a) Blame the manufacturer for a poor quality product and then purchase an exact copy from another supplier only to find that the new coil fails as well?

b) Or, check out if your steam system, or the way that you are operating the heat exchanger, is causing the damage?

This article provides a list of possible causes of failures, plus some calculations that allow the user to check some of the common reasons for problems relating to steam coils.
 

Firstly, let us look at the heat exchanger itself:

1) Check the specification of the heat exchanger

i. For steam coils, the tube wall thickness should ideally be no less than 1.0 mm.

ii. The solder used should contain at least 50% silver, or should be brazed.

iii. The test pressure of the heat exchanger should be no less than 1.5 times that of the working pressure of the steam.

iv. The system steam pressure and temperature should match the figures used in the selection of the steam coil.

2) Header length – these should be kept as short as possible, especially when using long finned tube lengths. Copper elongates about 1 mm every 60 Deg C rise in temperature. The differential expansion directions between tube and header can break tube to header joints as the expansion forces are perpendicular to each other.

The copper tubes need to be brazed into the headers rather than soldered. Brazing provides the strongest joint.

Consider the use of a different material for the headers – for example steel tubing elongates around 1 mm every 90 deg C temperature rise.

General guidelines for steel or stainless headers are that up to 1000 mm length headers are generally ok for any steam coil with copper tubes. A coil with 1 row of tube should be able to take a header length of 1500 mm whilst with 2 or more rows then the headers need to be shorter (less than 1300 mm), to avoid issues.

If the coil has steel or stainless tube material then the length of the header should be kept shorter as these materials do not “give” as much as copper and so more stress is going to be exerted on the tube to header joints.

3) Tube velocity: If this is too high then tube erosion can occur. Although manufacturers use different rules, a tube velocity of 10 m/sec is usually an acceptable safe limit for copper tubes. To check the steam velocity you first need to know the heat duty that the coil is selected to perform. This should be available from the data sheet relating to the heat exchanger. If this information is not provided then there are some calculations at the end of this article that might help you (refer to “Appendix 1: Calculating the Heat Load of the Steam Coil”).

It is important to count how many tubes are connected to one of the headers. Once this has been determined by counting or by working out the coil model code, this number is known as the “circuits” (i.e. it is how many times the total steam flow is divided as it travels through the tube matrix). At the end of this article a method to calculate the minimum number of circuits required is provided. (refer to “Appendix 2: How to determine the Number of Circuits required”)

If the number of circuits counted are greater than indicated by the calculation then all is OK in the design in terms of steam velocity through the tubes.

If the number of circuits counted are fewer than indicated by the calculation then talk to the manufacturer to check the tube velocity and maybe ask them to increase the number if considering purchasing a replacement.

Having gone through the first three points above and if everything checks out, then it looks as though the coil is OK and so the repeated failure issue can be in the way that the coil is being used.
 

Next, A list of things to check in your system:

1) Steam Quality – is only dry saturated steam being sent to the coil?

Partially condensed steam in the pipes supplying the coil needs to be drained off. Water droplets can be picked up by the faster moving steam above the condensate layer and act like “bullets” when inside the tubes, smashing against tube walls causing pin hole erosion.

Water (condensate) entering the coil partially fills the tubes thus leaving less space for the remaining steam to pass through. The velocity therefore goes up and again you are looking at erosion of the tube walls from inside.

Additives in the steam can corrode the copper – check steam treatment chemicals being used do not attack the copper or the solder.

Corroding steam lines supplying the coil can add debris to the steam flow again causing erosion.

2) Is the steam trap working OK and are the condensate lines large enough / properly installed to drain away the water?

If condensate builds up inside the coil then some of the tubes are not going to be open for the steam to enter. Steam velocity then increases in the remaining tubes.

If the coil remains partially flooded after it has been shut down then this trapped water is slow to heat up when the coil is re-used. A temperature gradient is then set up between the open tubes and the flooded tubes. This is because the open tubes are accepting the hot steam whilst the flooded tubes are not. This temperature difference across the coil tubes can cause uneven expansion and can put the coil out of shape (the tube matrix starts to look like an “S” rather than having straight tubes). Over stressing of joints can occur and the coil eventually breaks.

Water hammer can occur inside a partially flooded steam coil at start up, and this alone can break the coil joints.

3) Is the Steam coil oversized for its intended duty?

Oversizing is generally thought to be a good thing as it promotes a “comfort factor” in the design of the plant machinery. However, running a steam coil at more than 30% lower capacity than it is capable of can result in the entering steam taking preferential pathways inside the coil. This is instead of entering all of the tubes available for the steam. When some of the tube circuits are missed out, the velocity inside the working tubes increases, plus you have a coil at different temperatures at various places inside the tube matrix. Temperature gradients / differences give rise to different expansion rates in different parts of the coil and over stressing of joints can occur.

4) Is the steam coil being starved of steam?

Similar to point 3 above, there is not enough steam available to supply the heat exchanger with the correct amount of steam to fill/use all of the available tube circuits in the heat exchanger. This time instead of the control valve throttling back the steam supply, there is not enough steam reaching the control valve in the first instance. The following can be investigated:

i) Check over the steam volume available from your boilers and compare against the needs of the heat exchanger.

ii) Check to see if the available steam is being used elsewhere on other pieces of equipment prior to supplying the heat exchanger in question.

iii) Check also to see if the steam supply pipeline is large enough to handle the volume of steam required. There may be enough steam pressure available, but if there are not enough volume/steam particles then the heat exchanger is going to suffer. At the end of this article there is a quick method of calculating the required steam pipe size to carry the required volume (refer to “Appendix 3: Steam Pipe Sizing”).

5) What is the steam pressure – is it set lower than the initial design condition?

In this energy conscious world, it may be thought that if the Plant can operate at a lower steam pressure than the design condition then less energy is needed to produce the steam in the first place. This is true, but a lower steam pressure has a higher specific volume than higher pressures. A higher specific volume inside the tubes means that for the same heat output, the velocity of the steam through the tubes is greater. High tube velocity equals a greater chance of erosion especially if the steam coil has return bends.

6) Is the coil physically allowed to expand and contract freely?

Rigidly fixing steam supply and return lines without the chance of any movement is asking for trouble. As mentioned before, the steam coil needs to be able to expand. Your connecting pipework must not restrict this movement. This is especially important if the steam coil has vertical tubes with a steam header at the top and a condensate header at the bottom. Rigidly fixing both ends means that when the coil gets hot and expands, this expansion causes the tubes to go sideways as they increase in length between the two fixed ends.
 

Conclusion:

It is always easy and sometimes more convenient to blame the quality of the product and/or the manufacturer. There are cases where the design or the construction is wrong. With more companies employing inexperienced sales teams, and the increased use of software that is not necessarily written to cover all aspects of the selection, then it is possible that the heat exchanger sold to you is not ideal for the application.

However, there are reasons why even a good quality heat exchanger is going to fail and this is due to the way it is being operated, not enough attention being placed upon giving the supplier the most accurate information, and issues with steam supply/removal.

So the moral of the story is – before making that angry telephone call to the supplier, make sure that the system and the way the heat exchanger operates are checked first !

In most cases, Suppliers and Manufacturers are keen to try and resolve any issues, after all it is their reputation on the line. But, as with most things, some knowledge of the potential problems can help the end customer develop a deeper understanding of the way that some items of Plant operate. This can perhaps save everyone time and money.

As a side point, it may be of benefit to consider an air pre-heating coil that uses the heat from the condensate coming off the steam heat exchanger to pre-heat the incoming air. In this way, the actual steam volume required is reduced because a proportion of the heating is being done by the condensate. This is especially useful where production has increased but the steam lines have not, due to costs etc.

Should you have any issues relating to steam coils where you feel that the supplier is not being totally honest with you, or if you want to consider an air pre-heating coil that uses heat from the condensate then send an email to tim@tlsheatexchangers.co.uk and we shall try to help out.

TimSmith Heat Exchangers Ltd – Not just a Heat Exchanger Supplier.

 

Article continues:

Appendix 1: Calculating the Heat Load of the Steam Coil

Appendix 2: How to determine the Number of Circuits required

Appendix 3: Steam Pipe Sizing

 

Appendix 1: Calculating the Heat Load of the Steam Coil

Information required:

1) air flow rate

2) air inlet temperature

3) air outlet temperature

The calculation can be performed a number of ways – we shall look at two calculations, each one uses different units.

1) Calculation Method: Using “Technical” units – these are:

m³/hour for the air flow rate

Deg. C for the temperatures

Kcal/hour for the heat load

Heat duty:

Kcal / hour heat load = AF(m³/hr) x SG (Kg/m³) x SPH (kcal/kg. C) x ΔT

Where:

AF = air flow

SG = specific gravity of air at mean temperature

SPH = specific heat of air at mean temperature

ΔT = temperature difference of air

For most calculations, the air flow is usually expressed as Normal, or Standard.

Normal = specific gravity at 15 C (1.225 Kg/m³)

Standard = specific gravity at 20 C (1.204 Kg/m³)

For most calculations, the specific heat can be taken as 0.24 kcal/kg C

Assuming that the airflow is given as “Standard” we have:

Kcal / hour heat load = AF(m³/hr) x 1.204 x 0.24 x (air outlet temp – air inlet temp)

2) Calculation Method: Using “SI” units – these are:

m³/sec for the air flow rate

Deg. C for the temperatures

KW for the heat load

kW = AF(m³/sec) x SG (Kg/m³) x SPH (kJ / kg. C) x ΔT

Where:

AF = air flow

SG = specific gravity of air at mean temperature

SPH = specific heat of air at mean temperature

ΔT = temperature difference of air.

For most H & V calculations, the air flow is usually expressed as Normal, or Standard.

Normal = specific gravity at 15 C (1.225 Kg/m³)

Standard = specific gravity at 20 C (1.204 Kg/m³)

For most calculations, the specific heat can be taken as 1.004 kJ/kG C

Assuming that the airflow is given as “Standard” we have:

KW heat load = AF(m³/sec) x 1.204 x 1.004 x (air outlet temp – air inlet temp)

Some Conversions:

cfm into m³/sec.	m³/sec = cfm / 2119
litre/sec into m³/sec	m³/sec = litre/sec / 1000
m³/hour into m³/sec	m³/sec = m³/hour / 3600
fpm into m/sec.	m/sec = fpm / 196.85
Lb into kG	kG = Lb / 2.2046
Kcal’s / hour into kW	kW = Kcal’s / 860
Pascals into mm w.g.	mm wg = Pa / 9.80392

 

Appendix 2: How to determine the Number of Circuits required

Number of circuits required can be calculated:

Number of circuits required = Heat duty (kW) / Coefficient

General “rule of thumb” coefficients to use when calculating the number of circuits required:

Steam Pressure set at 2 bar g:

3/8” tubes fitted to the coil: Coefficient is 2.205

0.5” tubes fitted to the coil: Coefficient is 3.969

5/8” tubes fitted to the coil: Coefficient is 6.3

Steam Pressure set at 4 bar g:

3/8” tubes fitted to the coil: Coefficient is 3.5

0.5” tubes fitted to the coil: Coefficient is 6.3

5/8” tubes fitted to the coil: Coefficient is 10

Steam Pressure set at 6 bar g:

3/8” tubes fitted to the coil: Coefficient is 4.69

0.5” tubes fitted to the coil: Coefficient is 10.269

5/8” tubes fitted to the coil: Coefficient is 13.4

 

Appendix 3: Steam Pipe Sizing

Here is a quick check that can be used as a guideline. Note – the following figures are based upon a maximum velocity of 35 m/sec through a mild steel or stainless pipe. In some cases it may be prudent to use a lower velocity. We recommend that a steam specialist should be consulted for further confirmation, especially if you have concerns over the quality of your steam.

Information required:

a) Heat load in KW being performed by the steam coil

b) The nominal bore of the steam pipe supply the heat exchanger

Calculation

Pipe bore diameter coefficient x correction factor = Maximum Heat load (kW)

Pipe bore diameter coefficient to use

Pipe bore inches (mm)	Coefficient
1.0” (25 mm)	95
1.25” (32 mm)	156
1.5” (40 mm)	243
2.0” (50 mm)	380
2.5” (65 mm)	643
3.0” (80 mm)	973
4.0” (100 mm)	1522

Correction Factor to use:

Steam Pressure (Bar g)	Correction Factor
1.0	0.44
2.0	0.63
3.0	0.82
4.0	1.00
5.0	1.18
6.0	1.35

Calculate the maximum heat load possible using the coefficient which relates to the steam pipe nominal bore installed and apply the correction factor for the steam pressure of the system.

Compare to the heat load that the steam coil was selected to perform.

If the calculated maximum heat load is greater than the heat load figure of the steam Coil then this is a good indication that the diameter of the steam supply line is OK and is not limiting the performance of the heat exchanger. If visa versa, then the steam line is not large enough and there is a risk of high pressure drops resulting in the steam condensing in the pipeline before it reaches the heat exchanger.

 
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