Download - PHE as Condensers

Plate Heat Exchangers as

Refrigeration Condensers

ISHRAE, February 25th

Pune, India

Dr. Claes Stenhede

Fouling.

The majority of condensers probably operate with cooling tower water. Treat-ment of cooling tower water depends on the actual water quality, the air qua-lity, and some times varies with the season and is normally best done by a specialized company, with experience of the particular conditions at the site.

The only special consideration is that a PHE is sensitive to fibre like particles, such as grass, seaweed, and leaves. Agglomerations of micro organisms, which can be found in cooling tower water, can also cause problems. A good screening with a mesh size about half the channel height is usually sufficient.

2

Do not mistake fouling for corrosion.

The figure shows a plate with hard, rust like deposits.

It was initially thought that it was corroded, but the plate was made of titanium, which simply does not corrode in the brackish water used.

Moreover, corrosion of titanium does not produce insoluble, rust like deposits.

A closer investigation showed that the deposits originated from the con-necting, steel pipe work.

3

Non condensable gases.

Sources of non-condensable gases (inerts, air) in a refrigeration system can be:

Insufficient removal before start up due to either a faulty vacuum pump or a faulty pressure gauge.

A part of the system is shut off from the rest during the initial evacuation of the air.

The evaporators operate below ambient pressure, especially in case of multiple and large unit coolers. The frost can easily break a tube in a unit cooler if the defrosting is not properly

Decomposition products. This is normally a minor source except in ammonia-water absorption systems. Under certain conditions, ammonia decomposes to hydrogen and nitrogen, especially if nickel is present.

This decomposition mixture has a lower density than ammonia, which can affect the vent position.

4

The effects of non condensable gases.

Pure vapour

Vapour with air

Temperature°C

The inert gases affect the temperature diffe-rence, the heat transfer coefficient and the condensing efficiency.

Heat load 100%

Both the heat transfer coefficient and the temperature difference are decreased, see the figure. The denomination “air” is used here, as this is the most common inert gas.

When the condensation proceeds:

The relative concentration of the vapour decreases and thus the saturation tem-perature and the temperature difference decrease as well.

There will be an increasingly thick layerof air saturated with vapour close to the condensing surface. The vapour hasto diffuse through this layer. The result is a decreasing heat transfer coefficient.

Some refrigerant remains in the vapour, regardless of the exit temperature.

0.1 bar), the temperature has decreased to 40 °C and 193 kg R22 remains at the exit.

Ex.:1000 kg R22, containing 1 % air, con-denses at 45 °C. After the condenser (∆P =

5

Causes

Observations

A. Air B. Condensate blocking of conden ser surface

C. Restric tion in refrigeration line.*

D. Fouling or blocking of water channels

E. Too small con den ser.***

1. Too high condenserpressure.6

Yes, if the expansion valve lets air pass or if a finite amount accumulates

Yes. See also § 6.D. Yes * * <= <=

2. The pressure continues to increase until the com pressor HP cut-out.

For small amounts, normally not. For larger amounts yes, but it takes time (≈ minutes) until enough con densate (air) has collected.

Yes * * Yes, but a slow process, (months, years)

Yes, rapid (< minute)

3. Discharge pressure >condenser pressure.

No <= Yes, if before condenser

No <=

4. The condensate exit temperature is much colder than the pure vapour saturation temperature.

Yes. Yes. See also § 6. D. No <= <=

5. A sight glass at the exit shows both a vapour and a liquid phase.

The presence of vapour/ liquid surface at the exit is no sure sign of air, it could simply be that the condenser is well drained and thus there is no large liquid level at the bottom.

No, condensates moves up into the surface but it could also be a small vapour bulk, which cannot escape.

Yes * * No, presumed a well drained condenser

<=

6. An unequal tem perature distribution along the plate pack. (Possibly more marked on the water side)(Use of a thermal camera)

No, not typical. <= Yes. If abnor mal ∆P in the port

Yes. If many blocked channels, the water temperature will not increase here.

No

7. Even temperature along the plate pack (refrigerant side) but a clear difference between top and bottom.

Yes, a fair temperature drop from the top to the bottom

Yes, possibly a marked change where the condensate level is.

* * If a vapour with a large superheat, the top part could be different from the bottom. This is valid for a correct PHE as well.

8. The discharge manometer needle vibrates, §3.3. Especially useful for halo carbons

Yes No <= <= <=

A. Troubleshooting matrix for refrigeration condensers, mainly valid for PHEs.

* Anywhere between the compressor and the expansion valve. * * A restriction can cause a condensate back-up, which then can have various effects, see column B.* * * Either because of incorrect physical properties, design errors, an incorrect duty specification, too high water temperature, etc.

6

Causes

Observations

A. Air B. Condensate blocking of conden ser surface

C. Restric tion in refrigeration line.*

D. Fouling or blocking of water channels

E. Too small con den ser.***

9. Venting from the top of the condensate exit pipe.

Gas or vapour emerges Liquid emerges, see also 5B.

<= * * Gas/vapour should emerge if well drained.

<=

10. Venting into a bucket with water (For ammonia)

Bubbles emerges. No bubbles, no air <= <= <=

11A. There is a temp. diff. on the water exit pipe just after the exit.Between left and right .This indicates a maldistribution over the channel width.

Yes, especially if low pressure drops on both sides.

No <= Yes, If uneven fouling over the channel width

No

11B. As 11A.Between top and bottom. This indicates a maldistribution along the plate pack.

No, not typical..

<= Yes. If abnor mal ∆P in the port

Yes, if water chan nels close to either frame plate are blocked. See 6D

No

12. Water pressure drop is too high.

No <= <= Yes, If several blocked channels.

Yes

13. Vapour pressure drop is too high.

Yes, especially if the con denser is a brazed PHE with a distributor (normally used as an evaporator).

Yes, but a conden sate level is the re sult, not the cause.

Yes No Yes.

14. Water exit temp. too low. Note: Check flow rate.

Yes <= <= <= <=

15. Condensate blocking part of the surface (This can be both a cause and a result)

No, normally not See fig. 7 A -C. Yes, together with a faulty EQ line, Fig. 7 A - C.

Blocked channels create an internal EQ line on the vapour side, Fig. 7.

No

17. Vapour emerges uncondensed. Yes No No Yes, but initially should be OK

Yes, from start

B. Troubleshooting matrix for refrigeration condensers, mainly valid for PHEs.

* Anywhere between the compressor and the expansion valve. * * A restriction can cause a condensate back-up, which then can have various effects, see column B.* * * Either because of incorrect physical properties, design errors, an incorrect duty specification, too high water temperature, etc.

7

Detection of noncondensables.

See the troubleshooting matrix in slides 6 & 7 and figures in slides 10 and 26 - 28.

It can be very difficult to distinguish if it is air or condensate, which is blocking the surface. A thermal camera is a good help but the pictures might be inconclusive.

An undampened, i.e. not filled with liquid, pressure gauge, type Bourdon, shows the small, rapid pressure variations, which arise when a vapour containing air is compressed.The needle vibrates, to the point that it can hardly be seen. Compressing a pure vapour is a stable process, thus there are no needle vibrations. It obviously means that the compressor exhibits these pressure variations but piston, screw and scroll compressors do. It is thus a good indicator on the presence of air.

Venting can be used – see the slides below how to install a vent – but it is a negative test: If the venting is done for a long enough time and there is no improvement of the performance, there was obviously no air present. Unfortunately, a lot of refrigerant has then escaped.

Ammonia is a special case. Ammonia is extremely soluble in water. If the vent is connected to a hose and the exit from the hose is the dipped into a bucket filled with reasonably cold water only air will emerge at the surface, it bubbles. Pure ammonia whether liquid or vapour dissolves in the water. There are no bubbles, only some noise and movements on the surface. See the case study in slide 26 - 28.

8

Venting.

Ammonia. Ammonia is easy to vent, see slides 10 and 26 - 28. Vent always into water to avoid the ammonia smell.

Halocarbons are expensive and usually forbidden to vent to the air, thus especially recovery units are used. It also a danger if venting into a confined space, where the vapours could collect in a low space and cause suffocation.

Hydrocarbons can be vented to the air, but collect in low places where they can ignite or cause suffocation.

Carbon dioxide also collects in low places and can cause suffocation. It can be vented to the air but the vent should not be connected to a downstream pipe as high-pressure carbon dioxide will solidify when the pressure suddenly decreases below 5.2 bar. This is an especially important consideration for emergency valves.

9

10Venting position.

The basic principle is that the vent should be placed at the end of the condensation, i.e. close to the water inlet or maybe vice versa. In case of a plate heat exchanger that is easy, the vapour inlet is one of the upper port and the water inlet the other side, lower port. The venting shall thus be done from the bottom port. As the condensate leaves a bottom port as well, the venting is thus done from the opposite port or after the condenser.

A S&THE is more complicated. Vents are practically always placed on top of the shell. However some-times the vent should be close to the bottom.

A.PHE, all types.

For all vapours, vent from:

A bottom port (the one opposite the conden-sate exit)

A liquid receiver

The pipe work

B. S&THE: Venting shall be done at the end of the heating surface, close to the exit.

a) Top vent (Inerts lighter than the vapour).

There is usually a vent at the top of the shell.

I. All vapours except ammonia & water.II. Ammonia, if hydrogen is present.

b). Bottom vent. (Inerts heavier than vapour)

A vent close to the condensate exit (but usually none present).

The pipe work.

A liquid receiver

I. Water.II. Ammonia, if no hydrogen.

C. Venting of ammonia (PHE or S&THE):

1. Let a hose from the vent discharge below the surface in a bucket with water.

2. Bubbles emerges => air

3. No bubbles => no air.

Do not vent from an upper port only.

The water inlet follows the vent: Top vent => TopBottom => Bottom

11Desuperheaters and subcoolers

Pinch point.

Desuperheater section

Condenser section

Condenser sectionDesuperheater

section

Note that there is pinch point, where the temperature difference approaches zero.The cooling media cannot pass this point and cooling of the water to a high tem-perature is not possible.

Use two separate heat exchangers.

In the desuperheater, tap water is heated to a temperature approaching the inlet temperature, albeit with a lower capacity corresponding to the superheat load.

In the condenser, the condensing heat is dumped into a suitable heat sink

Cooling of a superheated vapour.

12

1. The liquid level controls both the condensing surface and the subcooling surface but in opposite directions.

A simultaneous control of both the capacity and the condensate temperature is not pos-sible as both are dependent on the conden-sate level.

2. Air cannot escape.

If inerts are present there will be a gradual decrease of both the heat transfer coefficient and the temperature difference.

3. A liquid level is not allowed in ammonia or water condensers.

The excellent thermal properties of both fluids means rapid temperature changes, which can lead to thermal fatigue.

4. The design is thus somewhat questionable.

5. Better is a separate subcooler, possibly with different cooling fluids.

?

Condenser and subcooler in the same unit.

13Condenser and subcooler in separate units.

1. Separate control of the condenser and sub-cooler is now possible.

2. Air can now be vented.

3. The subcooler can sometimes be operated with a considerably colder cooling medium than the condenser, e.g. well water especially in case of an air cooled condenser.

The colder the condensing temperature is, the better the evaporator capacity and COP will be.

14 Condensate exit and liquid receivers. Some considerations.

Through liquid receiver.

Possible vent positions

The condenser is connec-ted to the liquid receiver via large condensate pipe.

This pipe is not filled of condensate.

Vapour can flow in counter current to the condensate.

Some refrigerant eva-porates because of heat influx from the surroundings


Through liquid receiver.

Possible vent positions

The condenser is connec-ted to the liquid receiver via large condensate pipe.

This pipe is not filled of condensate.

Vapour can flow in counter current to the condensate.

Some refrigerant eva-porates because of heat influx from the surroundings

No equalization line is permitted


Surge liquid receiver.

To overcome the pressure drop in the condenser a liquid column is created in the condensate pipe.

Condensate level

Equalization (EQ) line

Vent

h)

Vent

An oil cooler can be attached to a surge liquid receiver. The vapour recondensesvia the EQ line

Vent not possible


Through liquid receiver with equalization line.

The condensate enters in to the bottom of the LR below the refri-gerant surface.

Equalization (EQ) line

Vent

Vent

Vent not possible

A through LR (either type) is less sensitive to outside heating than a surge LR.

18Condensate blocking of the surface.

A. Contrary what is generally thought, the reason for a condensate back up in the condenser is normally not some obstruction in the condensate line. After all, there is a very large and efficient obstruction in the condensate line – the expansion valve –, which does not impede a proper flow of the condensate.

B. One reason for a condensate back-up is a misplaced equalization line (Slide 22-23) or a too high pressure drop anywhere in the system – and not only in the condensate pipe - between the equalization lines connection points and a LR not designed for this, see slides 15 – 17.

C. Another reason is that condensers of different pressure drops are installed in parallel and there is not sufficient height in the

adjoining condensate pipes to create the necessary equalizing liquid column, see slide 24 & 25.

D. The third reason is a too large refrigerant filling in the system, e.g. either temporary when e.g. condensers are shut down in winter

or by incorrect refrigerant charge. We can distinguish three cases:

19Condensate blocking of the surface.

Too large refrigerant filling:

1. A dry expansion system. The part of the system from the liquid re-ceiver to the compressor has a refrigerant filling, which varies very little. Consequently, this part cannot accommodate an excessive refrigerant filling. This leaves the part from the liquid receiver to the condenser to do this. A condensate blocking of the surface is then a risk.

2. A flooded system with multiple evaporators must have the expansion valve controlled by the liquid level in the LP receiver(s). The liquid filling will be more or less constant from the HP receiver to the compressor and a change of the refrigerant filling have to occur in the condenser-HP receiver part, with a flooded condenser as a possible result.

3. A flooded system with only one evaporator can have the level control in the HP receiver. The filling is now constant in the condenser – HP receiver part and possible variations in the liquid filling occur in the LP receiver. There is now no danger of flooding the condenser (i.e. from overfilling of refrigerant) but in extreme cases, the resulting refrigerant overfilling in the LP receiver can cause liquid droplets to leave this and damage the compressor.

20Some general considerations.

There can be a temporary condensate flooding of the condenser if the ex-pansion valve is placed far above the condenser. When the valve closes, condensate might flow back to the condenser and flood this. This can happen if the pipe is very wide, i.e. it cannot hold a liquid column or if vaporization starts because the pressure has decreased at the top of the column.

This can also lead to a decrease of the capacity of the expansion valve as a vapour enters together with the liquid.

In general, avoid placing the valve above the condenser. The system can work perfectly but troubleshooting in case something goes wrong can be close to impossible.

If a rooftop placed condenser is used, avoid long condensate lines, even insulated, on the roof, in the sun. During a plant stop, even a short one, vaporization occurs in the lines. When the plant restarts, the resulting vapour plug can easily pass a valve or other restriction. The subsequent liquid plug then accelerates and hits the valve and can easily destroy it.

Do not let vapour lines, especially HP vapour, pass through a cold store. This can easily happen if the entire building is a cold store. Refrigerant can condense in the lines and the resulting liquid plug can damage a valve or other equipment.

21Some general considerations.

Install a solenoid valve as close to the expansion valve as possible. If a large distance and especially if a large diameter, the liquid content between the solenoid and the EV is large. A pump-down of the evaporator can then cause freezing of a water-cooled evaporator.

Equally important, when the solenoid opens, the entering liquid plug will only meet vapour. The vapour easily passes the expansion valve and the evaporator but the liquid plug does not. Again, possible damage of the expansion valve can happen.

Be especially on guard when a saturated vapour is entering a subcooled liquid body. It should not happen in a well-designed system but it can happen, e.g. at a later extension, when the original layout maybe has disappeared. The vapour can then collapse – the phenomenon is equal to cavitation – and noise as by liquid hammering can occur with a risk of damage to the equipment.

All three liquid hammering effects can cause such large noises and vibrations that the pipes can be damaged.

22Troubleshooting case studies.

A flooded condenser.

Desuperheater

Surge LR

Condenser

Equalization line



Desuperheater

Surge LR

CondenserEqualization line connected to just before the condenser

h



S&T condenser S&T condenser SWPHE condenser

Flooding

Low P Low P High P

h



S&T condenser S&T condenser SWPHE condenser

Low P Low P High P

Compressor HP cutout


SWPHE condenserfor ammonia

21.0 C 21.0 C

Flooded condenser or inerts?




21.0 C 21.0 C

Vibrating manometer => Inerts




29.0 C 21.0 C

30 C

29Troubleshooting

case studies.

Occasionally flooding

of a condenser

The capacity of the condenser oscillated between 75 % and about 100 % with a period of about 15 min

Liquid receiver

SWPHE condenser

Surge drum

Expansion

valve

An investigation showed that the vertical feed to the expansion valve was almost 10 metres and a about 100 mm in diameter.

30Troubleshooting

case studies.


of a condenser

When the expansion valve closed, the liquid column in the pipe could not be maintained. The liquid then eneterd the LR and the condenser.

Liquid receiver

SWPHE condenser

Surge drum

Expansion

valve

31Troubleshooting

case studies.


of a condenser

The surge drum was lowered. This had the extra benefit that the evporator operated better.

Liquid receiver

SWPHE condenser

Surge drum

Expansion

valve

A water cooled condenser - CDEW 550/T - for R22 did not give the rated capacity.

The nominal thermal duty was:

CDEW 550/T S, 860 kWR22 70 °C -> 45 °C, ΔP = 13 kPaWater 39 °C <- 32 °C, Δp = 60 kPa, 106 m3/hr

The actual capacity was below 400 kW.

Moreover, the ΔP between the compressor discharge exit and the liquid receiver after the condenser was more than one bar, unclear where. The condenser should have only 0.13 bar.

This corresponds to a decrease of the actual condensing temperature to a little more than 43 °C and a corresponding decrease of the MTD, less 2 K.

The decreased MTD explains part of the capacity decrease but, by no mean, all.

Performance problem of a CDEW 550/T at a refrigera-tion plant for the Mauritius Freeport Development Co. Ltd.Claes Stenhede, Alonte, Sept. 21st, 2005

32

A too small heat exchanger. Unlikely, as similar units had performed well.

Too high water temperatures, too low water flow rate and/or too low con-densing temperature were ruled out as all were found to be in order.

Fouling of the water side. This was ruled out as it was a closed circuit.

Inerts in the vapour. Preliminary tests were inconclusive.

Flooding of the condenser.

Flooding turned out to be the most likely cause of the underperformancebut the reason for the flooding could not be found.

It was difficult to find the exact reasons for these phenomena.

33

A view of the cold storage.

Roof mounted air condensers.

34


During the cyclone (= Hurricane, typhoon) season there is a danger that the air condensers will be damaged.

That would mean a loss refrigerantand maybe a total break down

of the system.

35


The air cooled condensers are thus shut down and an emergency circuit for about half the capacity is activated.

This is composed of an evaporativecooler integrated into the wall and placed

inside the building. The water cools a S&Tcondenser also placed inside the building.

S&Tcondenser

Air exit

Air inlet

Evaporative cooler

36

Closed

Open

Controlled

Outline of the refrigerant circuit.

MP receiverTo/From storage LP receiver

To/From freezer

The main air cooled condensers

During the normal operation, the air cooled condensers operates at the full capacity, about 1760 kW.

37

Closed

Open

Controlled

Outline of the refrigerant circuit.

During the cyclone season the air cooled condenser is shut off from the system.

The valves to the emergency circuit open and connect the S & T condenser to the system.

The S&T condenser takes over for the emergency operation with about half the capacity.

Only one compressor works.

MP receiverTo/From storage LP receiver

To/From freezer

38

10/04/23 39

The essential parts of the condenser circuit.

Problem: The conden-ser is flooded or full of inerts.

To MP receiver

From MP “

That flooding was the basic problem and not inerts became clear when liquid refrig-erant left the vent during a venting.

The flooding could be caused by the much larger liquid volume in the air condensers entering the HP LR & the S&T condenser during the switch-over to the emergency circuit when a cyclone approaches.

As the HP receiver is drained into the very much larger MP and LP receivers by the level controlled HP expansion valve, this was probably not the problem.

It was possible to drain the condenser completely and almost regain the nominal capacity by a forced opening the HP valve.

39

10/04/23 40

The essential parts of the condenser circuit.

Problem: The conden-ser is flooded or full of inerts.

To MP receiver

From MP “

A very high pressure drop was measured between the discharge exit and the HP receiver.

The conclusion was that the condensate exit was too small and this caused the high pressure drop and condensate back-up in the condenser.

40

10/04/23 41

1st solution:

A new condenser with double condensate exits was installed.

No result, the condenser was still flooded.

To MP receiver

From MP “

Double exit condenser.

41

10/04/23 42

To MP receiver

From MP “

Double exit condenser.

2nd solution:

The old condenser was fitted with two, double as large exits.

No result, the condenser was still flooded.

42

10/04/23 43

A closer inspection of the system.

Under condition that:

The equalization line is closed.

The condensate remains subcooled in the LR, i.e. no evaporation from the liquid surface.

The expansion valves keeps the liquid level constant.

The condenser will then drain and there will be a smooth condensate flow from the condenser to out of the LR, despite a high ΔP somewhere in the flow path.

43

10/04/23 44

A closer inspection of the system.

An inspection at the site showed:

The oil coolers of both the HP and LP com-pressors were cooled by refrigerant from the HP receiver in a thermosiphon loop.

The vapour then produced in the HP re-ceiver is supposed to leave by the equali-zation line back to the condenser for recondensing.

The pressure at the condenser inlet is higher than in the LR due to the high ΔP.

To overcome the pressure difference, a liquid column has to build up in the condensate line.

Liquid then floods the condenser.

:

:

Height of liquid column

44

10/04/23 45

Determination of the site of the high pressure drop.

M1

M2

M3

M4

V1

The measurements showed:

M2 – M3 = 1.0 bar

M3 – M4 = 0.1 bar

M4 – M1 ≈ 0 bar

To check the inlet ΔP, the valve V1 inte-rior was removed and the tests repeated:

M2 – M3 = 0.7 bar (without V1)

Over V1 = 0.3 bar (Calculated)

The result showed that the inlet had a very much larger pressure drop than foreseen and this caused the flooding.

45

10/04/23 46

Reasons for the high inlet pressure drop.

The nominal 50 mm inlet was somewhat too small for the required 860 kW

The internal diameter was further reduced to about 43 mm by the threads used at the pressure tests

The resulting velocity was more than 50 m/s, far too high for a high density vapour

This high velocity vapour hit an impingement plate and forced to deflect 90 °.

The result was the very high inlet pressure drop.

46

10/04/23 47

Suggested solution I.

h

By increasing the level difference between the condenser and the LR, the effect of the high ΔP – the flooding - is removed.

47

10/04/23 48

Suggested solution II.

By connecting the equalization line to the service connection, the effect of the high ΔP – the flooding - is removed.

h

48

10/04/23 49

Suggested solution III.

The old and the new condensers are mounted in parallel.

The equalization line is connected to the service connections.

h

49

10/04/23 50

Suggested solution IV.

The equalization line is be connected to the service connection on the S&T condensers.

Either one of the old condensers or a completely new one are fitted with an at least double as large inlet.

h

50