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Hi Guys
I want to share this consep with you regarding the control of van speed on a evaporative condensing unit.
i want to control the discharge temp via ambient temp by keeping the discharge temp just above ambient temp, this will ensure that no sub-cooling takes place.
So the discharge pressure is not a set-point any more but changes constantly as the ambient temperature changes.
So what do you think?
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But sub-cooling is good, why no sub-cooling?
rob
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Lodiev,Quote:
Originally Posted by Lodiev
Has been done for a number of years controlling off wet bulb temperature compared to dry bulb.
Can buy controllers to to do it, or develop your own PLC formula/logarit
Properly designed and operated evaporative condensers don't have subcooling.
To save energy you should control wet bulb approach(temperature difference between condensing temperature and wet bulb temperature). This approach shows the balance between compressors and condensers capacities. At optimum approach, these capacities are balanced and total (compressors + condensers) energy use is minimal.
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Oh OK, I didn't know that ��
Is that for flooded systems only?
Would it include direct expansion?
��
taz
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Any system should be run at optimum condensing pressure. Systems with air condensers should control dry bulb approach ( temperature difference between condensing temperature and ambient dry bulb).
However, every plant has minimum allowable condensing pressure. The lower this pressure is the better efficiency of the plant. TEVs are one of the barriers to reduce condensing pressure. This barrier can be solved by installation of refrigerant liquid pumps or electronic expansion valves.
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Hi Segei,
I understand that, but optimum condensing temperature
does not always guarantee good sub cooling, does it?
I understand about the difference between wet and dry bulb,
air cooled or evaporative, what I'm struggling with is liquid
at saturation temps??
I was always told that sub-cooling is good (taking condensing temps
and pressures into account). Good sub-cooling leads to good quality
liquid with lower temperature differences over the metering device.
If you have liquid entering the metering device still saturated and with
more heat, that heat surely has to be rejected across the metering device
and that will lead to more liquid flashing off (evaporating) just to reduce
the temperature of the liquid it's self??
Liquid at say 25 degC and evaporating temps at 0 degC = 25 degs
difference so the first part of the refrigeration effect is only cooling
the liquid itself? Lower the initial temp of the liquid entering the
metering device and you reduce temp differences over the valve?
I understand about low discharge pressures and liquid amplification, restrictions
over valves and metering devices, but is it not better to have the liquid entering
the expansion valve as cold as physically possible (within permitted limits) to
ensure that as much of the liquid as possible is in the evaporator, doing it's job?
Regards
Rob
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Hi, Rob.
Subcooling is good thing to have but it is not essential for energy efficiency. It is important how you get this subcooling.
1. Cooling saturated liquid by ambient air(wet or dry). For evaporative condensers optimum wet bulb approach is 9-11 F or 5-6 C. Very limited(2-3 C) subcooling can be done at 5-6 C approach.
2. Use heat exchanger to reduce temperature of saturated liquid. It can be done but no benefits to energy efficiency.
3. Increase liquid pressure by refrigerant pump. It can be done but no benefits to energy efficiency.
There are manors and majors energy savings measures. Keeping optimum condensing pressure is major one. Subcooling is minor measure. It is good if you have it. It is not big deal if you don't.
Hi!
I'm following this interesting conversation and then I was looking for some old thread regarding sub-cooling and energy savings discussed here on RE forums many years ago ... I hope you'll find some interesting thoughts ...
http://www.refrigeration-engineer.co...p-Refrigerants
Best regards, Josip
Quote:
Originally Posted by Segei
Cheers guys.Quote:
Originally Posted by Josip
Subcooling is a minor measure, I like that and I understand that :D
All the best Rob.
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Let me clarify my statements.
Many years ago I visited one plant where operators kept condensing pressure 150 psig all year around. They thought that this is optimum pressure because it match compression ratio for screw compressors which had fixed compression ratio. Reduction of condensing pressure to 130 psig save 10% of energy. At the same time wrong compression ratio will increase energy use by 1%. Net energy savings is 9%. It is more efficient to keep right condensing pressure than right compression ratio. We should improve system efficiency. Very often we should spend some energy in one part of the plant and save energy in another part. If savings are greater than spending, we are going to the right direction.
Example. We run additional condenser fan of 10 kW and due to lower condensing pressure save 30 kW of compressor power (high wet bulb approach). Net savings is 20 kW. We are going to the right direction. If the same 10 kW of additional fan energy saves, 5 kW of compressor power(low wet bulb approach). Net savings is -5 kW. We are going to the wrong direction.
To my understanding subcooling can also save energy used for other than flash gas in high pressure liquid line.
Obviously in single stage economised plant for low temp application, also if motor power increases, but bigger refrigeration capacity., smaller compressor with same mass flow as bigger one.
In Josips link Andy uses heat recovery, when used on screw compressor oil cooler is best way to recover heat, instead of discharge line with pressure drops etc.
All these things can potentially save power as long as extra circulating pumps etc do not cancel it out.
If electronic or other expansion valves are used on ammonia, obviously same as synthetic/***** fridge plants, you need subcooling, but to much can also cause problems.
The more expensive power becomes subcooling on even high temp systems may be used, as well if compressor selection is on border it can bring it up in capacity (maybe bigger motor which can be less expensive).
Condenser with separate subcooling circuit in condenser can be used, but extra expense for condenser.
There are many ways to save energy. However, before initiating energy saving process two questions should be answered. How much energy can be saved? How much will it cost?
Look at US Iceman post. At 25F(14C) subcooling capacity of TEV will increase by 5%. It means efficiency will improve by 5% if subcooling heat removed by ambient air. Subcooling 2.8C will save 1% of energy. It means that if we put subcooling coil into evaporative condenser 1% of energy can be saved at optimum condensing approach of 5-6C. The same 1% can be saved by lowering condensing pressure by 2 psig.
Heat recovery. To recover heat we should know two things. Quantity and quality of the heat. Quality is at what temperature heat can be recovered. Typical ammonia industrial refrigeration plant with screw compressors has around 90% of low temperature heat(condensation) and 10% relatively high temperature heat(superheat). I believe additional oil cooling has less than 10% of the total heat. Another important issue is time. We have a lot of heat during summer operation but requirement for heat is low. We have limited heat in winter time but we need it a lot. All these factors should be evaluated during design stage of the plant.
Hi Guys sorry for coming back so late on this subject.
I should have said that it is on a ammonia flooded system single stage at -10 deg.C suction and desighned at + 35 deg.C discharge.
We tested this concept and found that it does indeed save a lot of energy in winter time but in summer not.
we programmed the controls of the discharge to work in auto set-point change only below an ambient of 28 deg.C and fixed set-point above. also at the specific plant there is 5 x EVAPCO VXMC 420 condensers.
The water pumps runs always and the fans ramps together according to pressure set-point.
only when the discharge is below set-point one by one condenser fan will switch off, or on and ramp again.
total saving resulted in 12%. the problem with sub-cooling is that the condensers try to cool down the ambient temperature around all the liquid piping and at this plant it is a lot.
It is not clear how it works. Condenser fans switch on and off based on condensing(discharge) pressure or based on discharge temperature as you mentioned in first post? Can you give su real life numbers?
Hi guys,
Let me add two new points to this discussion:
1) I have already implemented this type of condenser control in various types of refrigeration systems, without any problem, but this week, a customer asked me if, with this control, the oil return could be impaired. What do you think?
2) How would this control work in a system with plate condenser + cooling tower?
Tks, Renata
For me it is not clear how control based on discharge temperature works? Can you explain us?
Segei,
I think he is referring to SCT.
RANGER1
Most likely you are right. Unfortunately, many people operate this control but they don't know how it works. Sometimes companies install mentioned controls like "black boxes" and operators don't have opportunity to check the set points. At the end of the day all these plants use much more energy than required.
I have applied the control according to the saturated condensation temperature, but this set point fluctuates according to the actual wet bulb temperature (WBT), the set point is usually WBT+10ºC.
10degC wet bulb approach is good starting point. However, every plant is unique and optimum wet bulb approach will depends of efficiency of evaporative condensers.