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Boiler Lagging Tests

Chuck Hackett, 12/1/01


I ran these tests to determine the effects of adding a jacket and lagging (insulation) to my locomotive.  Most people add a jacket to their locomotive.  One reason is that the prototypes had them, another is that it adds a finished look.  Most people add lagging also since they believe that it conserves heat (fuel) and thus a better steaming locomotive.  Some do not add lagging as they believe that the benefit isn't worth the trouble.  So ...

  1. What is the reduction in heat loss realized by adding a jacket to the boiler with an air gap between the boiler and the jacket?
  2. What is the reduction in heat loss realized by adding lagging under the jacket?

To answer these questions I ran some tests while in the process of replacing the boiler on my locomotive (see UP 844 Catches The Flu(e) to see why it had to be replaced).


The Test Setup

The boiler was off the locomotive frame and setup on one of my engine stands.  This makes access to the burner, jacket and lagging a lot easier.

All water openings were plugged except that the two safety valves were installed and the feed to the cab turret was fitted with a 90 degree turn valve and piping to lead overflow away from the boiler/jacket, etc.

The smokebox was installed including the smokestack assembly and the smokebox door.  The opening in the bottom of the smokebox that accepts the exhaust nozzle was covered with duct tape.  This left a normal gas path and smokebox configuration except that there were many 5-40 size bolt holes left open (smokebox door bolts, etc.).  I didn't feel that these would have a large effect on the outcome.

To provide draft I fitted a U-shaped piece of copper tube into the stack fed from my shop air compressor via an adjustable regulator.

I fitted a type "J" thermocouple probe in the crown sheet inspection port (1/2" NPT) in the backhead.  The tip of the the probe was just above and to the rear of the crown sheet.  This is probably the hottest part of the boiler.  I would have liked to have fitted another thermocouple to measure the water temperature at the front flue sheet also but I didn't have time to set that up.  My thermocouple meter is calibrated in degrees C and only shows whole digits (i.e.: no digits after the decimal point).  If the reading was flipping back and forth I read this as ?.5.  So the data has errors due to the lack of resolution of the meter.


Test 1

For the first test there was no jacket or lagging installed on the boiler.

Test Sequence:

bulletI moved the boiler outside the shop.  The outside temperature was about 50 degrees F.
bulletI filled the boiler until water ran out of the cab turret piping
bulletThe turret valve was left open so there was no (minimal) pressure buildup in the boiler
bulletI lit the burner and set it for a medium-high flame and adjusted draft to suit.
bulletWhile the temperature rose the water in the boiler expanded and the overflow was discharged through the turret piping.
bulletI waited for a steam-only discharge from the turret piping (i.e.: no liquid water being discharged)
bulletI then lowered the burner and draft to a low fire setting and left it at this setting for several minutes.  Hopefully this let the temperature throughout the boiler equalize.
bulletI then turned off the burner, removed the blower tube from the stack and immediately plugged the stack with 2 shop rags and turned off the turret valve to prevent the loss of heat via escaping steam.  There were no observed water or steam leaks from the boiler - hey, there better not be, it's a new boiler!  :-)
bulletI rolled the boiler into the shop as quickly as I could and closed the door
bulletI then recorded the time, boiler temperature and shop temperature (about 62 degrees F) at approximately 5 minute intervals for about 2 hours


PIC00004.JPG (107281 bytes) Here you can see the general test setup with the thermocouple probe fitted in the backhead as well as a flow meter (Plexiglas device) on the propane line.  The boiler is heating up and water is coming out of the turret pipe.
PIC00006.JPG (77165 bytes) This is just before I turned off the burner, plugged the stack and moved the boiler into the shop.

This is the first steam from my new boiler!

Test Results:

Time Time From Start Boiler Temp (C) Boiler Temp (F) Shop Temp (F) Delta T
12:20:00 0:00:00 100.0 212.0 61.0 151.0
12:25:00 0:05:00 99.0 210.2 62.0 148.2
12:30:00 0:10:00 98.0 208.4 63.0 145.4
12:35:00 0:15:00 97.0 206.6 64.0 142.6
12:40:00 0:20:00 95.0 203.0 64.0 139.0
12:45:00 0:25:00 94.0 201.2 64.0 137.2
12:50:00 0:30:00 92.0 197.6 64.0 133.6
12:55:00 0:35:00 91.0 195.8 64.5 131.3
13:00:00 0:40:00 89.0 192.2 65.0 127.2
13:05:00 0:45:00 88.0 190.4 65.5 124.9
13:10:00 0:50:00 86.5 187.7 65.5 122.2
13:22:00 1:02:00 82.0 179.6 66.0 113.6
13:25:00 1:05:00 81.0 177.8 66.1 111.7
13:30:00 1:10:00 79.5 175.1 66.0 109.1
13:35:00 1:15:00 78.0 172.4 66.0 106.4
13:40:00 1:20:00 77.0 170.6 66.0 104.6
13:53:00 1:33:00 73.5 164.3 66.0 98.3
14:07:00 1:47:00 70.0 158.0 66.0 92.0
14:32:00 2:12:00 65.0 149.0 65.5 83.5

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Test 2

For the second test I installed the jacketing but no lagging.  This jacketing covers the entire boiler except for the backhead and about the last 3 inches of the rear outer wrapper.  This is where the cab will be installed later.

The jacket is held off the boiler by steel rings at about 8 inch intervals.  The air gap ranges from about .25" at the smokebox end to about .625" at the cab end.  The lower portion of the firebox (except for the rear, under the cab) is also covered by riveted (to simulate scale staybolts) false firebox sides.  There is an (essentially trapped) air gap of .25" on the lower firebox.

Test Sequence:

The test procedure for this test was identical to test 1.

PIC00008.JPG (95964 bytes) Test 2 warming up.  Entire boiler is covered by jacketing except for the area which will be covered by the cab.
PIC00010.JPG (97461 bytes) A better view of the thermocouple probe installation and the flow meter.  The flow meter is calibrated 0 to 10 GPH of water but it worked fine for setting the burner flow the same in both tests.  Sometime in the future I'll calibrate it in CFM of propane.
PIC00011.JPG (88889 bytes) Test 2, ready to roll it into the shop and start recording numbers

Test Results:

Time Time From Start Boiler Temp (C) Boiler Temp (F) Shop Temp (F) Delta T
16:06:00 0:00:00 100.0 212.0 64.5 147.5
16:10:00 0:04:00 99.5 211.1 64.5 146.6
16:16:00 0:10:00 99.0 210.2 65.0 145.2
16:23:00 0:17:00 97.5 207.5 65.1 142.4
16:27:00 0:21:00 97.0 206.6 65.0 141.6
16:35:00 0:29:00 95.5 203.9 65.5 138.4
16:38:30 0:32:30 95.0 203.0 65.8 137.2
16:45:00 0:39:00 93.5 200.3 65.8 134.5
16:54:00 0:48:00 92.0 197.6 66.0 131.6
17:01:00 0:55:00 91.0 195.8 66.0 129.8
17:06:00 1:00:00 90.0 194.0 66.0 128.0
17:10:00 1:04:00 89.0 192.2 66.0 126.2
17:21:00 1:15:00 87.0 188.6 66.0 122.6
17:25:00 1:19:00 86.5 187.7 66.0 121.7
17:45:00 1:39:00 83.0 181.4 65.5 115.9
18:01:30 1:55:30 79.0 174.2 65.0 109.2
18:17:45 2:11:45 76.0 168.8 64.5 104.3

Note the shop temperature rise in both tests.  The furnace never came on, the boiler was heating my shop which is an insulated 2 car garage!  

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Test 3

For the third test I installed the lagging under the jacketing.  The lagging consists of a powdered form of asbestos that turns to a thick paste when water is added and hardens to something like a grainy, fibrous Plaster of Paris.  It is not dangerous when in its paste form and is entirely enclosed by the jacket after it has been installed.  Again, the jacketing (and lagging) covers the entire boiler except for the backhead and about the last 3 inches of the rear outer wrapper.

The lagging varies in thickness from about .25" at the smokebox end to about .625" at the cab end of the boiler.  The lower outside portion of the firebox is also covered to a thickness of approximately .25".

Test Sequence:

The test procedure for this test was identical to tests 1 and 2.

To see how the lagging was installed you can click here.


Test Results:

Time Time From Start Boiler Temp (C) Boiler Temp (F) Shop Temp (F) Delta T
21:20:00 0:00:00 100 212.0 63 149.0
21:25:00 0:05:00 99 210.2 63.5 146.7
21:30:00 0:10:00 98 208.4 63.7 144.7
21:35:00 0:15:00 97 206.6 63.9 142.7
21:40:00 0:20:00 97 206.6 63.9 142.7
21:45:00 0:25:00 96 204.8 64 140.8
21:50:00 0:30:00 95 203.0 64 139.0
22:00:00 0:40:00 93 199.4 64 135.4
22:11:00 0:51:00 91 195.8 64 131.8
22:29:30 1:09:30 88 190.4 64.5 125.9
22:51:00 1:31:00 84 183.2 65 118.2
23:11:00 1:51:00 81 177.8 65 112.8
23:34:30 2:14:30 78 172.4 65 107.4

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Comparing Tests 1, 2 and 3

I have combined the graphs from the three tests and shifted them in the time domain to bring the 100 degree centigrade boiler temperature point to time 0.

It's obvious that adding the jacket (forming a trapped air gap) made a difference.  It's also very obvious that adding the lagging material added no benefit in reducing heat loss.  It was no better than a trapped air gap.

Two additional data items needed for this calculation are the fact that there is approximately 500 pounds of steel (and minor amount of copper for the flues) in the boiler and for each test there was 15.4 gallons of water in the boiler.



It is of benefit to add at least a jacket around the boiler to form a trapped air gap.

Even though the lagging is not providing me any benefit in thermal terms it does provide a hard foundation for the jacket and minimizes chances of inadvertently denting the jacket.


Next Steps

I still need to calculate the R-value of the jacket/air gap combination and the expected fuel savings.

I'll post the results here when I get them done.  Email me if you would like to be notified when the page is updated.


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