One Degree Chiller

In many industries, particularly in food & beverage, dairy and pharmaceutical sector, the required process temperatures are below 5OC at various points along the manufacturing course. It may involve cooling a process fluid to lesser than 5OC or maintaining a storage/packaging area at temperatures below 5OC.

For such cases, the cooling media has to be circulated at even lower temperatures, i.e., lesser than ~3OC, to enable the heat exchange. Earlier, for temperatures lower than 3.5OC, glycol solutions were chosen as cooling media to avoid freezing issues associated with use of pure water. However, use of glycol solutions has its own set of drawbacks.

• Process compatibility: In many processes, mainly F&B applications, glycol is not acceptable due to toxicity issues. In such cases, indirect heat exchange circuits or ice bank tanks have to be included between the chiller and the actual process.

This not only further reduces the outlet temp required from the chiller [to allow approach in the heat exchanger circuit] but also adds to the no. of required accessories ultimately increasing initial and operating costs.

• Heat Transfer: Heat Transfer Co-efficient (HTC) observed for glycol is lower than pure water owing to its thermophysical properties. The reduced HTCs have to be compensated for by providing excess area for heat transfer. Thus, equipment’s initial cost increases.

• Corrosion: Glycol solutions are more corrosive than water. Also, glycols degrade to acids over a period of use that adds to the corrosive nature. Therefore, on glycol addition, it becomes necessary to use corrosion inhibitors and buffers.

• Pressure drop: Due to higher viscosities, required pump head and pumping costs also increase.

• Recurring cost of glycol solution replacement: As glycols degrade over a period of time and use, the solution has to be replaced after definite intervals adding to operational costs & maintenance work.

All these glitches can be avoided by using pure water as the cooling media. The question remains, how to tackle the freezing?

Freezing phenomenon

We expect a material to solidify when liquid cools to just below its freezing temperature. However, crystallisation phenomenon follows process of nucleation i.e., initially small crystals, nuclei, are formed by clustering of atoms throughout the body of the liquid. Freezing is observed once these nuclei stabilise and start growing.

The minimum size of a crystal that must be formed by atoms clustering together in the liquid before the solid particle is stable and begins to grow is called as critical radius. This critical radius is a function of undercooling (cooling achieved below freezing temperature). As the extent of undercooling increases, the critical radius also reduces.

When there is continuous flow of liquid, the formation of stable nuclei for freezing is disturbed due to the flow turbulence and the onset of freezing is possible only after a certain amount of undercooling has been attained i.e., when the critical radius is sufficiently reduced.

That is, required undercooling for onset of freezing for certain flow regimes, and velocities can be ascertained by actual testing.

Anomalous expansion of water

Another fear entertained against use of pure water below 3.5OC temperature is the phenomenon of anomalous expansion displayed by water below 4OC. However, the decrease in density of water between 4OC and 0OC is nominal. As can be seen in fig.1, density of water at 1OC is same as that at 7OC.

As such, there is no need to worry about increase in the circuit pressure as long as water is in liquid state.

Subzero evaporating temperatures

If the required chilled water outlet temperature is below 3.5OC, the evaporating temperature for the refrigerant will have to be 0OC or even below. As for the LiBr- water systems, the refrigerant is water, for these evaporating conditions the refrigerant itself will freeze. That is, even the refrigerant freezing point has to be depressed. This can be achieved by intentional contamination of the refrigerant – intentional addition of LiBr to refrigerant. However, maintaining the extent of contamination in the desired range is critical for efficient chiller performance:

• Higher concentration – poor transport properties, lower heat transfer, reduction in cooling capacity
• Lower concentration – Freezing of refrigerant

Ground work

Thermax conducted extensive testing to ascertain the acceptable working conditions for hassle free use of pure water below 3OC.

• Freezing point depression of water was determined for different flow velocities in the evaporator tubes and headers
• Increase in circuit pressure in event of freezing was monitored
• Simulation of abnormal operating scenarios and corresponding corrective control actions were determined.

Thermax already has established design and control philosophy required for operating at subzero evaporating temperatures; this was also developed through exhaustive experimentation. These features too, were integrated and monitored during the 1 Deg concept finalization testing.

1 Deg chiller – design

Avoidance of freezing in headers and tubes:

• By ensuring certain minimum velocity of chilled water in the tubes, freezing point is lowered such that there is safe margin between operating condition and freezing condition
• The required velocities at rated conditions are ensured during heat exchanger design itself
• Online measurement and monitoring of flow velocity in evaporator tubes and headers
• Inclusion of Required corrective actions in control logic in event of deration of flow velocities
• SS tubes in evaporator for better strength at lower temperature operation
• Online monitoring and accurate control of refrigerant contamination
• Provision of sufficient controls to ensure smooth and efficient operation.

One Deg Chiller – advantages

• One Degree Chiller allows use of pure water in chilled water circuit till desired chilled outlet temperatures as low as 1OC
• As pure water is being used, optimum heat transfer co-efficients are observed in the chiller and cost/ TR reduces
• As the chiller sizing is optimum, COP of the VAM for a given duty is higher by 3 to 5% in 1 Deg Chiller as compared to use of glycol solutions
• Also, as indirect heat exchange is no longer required, the process becomes simple to erect, operate & maintain
• Operational savings would be higher due to better COP, lower pumping and maintenance costs _ ensuring a more attractive payback time.

Applications

A typical schematic of chilling systems in dairy

• As seen, the compressor is catering to the chilling load of cooling from 😯 to 1OC, with maintenance of ice bank tank
• With 1 Deg chiller, the PHE and associated scheme can be done away with
• Also, the compressor load will be greatly reduced; the compressor operation will be limited to only IBT maintenance as and when required.
• That is, the power consumption will reduce substantially. As a thermally activated vapour absorption machine is catering to a larger portion of the chilling requirement, either steam from biomass boiler or more waste heat recovery will be feasible. The overall operation costs will greatly reduce.

• The above mentioned scheme has already been supplied to a dairy company. The following was the scheme revision:

Food, beverage and pharma industries

In food, beverage and pharmaceutical industries, indirect circuits are added due to toxicity issues of glycols.

• As can be seen, due to an additional indirect circuit with glycol, the required cooling media temperature from the compressor is further reduced.
• As such, the COP of the chiller reduces further and the power consumption increases. The additional pump required for the glycol circuit also adds to the power requirement of the process.
• 1 Deg chiller will make the system very simple.

• Now, as a vapour absorption machine is catering to the chilling requirement, the system will be thermally driven, i.e., steam from biomass boiler or waste heat from any source can be utilised.
• Obviously, the required power and subsequently the optn. costs of the chilling system will reduce greatly.

Similarly, the concept can be used to simplify various chilling systems which have become tedious owing to their lower temp requirements and thus have higher initial and operating costs.