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Floating Suction and Head Pressure
This article was inspired by a podcast episode featuring Jeremy Smith. Jeremy is a refrigeration mechanic and has contributed a lot of good material to HVAC School. You can listen to that podcast on floating suction and head HERE.
Those of you who drive your own vans are probably all too familiar with the following annoyance:
You hop on the highway to get to your next job, but you get stuck in stop-and-go traffic. Your speedometer will creep up to 15 mph (maybe 20 if you’re lucky) and then slam right back down to 0 as you reach yet another slowdown. All the while, your miles per gallon rate steadily falls; it was 23 MPG in the morning, and it ends up being below 20 MPG when it’s time to go home. If only traffic stayed consistent and allowed you to travel at a comfortable 55 mph for several miles and coast down the hills…
Supermarkets have to deal with something similar, but the issue is with their equipment’s ability to adjust its suction and head pressure to various temperature conditions. Instead of worrying about around $50 worth of gas money, they have to worry about massive energy bills. Depending on the store size, specialty, and climate, those bills can exceed $40,000 per month!
However, those stores can use floating suction or head strategies to adjust those pressures to ideal levels for the current ambient temperature. When we find a way to control suction and head pressure, we can impact the equipment’s compression ratio. Controlling the compression ratio boosts efficiency and can reduce energy bills, potentially saving hundreds of dollars per month.
Compression ratio and efficiency
A vital indicator of a system’s efficiency is its compression ratio. The compression ratio determines how many times greater a system’s absolute discharge pressure is than its absolute suction pressure.
We can find the compression ratio by dividing the discharge pressure by the suction pressure. Let’s say we have a medium-temp R-404a rack. A compressor may have a discharge pressure of 290.6 PSIG and a suction pressure of 62.3 PSIG. We’d find our compression ratio using the following steps:
1. Add atmospheric pressure to both gauge pressure values to get the absolute pressures.
290.6 PSIG + 14.7 PSI = 305.3 PSIA
62.3 PSIG + 14.7 PSI = 77 PSIA
2. Divide the absolute discharge pressure by the absolute suction pressure.
305.3 PSIA / 77 PSIA = ~3.96
Our compression ratio would be expressed as 3.96:1. Medium-temp coolers tend to have compression ratios between 3:1 and 5.5:1, so the compression ratio is within the typical range. We typically see higher compression ratios on low-temp equipment and lower ratios on residential A/C units.
If the compression ratio were more than 6:1 on the R-404a unit in question, then we’d have to start questioning why the discharge pressure is SO high relative to the suction pressure. Although we could be dealing with oil breakdown, we could also simply be looking at a system that has a hard time handling fluctuations in ambient conditions. In the latter case, we’d have to find a way to control compression to make the compressor perform better under various ambient conditions.
A brief history of suction pressure controls
When we first started using parallel rack systems, we didn’t have many controls at our disposal. We only had mechanical low-pressure controls that could manipulate or operate based on the suction pressure. Over time, electronic controls were developed and entered the mainstream.
The problem with those new electronic controls was that they still only focused on the suction pressure. As we just covered, the suction pressure is only part of the compression ratio (and efficiency) equation.
So, there was a demand for a more holistic control that could manipulate the suction pressure, case temperature, and condensing temperature. Electronic controls evolved to address those needs, and we began seeing an early version of today’s floating head and suction strategies.
As we began using more electronic controls, we gained access to a broader range of operational data, including case temperatures. Since we had actual data, we could start integrating it into our operations. It was still practical to use the cut-in and cut-out functions we had from the beginning with our mechanical low-pressure controls, but we had ways to look at the case temperatures. With that information, we could manipulate the suction pressure when cases make the desired temperature.
Based on what we know about compression ratios, we would be able to maximize our efficiency by raising the suction pressure whenever possible. However, raising suction pressure too much can drive up the case temperature, which may cause food products to spoil.
Before the advent of electronic controls that could measure case temperatures, raising the suction pressure would have been very labor-intensive. Someone would constantly have had to record the case temperature and use those readings to raise or lower the suction pressure mechanically.
Nowadays, we can use electronic controls to allow our suction pressures to “float” up when the case maintains temperature. Allowing the suction pressure to float reduces the compression ratio, which improves the efficiency of the rack.
When the rack becomes more efficient, it may even shut down a compressor, which results in major energy savings. If done right, floating the suction pressure can save a grocery facility up to a few thousand dollars monthly. The energy savings become even more worthwhile when the compressors last longer because they don’t have to work as hard.
Floating head control strategies use the same idea as floating suction; it’s just that the head pressure floats down instead of the suction pressure floating up.
In the past, we’d use controls that turned condenser fans on and off at certain temperatures and pressures. Now that we have electronic controls that collect enough data to factor ambient temperature and discharge pressure into our operations, we can reduce the head pressure to bring down the compression ratio. The metering device poses the main limitation to floating the head pressure; the head pressure needs to stay high enough to maintain a proper pressure drop at the metering device.
While floating suction depends on the case temperature, floating head strategies mostly depend on the ambient temperature to reset the setpoint. Let’s say you set the ambient temperature to 68 degrees. In that case, the system will continue operating as though it were 68 degrees outside, even when the ambient temperature drops below that number. Floating head strategies also factor condenser TD into the equation to float the pressure setpoint with the changing ambient.
So, in markets that stay warm year-round, floating head strategies probably won’t save a ton of energy. However, floating head strategies may save energy and money in lower-ambient environments, especially ones prone to large temperature swings during the shoulder seasons of spring and fall.
By floating the head pressure, you may also notice some increased efficiency through natural subcooling. When you run the fans more often in cooler weather, you cool the liquid refrigerant even more without relying on mechanical subcooling.
Jeremy Smith has seen condensing temperatures as low as about 68 degrees on TXV systems with floating head controls.
Realistic compression ratios
We use floating suction and head strategies to keep the compression ratio as low as conditions permit by controlling the load.
However, if we reduce the compression ratio too much for the conditions, the expansion valves could start hunting, or the compressor may begin short-cycling. So, what does a realistic low compression ratio look like?
On a medium-temp R-22 system, the ideal operating conditions would give you roughly an 80-PSI differential between the head and suction pressures; the head pressure may be around 120 PSI, and the suction pressure may be 40 PSI (3:1 compression ratio). That’s about the lowest compression ratio we can expect for safe operation in that particular case.
Low-temp applications have higher compression ratios by nature. So, on an R-404a system with a box temperature between 0 and -10°F, a 6:1 compression ratio is a realistic “low” compression ratio for increased efficiency. If your compression ratio goes much more below that, then you might encounter those TXV hunting and short-cycling issues.