By Phil Boudreau
Refrigeration pros want to know – in this issue I provide answers to the questions I was asked most often during the past year. The questions and answers are not in any particular order.
1. Why is discharge superheat important and how do I check it?
Generally, the concept of evaporator and suction superheat are taught in refrigeration training programs. However, since the transition to HFC refrigerant and POE oil combinations several years ago, the discharge superheat has become a very important concept that must be considered during system commissioning and service.
Discharge superheat provides the technician with a metric that will help them determine the effectiveness of the lubrication system while ensuring there is sufficient suction superheat to support safe and long-term compressor operation.
Some manufacturers publish minimum discharge superheat values for their compressors. Ensuring that these minimum values are met is essential for long-term, reliable operation. For a given suction superheat, the resulting discharge superheat will be a function of the compressor design, the refrigerant type and the operating conditions.
A low discharge superheat provides us with a clear indication that the lubrication system will be considerably less effective since the oil will contain excessive refrigerant. High amounts of refrigerant in the oil reduce the oil viscosity, oil pressures and contribute to crankcase foaming, even with superheat present at the suction to the compressor.
The discharge superheat is checked by comparing the dew point of the discharge gas (pressure converted to dewpoint temperature) to the actual temperature read on the discharge line within three to four inches downstream from the discharge service valve.
An efficient way to meet the minimum discharge superheat requirement is to install a liquid-to-suction heat exchanger, which is sized to add enough superheat to the suction gas to ensure the discharge superheat will exceed the minimum.
2. Must R744 (CO2) be above both its critical pressure and critical temperature to be in a supercritical state?
If the CO2 is above its critical pressure, but below its critical temperature, the CO2 will be in a liquid state. If the CO2 is above its critical temperature but below its critical pressure, it will be a vapour state. If the CO2 is above its critical pressure and critical temperature, it will be in a supercritical state.
3. With vertical vapour lines, can I omit the trap if it is only a short rise, that is less than one foot?
To answer this question, let’s first discuss the purpose of the P-trap and inverted trap. The P-trap is intended to collect some oil. As the system load decreases, oil tends to separate from the stream of vapour refrigerant travelling up the riser. This oil then drops to the bottom of the riser, effectively reducing the cross-section of the area above the oil. This results in an increase in velocity, which promotes oil travel up the pipe. The inverted trap is installed at the top of the riser, to prevent oil from draining back down into a vapour riser. If there is no P-trap installed at the bottom of the riser, then the horizontal section of pipe just ahead of the riser will become the trap. The longer the length of this horizontal section, the more oil will be collected before the velocity increases enough to entrain the oil.
Note that some equipment manufacturers may state that P-traps are not needed for certain combinations of refrigerants and oil. However, it is very important that the capacity turn-down of the system be taken into consideration.
4. Can I operate a variable speed drive down to a low speed while unloading a reciprocating compressor?
Generally, we can unload a compressor or drive it with a variable frequency drive, but not at the same time. The main reason for this is there is a minimum mass flow requirement through the compressor to promote sufficient motor cooling. If we were to unload a four-cylinder compressor by 50 per cent and then reduce the speed of the compressor to 730 RPM or 25 Hz, then the mass flow rate will drop to 20.8 per cent. This will likely not produce enough motor cooling, especially in higher compression ratio applications.
On another note, the lower limit for the speed of a compressor is also due to the limits of the lubrication system. Oil pumps and centrifuges need to operate at or above the manufacturer’s minimum speed to ensure sufficient lubrication of all moving parts within the compressor.
The amount of unloading tends to be application specific while the VFD can generally vary the speed of the compressor through the full allowable range. Never unload a compressor more than what the compressor manufacturer allows for a given compressor, refrigerant type, and for a given set of operating conditions.
5. How do I check interstage superheat on a two-stage compressor?
Always follow the manufacturer’s instructions and never assume a typical evaporator superheat value of 10R or 5.6K is acceptable for a subcooler application. To achieve stable operation of the subcooler expansion valve or desuperheating expansion valve, the superheat must generally be higher than this. For example, a 35R to 40R total superheat entering the compressor’s interstage suction port may be required. Subcooler expansion valve hunting may take place if the superheat is set too low. This can also induce hunting of the expansion valve feeding the main evaporator if the amount of subcooling is also varying.
To check superheat on the interstage of the compressor, it is necessary to locate the correct interstage pressure port on the compressor. Generally, compressor manufacturers will provide a drawing with all the ports identified by location and description. Checking the interstage superheat is much the same as checking evaporator superheat.
Converting the interstage pressure to a saturation temperature and subtracting this from the temperature read on the interstage pipe, where the sensing bulb is located, is the correct way to check the interstage superheat. Never check the temperature at the subcooler outlet because the refrigerant at this point has not been mixed with discharge gas from the low-stage cylinders.