About the authorUnintended Consequences
Energy conservation is a very interesting field. With the advances of technology we have items like solar panels for energy generation or water heating, battery technology for energy storage and new forms of lighting like LED lamps. A recent article on LED lamps caught my attention recently and I thought it would be a good example for comparison to ammonia refrigeration systems.
The LED lamp article was about the consequences of utilizing LED lamps in streetlights. Apparently during a heavy snow, the snow collected on the streetlights rendering the specific light partially hidden or completely covered. Unfortunately, this created some traffic issues such as accidents and an increase in traffic citations. From what I have been able to learn the LED lamps burn much cooler than an incandescent light bulb. Without the warmth of the lamp to melt the snow, the specific color of the lamp was not visible to drivers.
Ammonia refrigeration systems can be affected like this also. In an attempt to lower the energy use, what are the underlying barriers?
Let’s say you decrease the discharge pressure. That saves energy, right? Yes, but what happens if the discharge pressure is too low and you cannot defrost the evaporators? Do you install head pressure control valves to elevate the discharge pressure for hot gas defrosts? Or, do you raise the discharge pressure so the defrost cycle can work as it currently does?
You want the discharge pressure to be low to save energy, however in some systems control valves are added to provide an artificially higher pressure. The need for the higher hot gas pressure could be for defrosting the evaporators or sufficient pressure to push liquid ammonia to the expansion valves. Another consequence could be additional oil carryover due the gas velocity exceeding the separator design limits.
One of the questions observed in the LED streetlight issue concerned the problem of the snow accumulation. If the lamp is too cool to melt the snow, how do you solve the problem? One statement was to add resistance heaters! It’s logic like this that is hard to argue with. Saving energy in one form and creating a need for energy to support the fundamental reason for the change in the first place?
So, when you are working with energy saving projects on ammonia refrigeration systems who pays for the resistance heaters when the plan does not work? If the projected energy savings are based on the lower discharge pressure, who pays for the difference when the dischare pressure has to controlled at a higher pressure to make the system work properly?
Something to think about…
Tags: cold storage warehouse, discharge pressure, energy conservation, energy reduction, food processing, industrial refrigeration, operating issues, System design
A good analysis of “false” economies. However, there is a solution for the ammonia head pressure deal and still enjoy the lower BHP/TR and increased capacity with lower head pressure.. As you note, a pressure regulating valve can be located in the compressor discharge line to provide enough pressure and temperature to regulate defrost. If the designer is smart he will place the regulating valve in only one of the compressors assuming the system has more than one compressor and will energize this valve to raise the pressure only when defrost is required. Of course,it goes without saying that the branch take off for the defrost gas is placed between the compressor discharge and the regulating valve. Another economic design is to pipe the return defrost condensate and attendant gas to an intermediate temperature level such as a flash economizer assuming there is one present rather than to the colder low pressure receiver.
Thanks for the reply Jon. The points you raise are good as they direct parties to look for other options. That is one of the reasons why this was posted. It is my feeling a lot of systems are designed in a similar manner, which perpetuates the inherent limitations. By looking at those barriers and understanding the underlying fundamentals we can bridge the gap between the way it’s always done to something that works well in all conditions. Sorry for the confusion on the posting. The spam blocker holds comments for approval.