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Today many people focused on energy savings in industrial refrigeration. However, it is not easy to prove that energy savings are real. To evaluate actual efficiency of the plant, refrigerant flow should be measured for every suction temperature. However, I didn't see any refrigeration plant that have refrigerant flow meter. How can we prove that energy savings are real?
Segei,
As you say hard to prove in a lot of cases.
Some indication could be
power bill is biggest indication if plant runs same like conditions as before unless maybe that was modified as well to benefit.
number of compressors running & at what capacity loading/speed.
plant discharge pressures.
plant suction pressure as high as practical.
how power bill is structured so you can work with it to benefit.
For energy efficient operation one has to maintain suction pressure at the design conditions and discharge pressure in line with ambient conditions.
To elaborate further:
Since the desired process temperature and the selection of evaporation temperature are fixed, the suction pressure is fixed. The operator has minimal control on suction pressure. He has to ensure that the suction pressure does not get depart from design conditions on lower side. At part loads the suction pressure may go up a bit.
The condensing temperature is decided to make plant work in worst ambient conditions. Therefore the maximum discharge pressure desired is in worst ambient conditions. Normally cooling tower and condenser automatically respond to ambient conditions and the discharge pressure varies with ambient conditions and the operator has to estimate continuously what should be the desired discharge pressure. Thus he has a dynamic target on discharge pressure side and he has to try to maintain this desired discharge pressure level.
If operator maintains design suction pressure and desired discharge pressure (arrived from prevailing ambient conditions), then the plant is said to be run efficiently.
RANGER 1.Quote:
Originally Posted by RANGER1
I know how to save energy. However, it is difficult to prove these savings to people who don't refrigeration (customers, utilities...). Usually, I try to convince them to compare energy use during short period of time(1 hour) to make sure that refrigeration load is steady. It is not easy, because everybody interested in long term energy savings.
I don't agree that suction pressure should be fixed. It depends of the plant. For production coolers and freezers suction pressure should be relatively steady. For holding freezers and coolers it can be changed in wide range. To get maximum energy savings, optimum suction pressure should be kept.Quote:
Originally Posted by sandybapat
I'm just curios. How should operator continuously estimate the desired discharge pressure?
Segie
I agree your comment regarding suction pressure. By fixed, I mean target suction pressure is fixed since the process temperature at which heat is to be removed is fixed.
How to ascertain desired discharge pressure:
We take a case of water cooled condenser with cooling tower and shell & tube type of PHE type condenser. Let the max ambient wet bulb at the place of installation is 28 C and the approach of 4 C and water temp range of 4 C is selected to arrive at 40 C condensing. (Corresponding saturated pressure is 14.8 bar approx.) This is very common in India for Ammonia plants.
The design difference between the wet bulb and condensing temperature is 12 C and design temp difference between water supply and condensing is 8 C. Normally a temperature gauge is installed at condenser inlet and outlet on water side. The compressor discharge gauge is having saturation temperature markings on the outer side of pressure scale. Operator can read the cooling water temperature and add 8 C to arrive at desired condensing temp and see in the discharge pressure gauge to locate the desired condensing temperature on the sat temp scale and the pressure at that temp should be the target discharge pressure. If the arrow is on right side , it can be concluded as high discharge pressure.
Here it is ignored that the actual heat rejection may reduce due to lower discharge pressure. With out much instrumentation operator can very easily ensure that the plant is responding to favourable ambient conditions.
Segei
Sorry I spelled your name wrongly in my earlier reply.
SegeiQuote:
Originally Posted by Segei
I have seen a vfd supplier using energymeter on the existing refrigeration plant. He installed a energy meter to measure the energy consumed for 15 days at the power input to refrigeration plant. Thereafter he changed the star delta starters to VFD and step capacity control arrangement (two stage reciprocating compressors) to step less capacity control with new vfds and measured the energy consumed for 15 days and compared the two figures to prove the energy saving. A record of variation in ambient conditions and production pattern was also kept simultaneously, to check out abnormal conditions.
Here in North America majority of the plants have evaporative condensers. I have a lot of info about operating these condensers.Quote:
Originally Posted by sandybapat
In your case, tower and condenser are separate. Assume wet bulb temperature is 20 C, water should be 24 C and condensing temperature should be 32 C. How does operator adjust condensing pressure/temperature? What actually he should do?
I'm concern about your expression "favorable(desired) condensing pressure". Why temperature difference should be 12 C? May be 10 C or 14 C is better? What is criteria?
I'm surprised that vfd supplier who probably doesn't have refrigeration knowledge try to sell compressor vfds. Refrigeration plant is the system and system performance should be evaluated. There are many variables in this performance. Did they sell these vfds? Sometimes people suggest to install equipment and prove energy savings. If not proven energy savings, they will not pay you and you can take your equipment back. Actually, compressor vfds for reciprocated compressors do not save energy.Quote:
Originally Posted by sandybapat
Having suction pressure constant, the power varies with discharge pressure. Therefore while designing, the designer will try to keep the condensing temperature as low as possible to have proper balance between initial cost and operating cost. From that angle 12 C difference will give lower operating cost compared to that of 14 C.Quote:
Originally Posted by Segei
If I am designer, then I will try to select condensing temperature as close to ambient wet bulb as possible while designing. Also remember, while designing I will have to consider the maximum ambient wet bulb temperature for the plant to operate in all conditions.
I used word "favorable" for ambient conditions and not for condensing pressure. Let the operator know that the plant is designed for, say 15 bar pressure which is corresponding to design wet bulb of 28 C. This condition may be prevailing for 2 to 3 days in a year. For rest of the period the wet bulb may be much lower. The operator is told that if the discharge pressure is higher than 15 bar then he should trouble shoot for high discharge pressure condition. However when the ambient wet bulb is say 20 C, then the discharge pressure should be around 11 bar. Now if the actual discharge pressure is 12 bar then the operator should understand that this is the high discharge pressure condition and should not wait for plant discharge pressure to go above 15 bar. This is called as taking advantage of favorable ambient conditions.
The capacity control of reciprocating compressor is in stages. For a six cylinder compressor typical capacity control steps are 33, 66 and 100%. At 66%, two cylinders are unloaded. The gas is compressed and given back to crankcase. Some work is done on that which is lost. If this compressor capacity control is switched over to VFD to make it step less, all the cylinders will be always loaded and the speed of the compressor is modulated to load requirements. Therefore the energy requirement is proportional. The energy saving will be when your load requirement is in between the two steps of compressor.Quote:
Originally Posted by Segei
I was told, the motor efficiency also is kept at a higher level under part load conditions with vfd.
These are tangible advantages.
Other intangible advantages are less wear and tear of compressor, smooth starting of compressor.
With two stage compressors, the intermediate pressure varies with step capacity control and the liquid temperature to evaporator (sub cooling) also varies. With vfd this is avoided.
Usually, I talk about optimum condensing pressure. At optimum condensing pressure total energy use (compressors + condensers) is minimal, because we save energy for the refrigeration system not for compressors. Lower condensing pressure is always better for compressors but it is not always better for the system. How can we keep optimum condensing pressure? Look at evaporative condensers. Assume that wet bulb temperature is 75 F (24 C). Operator should know(I hope so) optimum wet bulb approach. It is very important to know this approach. Assume the this approach is 10 F. Optimum condensing temperature will be 85 F or optimum condensing pressure 150 psig or 10 bars. Some PLCs have wet bulb approach feature others don't. Condensing pressure is regulated by cycling condenser fan and sometimes pumps.Quote:
Originally Posted by sandybapat
How should your operators regulate condensing pressure to get favorable one? I mean fan cycling, regulate water flow...
1. Compressor unload by holding open the suction valve on a cylinder. No compression, no work done. Efficiency does not suffer.Quote:
Originally Posted by sandybapat
2. If load between steps, one cylinder will cycle on and off. In average the same capacity like with vfd. Actually, load always goes up and down.
3. For motor there is no difference that load is vfd or just compressor. Probably, you was not told that vfd itself use 3-3.5% of additional energy.
4. For two stage compression, high and low stages should load and unload in parallel. If variation of intermediate pressure will happen, it has little effect on efficiency of the plant.
Must admit it would be very smooth control on a compound recip which would have benefits against chunky steps of capacity.Quote:
Originally Posted by Segei
It could assist in running suction pressure at optimum.
SEGEI
I do agree with your view point. This type of control is possible with sophisticated PLC based control system with a proper algorithm fed to PLC. In the absence of this, the refrigeration operator (who may not be having elaborate knowledge of refrigeration) can check whether the discharge pressure is varying as per ambient conditions or not. Based on that he can take corrective actions.
1. When the suction valve is kept open while the piston moves inwards it sucks the gas and in the discharge stroke the gas moves out to crank case. In this process some energy is lost, which may be around 2 to 3% of the power required for compression in that cylinder.
2. Take a condition of 80% load on refrigeration system with a compressor having 33, 66, 100% capacity steps. In this case the compressor works most of the time at 100% with a short period of 66%. Observation is that it consumes more energy. The process parameters also does not remain steady. With VFD the capacity of the compressor can be adjusted at 80% capacity. With VFD there is a limitation of lowest speed from oil pressure development.
3. Electric motor has 95 to 96% efficiency at 100% load, at 75% load the efficiency drops to 91-92% at 50% load it goes down further. I was told with VFD, the electrical flux required for the motor is continuously monitored by vfd and the motor efficiency is kept higher. Therefore at part load the motor efficiency is maintained at higher level thus reducing the losses.
4. As regards to two stage compression, I forgot to mention compound compressor. Ranger has pointed out correctly.
I agree that it might be smoother operation, but is going to invest in this issue without financial benefits. Every refrigeration plant has fluctuation of the suction pressure. Load up and down, flash gas on and off... Steady suction pressure will not improve energy efficiency of the plant.Quote:
Originally Posted by RANGER1
Any investments should be financially justified. In US people don't invest if payback is longer than 2 years. To get this payback energy saved by vfds should be at least 10-15% or greater. Sometimes we can get this savings by investing in screw compressor vfds, but investment in reciprocated compressor vfds will give us very limited(if any) energy savings and very long payback.Quote:
Originally Posted by sandybapat
To keep optimum condensing pressure, you don't need sophisticated PLC. Sometimes I give to my customers a table of optimum condensing pressures. They measure web bulb temperature, add optimum wet bulb approach and have optimum condensing pressure. They should change this setting a few times per day and this is not complicated. I still don't understand your suggestion how keep required condensing pressure for the plants with cooling tower and separate condensers.
I think that many people over focused on trying to keep optimum condensing pressure by floating this pressure. This floating saves condenser energy not compressor energy. It means that saving are very limited.
During period of cool weather, a lot of energy can be saved by lowering condensing pressure, because we will save compressor energy. In Canada, during winter operation optimum condensing pressure can be as low as 50-60 psig, but typical minimum condensing pressure is 110-120 psig. We have a lot of room for improvement.
Everything does not have to have a financial benefit, it can be used as part of a good accurate control system or customer requirements.Quote:
Originally Posted by Segei
PHE condenser only for particular requirement as not as efficient as evaporative condenser.
You would not install a PHE in parallel of evap cond as still need a cooling tower & pump/fan, but PHE needs big pump with high flow, uses more power than necassary.
This seems to be a general post, not sure where it's heading.
Each plant is unique so can only generalise.
We had one US coolstore engineer suggesting turning off some evap fans & blanking off temporarily in winter, as if people have the time!
You could do a lot of things but labour is also not cheap, management runs with absolute minimum staff.
People have to accept some things are what they are as you say due to payback.
A good salesman sells his idea, but does he have to prove it works in practice, what if something else changes in plant, or power bill goes up.
A lot of improvements are common sense & no brainers, doesn't mean client will do it.
Most won't even invest in redundant equipment like compressors, apparently we make them to reliable!
In my opinion, initially operation of every refrigeration plant should be optimized. Typically, this optimization has payback 2-3 months. Optimum set points and optimum operating strategies should be chosen and implemented. However, to do that people should understand optimization but this is not simple. People invest in LED light and motion sensors because these are obvious energy savings. I saw many examples of poor investments in energy efficiency in industrial refrigeration. To avoid these mistakes, companies (including huge multinational food companies) should have good independent advisers. Otherwise a lot of money can be wasted. Recently, I was asked to look at one plant. Contractor proposed to save 1,000,000 kWh annually but required $1,000,000 investment. I found that way to achieve the same energy savings by investing $50,000. We can find many ways to save energy but first attention should be given to major energy savings. Lowering condensing pressure is major one and very often can give us up to 50% of total energy savings including VFDs. However, there are several barriers to lower condensing pressure but every barrier has a solution.Quote:
Originally Posted by RANGER1
Go back to verification. At this point I see only one way to do that. Compare energy use before and after within relatively short period of time. Refrigeration load should be as steady as possible. Production should be steady, try to avoid defrost or number of defrosts should be equal during testing before and after. Several tests should be done to get average energy savings. Later testing period can be increased.