Managing the cost to operate a large industrial refrigeration system has never been more important than it is now. The rate for cents/kWh has reached the highest rates to be registered since the beginning of the century. Compressor Lift/Compression Ratios is what can be used to find the optimum performance range for compressors. Reducing costs to operate the mechanical refrigeration system is no different than how you would try to lower your own electricity bill at home. The two factors that determine compressor lift are suction pressure and discharge pressure (compression ratio).
· Raising suction pressure (saturated evaporation temperature) is going to increase capacity (tonnage rating) for each operating compressor.
· Lowering discharge pressure (saturated condensing temperature) is going to use less power from the drive motor (brake horsepower) for each operating compressor.
Reduced compressor lift = reduced compressor work. This is no different than someone in the gym doing squats with 450 lbs of weight on the bar vs 250 lbs of weight on the bar. Less weight to lift = less work required. For a compressor, instead of lifting actual “weight”, compressors push against pressure and the higher the pressure, the harder the motor is going to work to push ammonia vapor out of the screw compressor.
Compressor lift can greatly impact compressor operating costs. One of the quickest ways to calculate compressor costs at different compressor lift pressures is to use the following rule of thumbs.
· Suction Saturation Temperature = 2% compressor operating costs savings per °F in suction temperature increase (raise suction pressure).
· Discharge Saturation Temperature = 1.5% compressor operating costs savings per °F in condensing temperature decrease (lower discharge pressure).
Most industrial refrigeration systems operate their compressor suction setpoints too low for what is needed for their process. This can be low-stage compressors & high-stage compressors. If your high-stage compressor(s) for example, operate at a suction pressure setpoint of 20 PSIG (5°F). And the connected evaporators on the High Temp Recirculator (HTR) maintain room temperatures only low as 30°F. The high stage compressors are doing wasted work ($$$) maintaining a suction setpoint of 20 PSIG when the suction setpoint only needs to be around 30 PSIG (17°F). Might have to raise or lower the setpoint a few pounds to account for system pipe pressure drops, oil fouled evaps, dirty evap coils, and other variables for each system design. The same principles apply to the discharge pressure setpoint of the facility if the current discharge pressure setpoint is 165 PSIG (90°F). The discharge pressure setpoint could be adjusted to 135 PSIG (78°F) and this also provides big cost savings opportunities. There is a point where discharge pressure can be too low, and this affects evaporator defrost operation, LIC oil cooling on compressors, coalescer filter differentials, TXV operation, etc. Most facilities don’t analyze the suction pressure and discharge pressure conditions to determine the most efficient setpoints, but an educated operator can make small adjustments to operating parameters that provides big cost saving potential for the facility. Modern day kWh costs for electricity are higher than they ever have been in history and it’s crucial to operate large mechanical refrigeration systems efficiently.
We’re going to calculate how much energy costs can be saved by operating your system at the right pressures for your process application. The following calculations are only going to express how much "wasted" costs for operating at unnecessary pressures and does not express the total costs for the machine to operate. For this calculation we must use the saturated temperature, but we control the saturation temperature of the refrigerant by controlling the pressure the refrigerant is operating under.
Example A:
If you have a high-stage compressor operating with a suction pressure of 20 PSIG (5°F saturated suction temp) and we raise the suction pressure setpoint to 30 PSIG (17°F saturated suction temp). Let’s calculate costs savings potential using the suction compressor lift rule of thumb (2%).
Note: 3 Phase kW = {(Amps x Volts x Power Factor x 1.73) / 1000} x hours
1. Saturation chart for R717 at 20 PSIG = 5°F
2. Saturation chart for R717 at 30 PSIG = 17°F
3. Saturation temp delta T (17°F - 5°F) = 12°F increase in suction temperature.
4. Use compressor motor nameplate data to calculate kWh, for this example we’ll use a 350 hp screw compressor, 384A x 460V x .89 x 1.73 / 1000 = 272 kWh.
5. Now multiply 272 kWh by the number of hours chosen, we’ll do 24 hours a day for 30 days (720 hours) = 195,840 kWh
6. Multiply 2% “rule of thumb” by saturation delta T (2% x 12°F delta T) = 24%
7. Multiply 24% by the kWh usage (195,840 kWh) = 47,002 kWh.
8. Assume $.15 cents per kWh ($.15 x 47,002 kWh) = $7,050.30 savings per month for one compressor.
The suction temperature increase (raise suction pressure) has a larger return on savings compared to decreasing condensing temperature (lowering discharge pressure). These cost savings numbers can grow rather quickly when you add them together and if multiple compressors are operating your process. Each of the previously calculated savings per month would need to be figured out for all operating compressors to get a total savings per month for the whole machinery room. Before making setpoint changes on your mechanical refrigeration system, always follow all management of change (MOC) procedures to verify no unwanted results would occur from making changes to your site-specific setpoints on your system.
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