermetic compressor, the capacity of the medium temperature system increased to 800 kBTU/H. The additional 81 kBTU/H is available to provide liquid subcooling of the liquid in the LT system before it reaches the expansion valves. To accomplish this, a brazed-plate heat exchanger (BPHE) is typically used. The BPHE can be installed on either the LT or the MT system. However, by installing it on the LT rack, the overall LT refrigerant charge should be less since the liquid line does not have to be piped over to the MT system and then back to the LT system.
With the MT system carrying the additional 81 kBTU/H, capacity of the LT system has increased by approximately the same amount. Also, the EER of the LT system increased from 4.89 to 6.69. This represents an efficiency increase of 34.8 per cent. However, the MT system now draws additional energy to provide the additional cooling in the LT circuit. Note the input power of the MT system increased from 76.1 kW to 84.5 kW. This represents an increase of 11 per cent which is a very good trade-off.
If we compare the total energy efficiency ratios of both the LT and MT groups with and without the additional subcooling load on the MT system, we can see that the EER has increased from 7.83 to 8.53 (925 kBTU/H ÷ 118.2 kW = 7.83 and 1,087 kBTU/H ÷ 127.4 kW = 8.53).
This represents a total EER increase of 8.9 per cent. One thing to point out with this approach is that the additional MT compressor that was added to handle the LT subcooling load operates at +18°F SST since it is piped in parallel with the other 4x semi-hermetic compressors. This could be made even more efficient by allowing the semi-hermetic compressor to operate at a higher saturated suction pressure.
However, this is not quite as simple as it may sound. If the semi-hermetic compressor is configured such that it will operate at the higher SST, that is 35°F or so, then it becomes a dedicated subcooling compressor. Since the subcooler load will vary with ambient temperature and the LT load itself, the dedicated subcooling compressor will have to have a wide capacity control range. This of course can be accomplished by using a variable frequency drive or a mechanical capacity control approach where the unloading allows the compressor to unload down to as low as 10 per cent.
If the semi-hermetic compressor operates at the same SST as the other four compressors on the MT system, it still follows that one of the compressors should be equipped so that it has a high-capacity range. In this case, the capacity control can be applied to both the MT load and the LT subcooling load simultaneously.
An electronic expansion valve is recommended for the BPHE. It is very important that this valve is loaded up as much as possible and not too large. Additionally, by operating the BPHE at a higher temperature difference, it is possible to achieve good stability of the subcooler circuit.
A little more on the topic of subcooling
Subcooling is higher at the bottom of liquid line drops, in the bottom of vertical pressure vessels. Some degree of subcooling is needed to offset inevitable pressure losses that take place as the liquid travels from the condenser or receiver to the expansion valve. If there is a possibility for the liquid line to absorb some heat during its travel, then this will tend to reduce the amount of subcooling also.
Many split-type refrigeration systems include an optional liquid line kit at the condensing unit. This liquid line kit typically consists of a drier, sight glass and shut-off valve. However, having a sight glass near the inlet of the expansion valve is beneficial in that it makes it possible to ensure there is no flash gas present at this point in the system. Without a sight-glass at this location of course, one could always compare the temperature of the liquid line to the saturation temperature that corresponds to the pressure.