Floating condensing pressure

[Sandro Baptista]

Hi Guys,

I'd like to hear your opinion about the floating condensing pressure. The purpose is to minimize the electric power consumption of the sum (Compressors + Condenser).

On a "conventional system" I would run the evaporative condenser fans inverter to delivery the maximum speed if the the condensing pressure is above the minimum acceptable pressure for the good feed of liquid separators. In this way it will always ensured the compressors will have the minimum possible power consumption and less wear (higher service intervals) and a little more of cooling capacity.

My opinion about that "miraculous" floating condensing set-point if it's not well done, if there is not measure of all important parameters and combine with them it could happen that we would spend more electric power. The problem is that when someone/Clients hear about floating set-point condensing they get very happy but don't know the real significance and future implications.

[Rob White]

.

For some applications it does make sense to float the head pressure.

Keeping the discharge as low as possible keeps the compressor running costs
down as long as everything is within the design parameters.

What a lot of companies are starting to do now is lower the condensing pressure
as low as possible and then use a hydrolic pump to lift the liquid line pressure
above the minimum required for the metering devices to work.

They even run the compressors on inverters to control the back pressure to
the correct settings.

I think energy saving is such a big influence these days and floating the head
pressure is one way to reduce the bills.

Regards

Rob

.

[Segei]

Sandro.
I study this issue for several years. Unfortunately, many people have limited information about this "miraculous" floating. However, I try to educate people around the world about this issue.
First. Your statements.
- Good feed of liquid separator. This issue very easy to solve. Increase size of metering device. You can put bigger solenoid or 2 solenoids in parallel.
- Less wear. I'm doubt that lower head pressure will increase service interval. You can ask compressor manufacturers about this issue.
As I mentioned in one thread, floating condensing pressure saves condenser(fans, pumps) energy. This saving is not significant unless you significant fluctuation of the refrigeration loads. From my experience, ice cream production plant can be good candidate for this floating.

[Sandro Baptista]

.

For some applications it does make sense to float the head pressure.

Keeping the discharge as low as possible keeps the compressor running costs
down as long as everything is within the design parameters.

What a lot of companies are starting to do now is lower the condensing pressure
as low as possible and then use a hydrolic pump to lift the liquid line pressure
above the minimum required for the metering devices to work.

They even run the compressors on inverters to control the back pressure to
the correct settings.

I think energy saving is such a big influence these days and floating the head
pressure is one way to reduce the bills.

Regards

Rob

.

Dear Rob

"Keeping the discharge as low as possible keeps the compressor running costs
down as long as everything is within the design parameters."

To do such feature you don't need to float the head pressure. Just set the set-point of the head pressure the minimum possible and the fans of the condenser will run at 100% on the summer.

[Sandro Baptista]

Sandro.
I study this issue for several years. Unfortunately, many people have limited information about this "miraculous" floating. However, I try to educate people around the world about this issue.
First. Your statements.
- Good feed of liquid separator. This issue very easy to solve. Increase size of metering device. You can put bigger solenoid or 2 solenoids in parallel.
- Less wear. I'm doubt that lower head pressure will increase service interval. You can ask compressor manufacturers about this issue.
As I mentioned in one thread, floating condensing pressure saves condenser(fans, pumps) energy. This saving is not significant unless you significant fluctuation of the refrigeration loads. From my experience, ice cream production plant can be good candidate for this floating.

Lower head pressure it real can increase the service intervals because there are less load/tension on the moving parts. See the manual service of SABROE piston compressors.

"As I mentioned in one thread, floating condensing pressure saves condenser(fans, pumps) energy"
Okay, I understand and agree with you if the PLC have a good control and have several inputs from the field. Nevertheless you must have in mind that when you are saving fans and pumps you are increasing a little or a LOT the power consumption of the compressors.

[Segei]

I'm not sure about reciprocated compressors, but majority of industrial compressors are screw. I know one Frick compressor that run 125,000 Hrs without overhaul. Any way it is minor issue.
Many years ago I start testing one plant. Assume that it was operated 150 psig(10 bars) cond. pressure. Refrigeration load is constant. Condenser operates on manual. turn on additional fan 20 HP. In several minutes cond. pressure settled and energy use of compressors was lowered by 50 HP. Total benefit is 50-20=30 HP. It means that new lower cond. pressure is better than initial 150 psig. Next step is additional fan of 20 HP. When cond. pressure settled, compressor energy savings will be 30 HP. Total benefit is 30-20=10 HP. Third step. Additional fan 20 HP and 20 HP of compressor power saved. Benefit of this step is 20-20=0 This is optimum condensing pressure. Assume that we run additional fan 20 HP. Compressor energy saving of this step will be 10 HP. Total benefit is 10-20= -10 HP. We overspent condenser energy. When condenser energy overspent, floating cond. pressure control will be initiated and cond. capacity will be reduced to keep cond. pressure at optimum level.

[Sandro Baptista]

Many years ago I start testing one plant. Assume that it was operated 150 psig(10 bars) cond. pressure. Refrigeration load is constant. Condenser operates on manual. turn on additional fan 20 HP. In several minutes cond. pressure settled and energy use of compressors was lowered by 50 HP. Total benefit is 50-20=30 HP. It means that new lower cond. pressure is better than initial 150 psig. Next step is additional fan of 20 HP. When cond. pressure settled, compressor energy savings will be 30 HP. Total benefit is 30-20=10 HP. Third step. Additional fan 20 HP and 20 HP of compressor power saved. Benefit of this step is 20-20=0 This is optimum condensing pressure. Assume that we run additional fan 20 HP. Compressor energy saving of this step will be 10 HP. Total benefit is 10-20= -10 HP. We overspent condenser energy. When condenser energy overspent, floating cond. pressure control will be initiated and cond. capacity will be reduced to keep cond. pressure at optimum level.

I plenty agree with that. However, what I said on the first post and I'm trying to say is that if there is no input data to the PLC and a good algorithm you will not have chance to automatically do what you have did manually at many years ago.

[Segei]

In this process our goal is to keep balance between compressors and condenser capacities.
For example. A refrigeration plant operate at optimum condensing pressure. Load is 100%. 4 equal compressors are on and 2 equal condensers are on. When load is 50%, 2 compressor will be on and to keep the balance 1 condenser should be operated.
How to keep this balance? Refrigeration plants with evaporative condensers have modern PLCs with wet bulb approach feature. This is temperature difference between condensing temperature and web bulb temperature of ambient air. This feature is useful to keep certain balance between compressors and condensers capacities. At optimum wet bulb approach, balance is optimum and condensing pressure is optimum as well. Typically, summer optimum wet bulb approach is 8-12 degF.

[desA]

What happens to the evaporator operation, as the condenser operating pressure lowers?

[Tesla]

Sadnro another saving comes in when the ambient is very low and the pressure drop across the metering device is too low for the refrigeration effect. All this above requires complex control which is slowly taking place. If we look back 20 years almost ever thing was fixed - now we are moving to varying this that and the other.

[Sandro Baptista]

In this process our goal is to keep balance between compressors and condenser capacities.
For example. A refrigeration plant operate at optimum condensing pressure. Load is 100%. 4 equal compressors are on and 2 equal condensers are on. When load is 50%, 2 compressor will be on and to keep the balance 1 condenser should be operated.
How to keep this balance? Refrigeration plants with evaporative condensers have modern PLCs with wet bulb approach feature. This is temperature difference between condensing temperature and web bulb temperature of ambient air. This feature is useful to keep certain balance between compressors and condensers capacities. At optimum wet bulb approach, balance is optimum and condensing pressure is optimum as well. Typically, summer optimum wet bulb approach is 8-12 degF.

My opinion is that if we want a good control and realistic then it's not enough just to read a HP pressure sensor (to get the condensing temperature) and the wet bulb temperature. Another read should be get to PLC such as power meters of the equipment, operating conditions of the compressors, condenser motor power installed...if not as I start to say at the first post we could easily get wrong about the energy savings.

[Segei]

I don't know what you mean about power meters of the equipment. However, our goal in energy savings is minimize energy use of whole refrigeration system. It isn't easy. Floating condensing pressure is only one small part of energy saving process. Lowering cond. pressure during winter operation is major energy saving measure. However, there are several barriers to implement this measure. Hot gas defrost, liquid supply, liquid injection oil cooling for screw compressors, oil carry- over are the barriers, but every barrier has a solution or several solutions.
Screw compressor shouldn't be operated at capacity below 50%. It is very inefficient. Condenser fan and pump power influence on wet bulb approach. The greater this power is, the greater optimum wet bulb approach is.

[Segei]

What happens to the evaporator operation, as the condenser operating pressure lowers?
For industrial refrigeration plant, no significant difference. Majority of them have liquid pump to supply liquid to the evaporators.

[Sandro Baptista]

I don't know what you mean about power meters of the equipment. However...

A device to measure the power consumption or indirectly and less precise by measure the current absorved.

[Segei]

Sometimes people install power meters for compressors, evaporators, pumps... I think that we need one power meter for whole refrigeration plant. Ideally if we have meter to measure refrigeration. By saving energy, we improve energy efficiency of the plant. It means energy use per unit of refrigeration.
E=P/RE p-power, re-refrigeration.

[Segei]

Sadnro another saving comes in when the ambient is very low and the pressure drop across the metering device is too low for the refrigeration effect. All this above requires complex control which is slowly taking place. If we look back 20 years almost ever thing was fixed - now we are moving to varying this that and the other.
Pressure drop across the metering device doesn't influence refrigeration effect. It influences on liquid supply.

[Sandro Baptista]

Sometimes people install power meters for compressors, evaporators, pumps... I think that we need one power meter for whole refrigeration plant. Ideally if we have meter to measure refrigeration. By saving energy, we improve energy efficiency of the plant. It means energy use per unit of refrigeration.
E=P/RE p-power, re-refrigeration.

You meant E=RE/P (Efficiency) :)

[Rob White]

Dear Rob

"Keeping the discharge as low as possible keeps the compressor running costs
down as long as everything is within the design parameters."

To do such feature you don't need to float the head pressure. Just set the set-point of the head pressure the minimum possible and the fans of the condenser will run at 100% on the summer.


Sandro, define what you think a floating head pressure is please.

Are you taking it to literally mean the head pressure floats around?

The idea of a floating head pressure is to maintain sufficient temp
difference (delta T) across your condenser to enable you to condense
all the refrigerant to liquid efficiently as possible.

So when I said

"Keeping the discharge as low as possible keeps the compressor running costs
down as long as everything is within the design parameters."

It is not as simple as setting the set point low, if you reduce the head pressure too
much, you will affect the liquid pressure and therefore you will struggle to maintain
correct temps because all the liquid will log in the condenser.

I'm not sure if you are trying to prove that low discharge pressures have no effect
on efficiency or not?

What are you trying to prove? Are you claiming floating head pressures (you description)
is ineffective, effective, good, bad or what?

I'm not sure you understood my reply because if you reread it and understand it
all I said was to control the head pressure can increase efficiency, within design
parameters, nothing about fixed speed fan control.

It might be my interpretation of "floating head pressure" but in refrigeration nothing
is fixed, you have operating parameters which you set for different applications but
temps and pressures are never fixed.

So that means if you control your head pressure to take into account ambient temperatures
you are floating the head pressure.

Rob

.

[r.bartlett]

Calvin becker is a member here as the owner of hysave is in the know or Marc O'Brien (various nom de plume here at various times) was assisting him on the research a few years ago

http://www.refrigeration-engineer.com/forums/showthread.php?1528-liquid-refrigerant-pumping

[Segei]

You meant E=RE/P (Efficiency) :)
Actually this is the same. To improve efficiency we should use more refrigeration per unit of energy or less energy per unit of refrigeration. I'm not sure about Europe, but in North America we use term of efficiency as horsepower per ton = 4.715/COP.

[Segei]

Sandro, define what you think a floating head pressure is please.

Are you taking it to literally mean the head pressure floats around?

The idea of a floating head pressure is to maintain sufficient temp
difference (delta T) across your condenser to enable you to condense
all the refrigerant to liquid efficiently as possible.

So when I said

"Keeping the discharge as low as possible keeps the compressor running costs
down as long as everything is within the design parameters."

It is not as simple as setting the set point low, if you reduce the head pressure too
much, you will affect the liquid pressure and therefore you will struggle to maintain
correct temps because all the liquid will log in the condenser.

I'm not sure if you are trying to prove that low discharge pressures have no effect
on efficiency or not?

What are you trying to prove? Are you claiming floating head pressures (you description)
is ineffective, effective, good, bad or what?

I'm not sure you understood my reply because if you reread it and understand it
all I said was to control the head pressure can increase efficiency, within design
parameters, nothing about fixed speed fan control.

It might be my interpretation of "floating head pressure" but in refrigeration nothing
is fixed, you have operating parameters which you set for different applications but
temps and pressures are never fixed.

So that means if you control your head pressure to take into account ambient temperatures
you are floating the head pressure.

Rob

.
Rob.
I'm not Sadro, but I can give you my vision of floating condensing pressure.
To save energy, cond. pressure/temperature should float up and down based on the temperature of ambient air(if we have air or evaporative condensers). For air condenser it should follow dry bulb temperature. For evaporative condensers it should follow wet bulb temperature. How close to follow? I don't have information about air condensers. Evaporative condensers should have optimum wet bulb approach 8-12 degF. Keeping this optimum approach minimum energy use of compressors+condensers will be achieved. However, we should know what is the optimum for the particular refrigeration plant. It depends of energy efficiency of the condensers. It means energy us per unit of heat rejection. Evaporative condensers with centrifugal fans fans use twice more energy compare to axial fans. So they should have different wet bulb approach.
As I mention early, one of the barrier to operate plant at low condensing pressure is liquid supply. Refrigeration plants with TEVs are very sensitive to this issue, because TEVs designed to operate in narrow range of cond. pressure. Solutions are EEVs, liquid pumps.

[Segei]

Calvin becker is a member here as the owner of hysave is in the know or Marc O'Brien (various nom de plume here at various times) was assisting him on the research a few years ago

http://www.refrigeration-engineer.com/forums/showthread.php?1528-liquid-refrigerant-pumping
Liquid pump is right step in the right direction. However, it will resolve only one barrier of liquid supply. However, several others barriers can arise.

[hookster]

http://www.sabroe.com/fileadmin/user_upload/Marketing/Brochures/Controls/3296controlsCPOscr.pdf

This is one of the condensing optimising solutions. As the wet bulb air temperature increases, the capacity of the evaporative condenser decreases. As the saturated condensing temperature increases your evaporative condenser capacity increases.

If your controller can allow head pressure to drop with the wet bulb temperature decrease, utilising the increase in condensing capacity from the evaporative condenser.
This decreases the compressor energy usage ie providing the savings.
There is a minimum that you can depress your head pressure before negative effects are achieved from defrost and expansion valves etc.

Of course any system designer worth his salt would have over sized the condenser already ;)

[Sandro Baptista]

It is not as simple as setting the set point low, if you reduce the head pressure too
much, you will affect the liquid pressure and therefore you will struggle to maintain
correct temps because all the liquid will log in the condenser.


Of course low discharge pressure have effect on COP of the compressors. But it could happen that fans power of the condenser start playing a important influence on the consumptions. Also it could compromise the operating of the expansion valves.

On the summer as I had reply to your first email you can run with the fans at 100% that the minimum condenser pressure will be always greater than the bulb temperature.

[Rob White]

.

Sandro, I might be being very dense here :o but are we not both
saying the exact thing?

Controlling the discharge pressure to a level just high enough to have
a temperature difference over your condenser is floating the head preasure?

If you drop to liquid pressure too low it will effect the metering devices, agreed
(but I stated that more than 20 posts ago) and if you run the condenser fans
harder there will be costs accounted to that also, but as I stated in my first post
if you run the compressor with as low a head pressure as possible (dependent on
ambients and parameter settings) you will save energy costs overall.

Condenser fans are not as expensive as compressors so run the conds hard and
the comps easy.... ;)

Regards

Rob

.

[Segei]

http://www.irc.wisc.edu/file.php?id=128 Read this paper. Page 16 shows that condensing pressure should be optimum not minimum.

[Rob White]

.

I think this is sort of turning into a debate about definition.

Page 16 shows that condensing pressure should be optimum not minimum.

But page 17 states:- An optimization algorithm designed to find the minimum total condenser and compressor power was used in conjunction with the refrigeration system model to identify optimum control points as a
function of outdoor conditions.

Tomatoes you say, I say tomatoes
Potatoes you say, I say potatoes

I know what I mean, I also know what you mean, I think we mean the same thing :confused:

Regards

Rob

.

[hookster]

"Page 20 The system arrangement that uses the least amount of
energy would have an over-sized (design condensing temperature of 85°F (29.4°C)) condenser with
a variable head pressure maintained with variable frequency drives controlling the speeds of the
condenser fans."

Seems to sum it all and its been fun reading Rob & Sandro playing word pong :D

I find on a lot of sites that the basics are always neglected! Everyone is trying to sell modifications and the really easiest payback would be a decent clean of the condenser.

[Rob White]

"


Seems to sum it all and its been fun reading Rob & Sandro playing word pong :D

I find on a lot of sites that the basics are always neglected! Everyone is trying to sell modifications and the really easiest payback would be a decent clean of the condenser.

Pong? PONG? there's nothing about me that pongs...... Sniff, sniff. Ah, erm, excuse me please. :o

Rob

.

[Peter_1]

Haven't read this whole thread but Sandro has a very valid point. But what's better and gives you even bigger savings is increasing the LP side as high as possible. Much more than decreasing the HP. But you must indeed make the necesaary calculations so that the electrical costs of running the condensor fans doesn't counteract the COP increase.

[Segei]

LP as high as possible, HP as low as possible. This is typical simplified approach to energy savings. Don't forget that to keep HP low we use condenser energy. At certain point energy use by the condensers will be greater than energy saved by lowering HP. Optimum is the lowest point on the page 16 of mentioned article. Actually suction pressure should be optimum as well. Assume that plant operated at suction temperature difference(between air and suction) of 10 degF. We decide to increase suction pressure/temperature and temperature difference will be 5 degF. To handle current load, evaporator capacity should be doubled. This is additional energy use of evaporators fans. This energy can be greater than compressor energy saved by increasing suction pressure.

[Sandro Baptista]

LP as high as possible, HP as low as possible. This is typical simplified approach to energy savings. Don't forget that to keep HP low we use condenser energy. At certain point energy use by the condensers will be greater than energy saved by lowering HP. Optimum is the lowest point on the page 16 of mentioned article. Actually suction pressure should be optimum as well. Assume that plant operated at suction temperature difference(between air and suction) of 10 degF. We decide to increase suction pressure/temperature and temperature difference will be 5 degF. To handle current load, evaporator capacity should be doubled. This is additional energy use of evaporators fans. This energy can be greater than compressor energy saved by increasing suction pressure.

Yes you're right about the energy evaporators fans, but at certain periods where the production has slow down and the cold stores have the doors closed and the inside product is already at the cold store temperature (for example in weekends) then the LP set-point could be higher. Of course the evaporators will delivery less capacity » more hours running but the COP increases.

[Segei]

Optimized suction pressure (1)

A majority of storage coolers and storage freezers have single speed evaporator fans. In the following 2 newsletters I will show you the opportunity to optimize suction pressure for these coolers and freezers.
It is common knowledge in the industry that raising the suction pressure improves the compressor’s efficiency (BHP/TR). Typical improvement might be 1% to 2% increase in saturated suction pressure per degree F. If all space temperatures are satisfied and evaporator coils are in a low load mode, the suction pressure usually increases to the maximum until some limiting temperatures are approached; thus, the efficiency of the compressors is improving. However, in order to operate the entire refrigeration plant efficiently, we have to pay attention to evaporator fans.
There are 4 steps to optimized suction pressure.
Step 1. Compressor efficiency.
Example. Freezer temperature is 0 deg.F; single stage screw compressor with economizer Frick RWB-II 134E; condensing temperature is 75 deg.F at 125.8 psig head pressure; saturated suction temperatures are – 5 deg.F, -10 deg.F, -15 deg.F, - 20 deg.F. Compressor efficiency at suction temperature of -10 deg.F

Ecomp= 221.1 BHP / 163.9 TR=1.349 BHP/TR

Table 1. Compressor efficiency at different suction temperatures

Suction temperature Compressor efficiency
deg.F BHP/TR
- 5 1.237
- 10 1.349
- 15 1.469
- 20 1.599

If we increase the suction temperature, our compressors are using less energy (BHP) per unit of refrigeration (TR) and their efficiency is improving.

Step 2. System efficiency.
To estimate efficiency of the system (compressors + evaporators) we have to add efficiency of compressors and efficiency of evaporators.
We have compressor efficiencies in the Table 1.
Efficiency of evaporators can be estimated as follows.
Example. Evaporator coil fan power is 15 HP; capacity of the coil is 20 TR at temperature difference (TD) of 10 deg.F.
Evaporator coil efficiency at TD of 10 deg.F (suction temperature – 10 deg.F)
Eevap (10) = 15 HP/ 20 TR = 0.75 HP/TR
Capacity of the coil is proportional to TD. At TD of 5 deg.F capacity of our coil is 10 TR. Evaporator coil efficiency at TD of 5 deg.F (suction temperature is -5 deg.F) Eevap (5) = 15 HP/ 10 TR = 1.5 BHP/TR.

Table 2. System efficiency at different suction temperatures.

Suction Compressor Evaporator System
temperature efficiency efficiency efficiency
deg.F BHP/TR BHP/TR BHP/TR

- 5 1.237 1.5 2.737
- 10 1.349 0.75 2.099
- 15 1.469 0.5 1.969
- 20 1.599 0.375 1.974

From this table we can see that efficiencies of the system are better (less BHP per TR) at suction temperatures – 15 deg.F and – 20 deg.F, than efficiencies at suction temperatures – 5 deg.F and – 10 deg.F. This means that the highest suction temperature (pressure) is not the most efficient for this refrigeration plant.
Optimized suction pressure (2)

Evaporator fan motors not only use up electricity, but all the energy used by the motors must be removed from the space as refrigeration load. This load is parasitic. In order to obtain an estimate of the useful refrigeration load we have to subtract this parasitic load from total gross refrigeration load.

Qnet = Qgross – Qfan

Useful efficiency of the system (compressors + evaporators) can be estimated as follows

Esystem useful = Ntotal / Qnet

Where: Ntotal = Ncomp + Nfan

Esystem – useful system efficiency, BHP/TR
Ncomp – compressor power, BHP
Nfan – fan power, BHP
Qgross – total refrigeration load on evaporator coil, TR
Qfan – heat added to refrigerated space by evaporator fans, TR

Step 3. Useful system efficiency.
Example. Gross refrigeration load is 100 TR; T.D. is 10 deg.F (suction temperature is -10 deg.F); Eevap (10) and Ecomp (10) from Table 2 (October 2005 newsletter)

Nfan =Eevap (10) x 100 TR = 0.75 x 100 =75 HP
Ncomp = Ecomp (10) x 100 TR = 1.349 x 100 = 134.9 HP
Ntotal =Ncomp + Nfan = 134.9 + 75 = 209.9 HP
Qfan = 75 HP x 0.212 TR/HP = 15.9 TR
Qnet = Qgross – Qfan = 100 – 15.9 = 84.1 TR
Esystem useful = Ntotal / Qnet = 209.9 BHP / 84.1 TR = 2.496 BHP /TR

Table 3. Useful system efficiency

Suction Temperature Useful
temperature difference system efficiency
deg.F deg.F BHP/TR

- 5 5 4 03
- 10 10 2.496
- 15 15 2.203
- 20 20 2.144

From this table we can see that useful system efficiency at suction temperature of – 20 deg.F (T.D. is 20 deg.F) is the best and almost twice more efficient than system efficiency at – 5 deg.F.
Optimum suction temperature depends of the evaporator fan power. If our coil would have fan power of 4 HP instead of 15 HP, optimum suction pressure will be – 10 deg.F (T.D. is 10 deg.F).

Step 4. Real life optimum T.D.
In real life optimum T.D. can be increased by the follow factors:
- suction pressure losses
- frost on the coils
- actual fan power usage. Real fan power can increase up to 25% due to increase of cold air density
- static pressure penalty
To get real life optimum T.D. we have to take the best T.D. from Table 3 and add the real life penalties, usually 2 – 4 deg.F.
This estimation has shown that a refrigeration plant has to run at an optimized suction pressure or at optimum T.D. regardless of the refrigeration load.

[desA]

Peter_1:
Haven't read this whole thread but Sandro has a very valid point. But what's better and gives you even bigger savings is increasing the LP side as high as possible. Much more than decreasing the HP. But you must indeed make the necesaary calculations so that the electrical costs of running the condensor fans doesn't counteract the COP increase.

Increase LP fan speed.
Oversize evaporator area.
Basically 'squeeze' the LP & HP closer together.

If liquid logging is a concern in the condenser, then perhaps look at enlarging the liquid line size, to reduce high-side pressure drop?

[Segei]

In refrigeration we have several misconceptions.
1. LP should be as high as possible and HP should be as low as possible. They should be optimum.
2. Pressure drop in hot gas line is the reason of poor hot gas defrosting at low cond. pressure. Very often is not an issue.
3. We should keep higher HP to supply liquid to far end of the plant.
I know 2 plants. First plant keep 125 psig HP (LP is 5 psig) to supply HP liquid to far end(100 m max)of the plant. Pressure difference is 120 psig and no lifting. Second plant is cold storage. Liquid pump has pressure of 30 psig. 15 psig required to lift liquid to the roof and 15 psig to supply to the evaporators. One plant require 120 psig and another 15 psig. Why? Issue is pressure drop in liquid line. First plant supply saturated liquid. Pressure drop in liquid line create a lot of flash gas and this gas will choke the metering device. Second plant supply subcooled liquid and no gas before metering device. First plant should subcool liquid as well to reduce HP. Liquid temperature can be lowered(HE) or liquid pump can be used.

[Peter_1]

Segei, in fact you say you need a high pressure due to a too small liquid line. You then need this HP to solve an installation problem. You eventually can like yuo said subcool the liquid to prevent flashgas. But then again, initial calculations must be right.
LP as high as possible/allowable and HP as low as possible/allowable. English is not my mother tongue you see.
We have a flowercooler and an ULO room where we reduce fan speed of the evaporators with a VFD the more we reach set temperature. Carel EEV's to prevent liquid flooding.

[Segei]

About allowable LP. Look at optimized suction pressure newsletter. This is information about actual cold storage. It was designed for suction temperature difference of 10 F or SST -10 F. During winter operation refrigeration load is 50% or lower of design load. SST can be increased to -5 F and plant will be able to keep required temperature. However, it will be using 4.03 BHP per TR. At SST -20 F plant will be using 2.144 BHP per TR. This is almost twice better efficiency.
About allowable HP. Look at this newsletter. Minimum allowable HP for this plant was 110 psig. This is real plant as well.
Wet bulb approach

Modern refrigeration plant control PLCs have wet bulb control feature. This is relatively new method to control head pressure of refrigeration plant. However, many operating engineers prefer operate plants at fixed head pressure. Do we really need wet bulb approach control?
Wet bulb approach is temperature difference between condensing temperature and wet bulb temperature. Condensing pressure is 150 psig, corresponding condensing temperature is 85 degF, wet bulb temperature is 70 degF. Wet bulb approach will be equal to 85 degF – 70 degF = 15 degF
Idea of wet bulb approach control is to balance capacities of condensers and compressors to minimize total (condensers + compressors) power use.
Example. Refrigeration plant has 5000 HP of compressor power and 500 HP of condenser (pumps and fans) power. Total refrigeration capacity of this plant is 2000 TR. This is production facility. It has production 24 Hrs per day and 5 days per week. During weekends only 200 TR (cold storage) or 500 HP of compressor power is required. Set point of condensing pressure is 120 psig.
One weekend wet bulb temperature was 70 degF. 500 HP of compressors were running to keep temperature in cold storage. Condensers were running at full capacity, but head pressure has reached only 125 psig. Total power use was 500 HP (compressors) + 500 HP (condensers) = 1000 HP.
Assume that we installed wet bulb approach feature for refrigeration plant PLC. This approach was set to 15 degF. New condensing temperature will be 70 degF + 15 degF = 85 degF. Condensing pressure will increase from 125 psig to 150 psig (85 degF). Compressor power will increase from 500 HP to 550 HP. Condenser power will reduce from 500 HP to 50 HP. Total power use will be 550 HP (compressors) + 50 HP (condensers) = 600 HP. Our savings of using wet bulb approach feature is 1000 HP – 600 HP = 400 HP.
Certainly, 400 HP imbalance is extreme. However, I saw very often imbalance of 50 – 100 HP, usually when set point of wet bulb approach is not optimal. Every 10 HP of energy saving at energy rate $ 0.1KWh/Hr will save you from $ 3000 to $ 5000.
To maximize energy savings, refrigeration plants with wet bulb approach should have an optimum setting. Every refrigeration plant has own optimum setting. My research has shown that this optimum can vary from 8 degF to 30 degF. Refrigeration plant runs inefficiently, if optimum wet bulb approach is 20 degF, but your setting is 10 degF.
Assume that we have found the optimum wet bulb approach. However, this approach should be change from time to time, because properties of wet air are different at different level of wet bulb temperature.
Wet bulb approach optimization is not simple task.