With summer upon fish farmers in the northern hemisphere, the thoughts of many aquaculturists in temperate climates turn to the use of chillers to keep water at the appropriate temperature.
For some, this simply means switching on the chillers and getting back to husbandry. Unfortunately many are not on great terms with chillers and view them as notoriously fussy creatures. Perhaps because some spend all winter outside in the cold, damp weather, or perhaps because chillers are not intuitive to operate…. Or, perhaps it has to do with the spike in power bills when chillers are turned on. Anyway, here are some tips for optimal chiller performance:
INSULATION AND HEAT RECOVERY
Not only can insulation and heat recovery decrease the load on your chiller, they can decrease your energy bill, and they can decrease the size, footprint, and capital cost of the chiller you need.
The rules of thumb for insulation and heat recovery are:
- You can’t over-insulate. R25 walls and R40 roof insulation is recommended in many new applications.
- Clean heat exchangers regularly. Fouling seems to occur at different rates on different systems and with different temperatures. Monitor heat exchanger performance by temperature or pressure differential and err on the side of too many cleanings.
- Use a separate heat recovery system for each separate temperature application.
- Get every drop of water back to the heat exchanger for heat recovery.
FLOW RATE AFFECTS PERFORMANCE
The simplest way to explain it is that heat transfer happens faster with turbulent flow than it does with laminar (smooth) flow. This is why chiller barrel manufacturers stipulate a minimum flow rate for their products. When the transition from turbulent flow to laminar flow occurs the capacity of the chiller barrel is reduced significantly. A 50% reduction is not uncommon.
The problem with flow that is too low is that the compressor does not have a corresponding reduction in energy that it applies to the system. So, power consumption remains the same, but capacity is reduced. This equals dramatic inefficiency. The fallout doesn’t end there either: If flow has been reduced too far, the compressor can overheat and fail as it is often cooled by the refrigerant returning from the chiller barrel.
However, not all chiller barrels are created equally. They can be manufactured to accept and function efficiently at low flow. But many aren’t. Be aware of the limit on your unit and follow the manufacturer’s guidelines.
TEMPERATURE AFFECTS PERFORMANCE
An Air conditioner is designed and rated to work at air conditioning temperatures (ie what is needed to cool room air): 16°C for example. A freezer is designed and rated to work at freezer temperatures: -16 for example. One cannot do the job of the other efficiently. A freezer at air conditioning temperatures would blow a fuse owing to high suction loading. An air conditioner at freezer temperatures would do next-to-nothing and then fail from overheating.
Your chiller must be designed to operate at the temperatures you are operating at.
A rule of thumb is that for every degree Celsius from the design temperature that you are operating at 2-5% of efficiency is lost.
A geothermal or water-cooled heat pump is more efficient than air-cooled or air-source, because the temperatures are generally closer to the process target temperature.
There are some visual indicators of chiller performance. For example, frost on the suction line to the compressor can indicate inefficiency owing to low flow and/or incorrect setting of superheat. Your refrigeration contractor should be able to set superheat properly.
An efficient chiller operate its compressor at 10-15°C warmer than the ambient air (if air-cooled). If your compressor is accessible measure the temperature of the top of the compressor. If it is significantly more than 15°C warmer than the air temperature, it is not operating efficiently.
To measure actual output of the chiller; measure temperature of water at the chiller inlet and outlet. Measure flow rate through the chiller. Then go to the online calculator at http://aquaproduction.ca/chilling-horsepower-calculator/ and plug in the numbers to get capacity and the quantity of electrical horsepower required by an efficient chiller to perform the same job.
FOr a pdf booklet with full detailed instructions on calculating a chiller’s performance go to www.AquaProduction.ca/predict.