Reducing the workload of compressors by “floating” their discharge pressure down, or their suction pressure up, is a tried-and-true method used by supermarket operators to save energy and extend the lifespan of these critical refrigeration components.
Fluctuating ambient temperatures offer grocers floating opportunities on the discharge side, while changing case conditions offer them opportunities on the suction side.
And yet, while most grocers use controllers to manage these scenarios, they often fail to reap the benefits of floating their suction pressures, according to Amrit Robbins, CEO of Axiom Cloud, a California (U.S.)-based provider of commercial refrigeration optimization software.
“This is a problem that dogs the industry, and has forever,” he said.
For example, a study completed by the Portland Energy Conservation (PECI) across 50 stores found that almost 40% of the existing controls were not floating head and suction pressures at maximum efficiency, either due to programming errors or missing components.
Axiom Cloud has found that more than 60% of the stores it works with don’t employ suction floating controls correctly.
How much are these stores losing because of their faulty controls? Axiom’s Lead Sales Engineer Turner Anderson in a blog post on the company’s website writes that properly functioning floating suction pressure controls have been proven to reduce refrigeration system energy consumption by 5% to 8% on an annual basis, saving a store between US$3,000 and 9,000 a year.
The root cause of the problem, Robbins noted, is the “set-it-and-forget-it” mentality used by technicians who install float controls. “One week later, something could change – a new case, hot dogs replacing ice cream – and they don’t go back and fix the floating suction algorithm. So it doesn’t work.”
Grocers don’t normally track how these changes affect suction pressures, but “if you can do it with computers and software, suddenly it’s cost effective,” said Robbins.
This is where Axiom Cloud can play a role. The company produces three apps for refrigeration systems. The Facilities Analyzer predictively identifies refrigeration problems and gives users multi-site visibility. The Virtual Technician app autonomously solves refrigeration maintenance and energy problems, while the Virtual Battery app transforms a frozen inventory into an intelligent thermal battery. The first two can be leveraged to flag a problem and then optimize suction pressure floating, either remotely or through technician intervention at the store.
All three apps works for both hydrocarbon and CO2 (R744) systems, though “some changes to the underlying thermophysical models and data analytics are necessary for CO2 stores,” according to Robbins.
Top 10 problems
In his blog, titled “The Top 10 Reasons Floating Suction Isn’t Currently Saving You Money,” Anderson explains in detail how and why many retailers are missing the mark.
Refrigeration systems that use only fixed suction pressure set points need to ensure that sufficient cooling reaches the cooled spaces during the highest peak loads. This means that, on most days, the compressors are working harder than they have to, thus wasting energy.
By contrast, with floating suction pressure control. suction pressure set points are automatically adjusted based on actual temperatures in display cases and walk-in freezers.
Axiom’s strategy is to look at the cases in a given refrigeration circuit – typically three – and take an average of their temperatures to determine whether to float the suction pressure for that circuit. If the average is below the set point, the suction pressure can go up slightly to save energy; if it’s above the set point, the suction pressure is adjusted down.
One of these cases is identified as the “lead” case, which is the first one to warm up if the rack provides insufficient cooling. The lead case is thus one of the most vulnerable points in the suction floating process. For example, it may have a dirty evaporator that is preventing the circuit from floating the suction up, and needs to be repaired.
In addition to “the lead case is unhealthy,” Anderson cites nine other reasons why floating suction isn’t doing its job:
– No cases are specified as the lead case
– The lead case Is improperly linked.
– A lead case has been decommissioned
– Float temperature setpoint is too low
– Float temperature setpoint is too high
– Case temperature setpoint is too high
– There is no suction float temperature setpoint
– The maximum suction pressure float is zero psi
– There are temperature sensor offsets
More details on each cause can be found here.
Floating suction controls were likely commissioned correctly when they were first implemented, Anderson noted. “However, subsequent technician visits, changes in case efficiency (causing temperature drifts), changes in store configuration, or other factors somehow lead to inadequate or nonexistent suction float in each scenario,” he added. “So, because there are many ways floating suction control can be rendered ineffective, measures like set point lockouts or retro-commissionings do not produce lasting results.”
Consequently, “continuous, intelligent monitoring of real-time inputs and outputs is required to truly optimize a refrigeration system and reduce energy consumption on an ongoing basis,” he wrote.
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