Energy Efficiency Reference/Refrigeration/Recommendations/Reduce Discharge Pressure
Compressor power decreases with pressure difference across the compressor. Reduce the minimum condensing temperature set point to save energy when outside temperatures are below the existing approach temperature.
When to Apply
When the minimum condensing pressure setting results in condensing temperature (missing word) than 80 degrees F. The corresponding saturation pressure at this temperature is shown below for common refrigerants. Gauge pressures given in the table assume an atmospheric pressure of 14.7 psia.
Saturated Condensing Pressure (psig)
- 60 degrees F: 57.6
- 80 degrees F: 84.0
- 100 degrees F: 117.0
- 60 degrees F: 101.7
- 80 degrees F: 143.7
- 100 degrees F: 196.0
- 60 degrees F: 57.4
- 80 degrees F: 86.7
- 100 degrees F: 124.1
- 60 degrees F: 116.
- 80 degrees F: 161.2
- 100 degrees F: 216.2
- Ammonia (R-717)
- 60 degrees F: 92.4
- 80 degrees F: 138.4
- 100 degrees F: 197.3
Key Engineering Concepts
- Compressor Power increases with compression ratio.
- Compression ratio (CR) is determined by condensing pressure (pd) and suction pressure (ps); CR = pd / ps.
- Decrease minimum condensing pressure / temperature settings to decrease compression ratio and compressor power.
- Condenser fan energy will increase to maintain the decrease in condensing temperature. Fan energy increase typically represents about 10-30% of compressor energy savings.
- Approach temperature difference is measured relative to wet-bulb temperature for evaporative condensers, and relative to dry-bulb for air-cooled units. Wet-bulb temperature is less than dry-bulb due to evaporative cooling effects.
- Power Meter
- Clamp-on Ammeter
- Refrigeration Data Sheets
- Minimum condensing temperature (Tm) - (from condenser fan setpoints, pressure gauge or compressor display)
- Compressor power (CP) - (measure directly or volts and amps)
- Total fan power (FP) - (measure directly or volts and amps)
- Wet and dry bulb temperature
- Condensing approach temperature difference (minimum of approximately 10 degrees F) - (measure with all condenser fans on)
1) Choose Target Minimum Discharge Pressure Lower condensing pressure saves energy, by the minimum condensing pressure is limited by system requirements. Recommend that the equipment operator set an obtainable minimum condensing pressure in the one of two ways; either experiment by dropping pressure slowly and observe reaction , or consult manufacturer's specifications.
Expansion valves require a minimum pressure difference to function properly. In adequate discharge pressure can limit refrigerant circulation through expansion valves, including the liquid injection expansion valve for compressor cooling the system won't function correctly, and the compressor won't be properly cooled if condensing pressure is too low. A minimum condensing temperature of 60 degrees is often achievable.
Sometimes system modifications are required to lower condensing pressure. Following are some common problems and solutions:
- System pressure drop limits minimum condensing pressure.
Problem: The expansion valve will not function properly unless the refrigerant passing through it is liquid. If excessive system pressure drop causes flash-gas after the condenser, the condensing pressure will rise until liquid is delivered to the valve.
Solution: Use a centrifugal pump or suction gas to sub-cool liquid refrigerant after the condenser. Sub-cooling is achieved by increasing the liquid pressure. Open needle valves, replace orifices or valves to increase flow. These replacements generally cost must less than the compressor savings.
- Liquid-injection oil cooling limits minimum condensing pressure.
Problem: Refrigeration injected into the compressor (for cooling) passes through an expansion valve. If the discharge pressure is not adequate, the expansion valve will not function properly and the compressor will not be adequately cooled.
Solution: Refer to Recommmendation #3, Thermosyphon Oil Cooling. Another option is to install a centrifugal pump in the liquid injection line. This will not save as much energy as thermosyphon cooling, but will be better than a higher average condensing pressure.
- Undersized condensers limit condensing pressure
Problem: Heat transfer form the condenser depends on the temperature difference, the physically determined coefficients, and the heat transfer surface area. A smaller area requires a larger temperature difference.
Solution: Install more condensers to increase the condensing surface area and decrease the minimum approach temperature difference. In general, condensers cost less than compressor savings, and the system performs better on hot days.
2) Calculate Condenser Fan Use Factor Condenser fans will operate longer or at higher capacity when the minimum condensing pressure is reduced. This is referred to as increased fan use factor. Bin data is used to model the increase in fan use factor with a decrease in minimum pressure. Calculate use factors for the fan at both existing and proposed conditions with the equations from section 5, Power & Energy, Condenser Fan Use Factor.
3) Calculate Condenser Fan Energy Use Calculate fan energy for existing and proposed conditions. Refer to the proper calculation box, depending on the strategy, in section 5, Power & Energy, Condenser Fan Use Factor, Condenser Fan Energy Use.
4) Calculate Fan Energy Increase A decrease in discharge pressure increases fan operating time. A larger overall heat transfer coefficient is required due to the decrease in average approach temperature difference (temperature difference between cooling medium and lower condensing temperature).
Total Fan increase is calculated below:
- Fan Energy Increase = proposed fan energy - existing fan energy
- FEI = FEproposed - FEexisting
5) Calculate Compressor Energy Savings Conservatively estimate 1% drop in compressor power for each degree condensing temperature is reduced. You can also calculate compressor power decrease with condensing temperature from compressor manufacturer's specifications. We do not calculate demand savings because peak demand will not change on hot days. Calculations shown are for a simple cycle.
Calculate the existing and proposed condensing temperatures for each bin temperature. Remember that the condensing temperature at the maximum refrigeration load can never be closer to ambient temperature than the minimum approach temperature difference (MATD). Condensing temperature (CT) will floar at MATD above ambient temperature until the minimum condensing temperature (Tm) is reached.
- Condensing Temperature = bin temperature + approach temperature difference
Calculate compressor energy savings for each bin temperature. Estimate 1% power (range 0.5%-1.5%) savings for each degree condensing temperature (Tm) is reached.
- Energy Savings % = (condensing temp change) x 1% x bin hrs x operating hrs / total bin hrs
Calculate total compressor energy savings (CES) from the sum of bin energy savings time existing compressor energy use (CE).
- Compressor energy savings = compressor energy x sum of (energy savings percentages)
6) Calculate Cost Savings Calculate cost savings using the energy cost from the utility bills and energy savings. Total Savings are compressor energy savings minus fan energy increase.
7) Estimate Implementation Cost There is essentially no implementation cost to change the pressure settings at the condenser. If system modifications are require, consider the cost of purchase and installation. Modifications may include adding more condensing capacity to decrease the minimum approach temperature difference, new pumps, or thermosyphon oil cooling.
8) Simple payback period is implementation cost divided by annual cost savings. We neglect effects of interest, escalation, and discount rates over the period because we only estimate savings. These effects are small with short payback periods.