In order to operate at peak efficiency, every steam heating coil in a dry kiln must fully
purge the condensate that forms within it. Failure to do so results in a condition known
as waterlogging, where a portion of the coil becomes flooded with condensate.
Waterlogging is recognized as an undesirable condition that leads to uneven heat
distribution and increases the corrosivity of condensate. This paper reviews a unique
method to detect waterlogging, and assesses its impact on the lumber’s moisture content

Brint, Marty

Carroll, Steven W.

Western Dry Kiln Association

Western Dry Kiln Association. Meeting (56th : 2006 : Portland, Or.)

Undetermined

ScholarsArchive@OSU

W estern Dry Kiln Association May, 200616DETECTION OF WATERLOGGING IN KILN COILSAND ITS EFFECT ON MOISTURE CONTENTSteve CarrollWatercare Industrial ServicesWashougal, WashingtonandMarty BrintHampton Lumber Co.Willamina, OregonIntroductionIn order to operate at peak efficiency, every steam heating coil in a dry kiln must fullypurge the condensate that forms within it.   Failure to do so results in a condition knownas waterlogging, where a portion of the coil becomes flooded with condensate.W aterlogging is recognized as an undesirable condition that leads to uneven heatdistribution and increases the corrosivity of condensate.   This paper reviews a uniquemethod to detect waterlogging, and assesses its impact on the lumber’s moisture content.The Thermal Properties of SteamSteam is a remarkable material for transferring energy from one place to another, and it’suseful to understand steam’s basic thermal properties to evaluate its behavior in a heatingcoil.The first important concept concerns sensible heat and latent heat.   As water is beingheated in a boiler, its temperature rises toward the boiling point.   For each pound ofwater, it takes one BTU (British Thermal Unit) of energy to raise the water’s temperatureby one degree Fahrenheit.   W e refer to this energy as sensible heat.   W hen we reachthe boiling temperature of the water, we have to provide about one thousand times asmuch energy to convert that same pound of liquid water to gaseous steam.   In effect, wehave supercharged the steam with this extra energy, known as latent heat.   So, latentheat is the energy associated with a material’s conversion between a liquid and a gas.Once we’ve produced the steam, we can pipe it wherever we need for heat.   W e thenrelease the steam’s latent heat energy by allowing it to condense back into liquid form .Another important characteristic of steam is that its temperature and pressure are closelyrelated and very predictable.   If we know the pressure of dry, saturated steam, we canaccurately predict its temperature, and vice versa.Steam in the Heating Coil, and Detection of WaterloggingFrom our discussion above, we can see that in order to transfer the maximum amount ofenergy to the heating coil, the steam has to condense within the coil.   Furthermore, youwant the steam to condense throughout the entire coil, to transfer latent heat acrossthe coil’s entire surface.   And, finally, because the steam pressure is considered to beequal throughout the coil at any given time, we should expect the temperaturethroughout the coil to be uniform .W estern Dry Kiln Association May, 200617These conditions form the basis for our waterlogging detection method:   In any region ofthe coil where condensate has accumulated, steam is excluded.   Therefore, no activecondensation is taking place in that region of the coil, and no latent heat is being releasedthere.   One should expect, then, that the coil’s surface temperature in the flooded regionof the coil will be lower than the region that contains steam.   Our study shows thatmeasuring the temperature differences between several locations on a coil confirms thepresence and relative position of the waterlogged area.Typical Causes of WaterloggingIn a coil that’s operating properly, any condensate that forms is quickly pushed throughthe steam trap at the bottom of the coil, leaving the entire coil available for steam.Basically, waterlogging occurs because condensate is unable to leave the heating coil.W e’ve identified a few reasons for this:1) Back Pressure on the CoilIf the condensate has to overcome excessive pressure downstream of the steam trap, thecoil can waterlog.   Condensate piping that runs uphill is the most common example.Another example involves a condensate header pipe that is shared by several kilns, wherethe piping isn’t adequately sized to handle the discharge from all kilns without pressurebuildup.   This condition is exacerbated by leaking steam traps which allow higher steampressures to enter the condensate header, creating more back pressure on other traps.2) Undersized and/or Plugged Steam TrapsIf the volume of condensate produced cannot pass through a restriction in the opening ofthe steam trap, the coil will waterlog.   The selection of the correct trap, as well as routinetesting of traps to detect failures, is crucial to their successful performance. 3) Insufficient Steam PressureThe condensate relies on the pressure of the steam to push it through the trap.  Lowsteam pressures can, therefore, cause waterlogging.   Almost all kilns are vulnerable tothis condition during idle periods, where a “closed” steam valve allows a small volume ofsteam to leak by.   Condensate will form in the coil, but there’s insufficient steam pressureto push it out.   Insufficient steam pressure is particularly troublesome if combined withconditions of back pressure and/or trap restrictions.Conducting a TrialFor purposes of this study, we selected a kiln with certain features that could besuspected to cause waterlogging.   Specifically, we chose a 104’ packaged, double-trackkiln with center booster coils having horizontal fin pipe.   These booster coils were of thetwo-zone variety, with upper and lower positions.   A lower coil was used for this test.Circulation fans were cross-shaft, moving approximately 1000cfm.  The kiln usesArmstrong #214 bucket traps.   The condensate piping involved an overall vertical lift of13’ 6” relative to the coil’s lowest piece of fin pipe.   The resulting net static head (backpressure) at the bottom of the tested coil was calculated to be approximately 5.6psi. Steam pressure to the heating valves was approximately 90psig.   Four thermocouple probes were secured to the coil at positions shown in Figure 1.Specifically, the probes were mounted on the top of the selected fin pipes, at the inlet(steam) side of those pipes.   It is felt that this positioning avoids the normal temperatureW estern Dry Kiln Association May, 200618depression expected on the bottom of the fin pipes as condensate runs along their length.In other words, if any of the probes detected significant temperature depression, it couldbe legitimately regarded as due to waterlogging.FIGURE 1.  Location of probes on coils shown with black squares.Temperatures were recorded throughout the lumber drying schedule (about 33 hours, 30minutes overall) at one minute intervals.   The results were very revealing.   Below is abrief summary of our observations and interpretations.   Pertinent graphs follow the text.There was relatively little error between the individual thermocouples.   That allowed usto confidently regard significant temperature differences as evidence of waterlogging.At the beginning of the schedule (Figure 2), at least 38” of coil height started out as beingwaterlogged (following an extended idle period).   It took almost thirty minutes for the coilto completely purge its condensate.  During that time, the bottom fin pipe wasapproximately 130/F cooler than the rest of the coil.FIGURE 2.  Temperature on coil during first hour of operation.  W aterlogging is evidentfor about 30 minutes.W estern Dry Kiln Association May, 200619As the dry bulb temperature (measured independently) in the kiln increased, the surfacetemperatures of the coil decreased.  This is consistent with normal observations of aheating valve, which throttles steam flow as the drying schedule progresses.   The lowersteam pressures associated with the reduced coil temperatures became a factor in thesubsequent waterlogging that was observed.W aterlogging of the coil again became evident less than 14 hours into the schedule(Figure 3), affecting at least the bottom fin pipe.   By the 17-hour mark, the bottom piperemained waterlogged for the remainder of the schedule, with the exception of steamspikes when fans were reversed.W aterlogging affected at least the bottom four fin pipes (16” of coil height) in less than 21hours; they remained largely waterlogged for the remainder of the schedule with theexception of the brief spikes of fan reversal (Figures 4 and 5).FIGURE 3.  Temperature on coil during entire schedule.  W aterlogging becomes evidentafter about 12 hours when the pressure in the coil decreases.FIGURE 4.  Temperatures on coil magnified.  At each fan reversal all four temperaturesbecome equal indicating that the coil drains when the valve opens and the pressure in thecoil increases.W estern Dry Kiln Association May, 200620During the final three hours of the schedule, at least 38” of coil height was affected bywaterlogging.   Since the “T2” thermocouple was intentionally placed at the approximateheight of the coil’s RTD temperature sensor, the RTD’s throttling of the steam valve couldbe readily seen in the graph.FIGURE 5.  Coil temperatures at end of schedule.  The water level reaches the thirdthermocouple from the bottom as the coil pressure reduces.Somewhat surprising was the unexpectedly low temperature drop associated with awaterlogged condition in later stages of the schedule.   W e initially expected a dropseveral times greater than the ~10/F observed, but in retrospect we realized thatcirculation of heated air (from the overhead coils) over the waterlogged coils wouldprevent a large temperature drop.  Nevertheless, we recognized that even a smalltemperature drop would have dire consequences on drying rate, as explained furtherbelow.Moisture Content EvaluationThe temperature measurements were made while drying 2” x 6” Douglas-fir, with dry bulbsetpoints of 170/F to 180/F and depressions from 10/F to 25/F.   The targeted moisturecontent during the hot check was 15-16%.After the lumber was pulled from the kiln and allowed to cool, moisture measurementswere taken of boards that had been adjacent to each of the thermocouples.   For eachthermocouple location, 24 boards were checked from each side of the booster coil.   Theresults of those tests are shown on Diagram 1, represented by eight pairs of numbers tocoincide with thermocouple height and side of the coil.   The top number of each pair isthe average moisture content of the 24 boards, while the bottom number is standarddeviation of that set.The range of measured moisture contents is significant, and the sequential increase inmoisture content of lower boards certainly implicates the observed waterlogging of thecoil.The waterlogging is further implicated by reviewing a table that compares dry bulbtemperatures and wet bulb depressions to determine EMC (equilibrium moisture content).For example, a dry bulb setpoint of 180/F with a depression of 15/F produces an emc of9.0%.   If part of the steam coil is producing an air temperature only 10/F lower than drybulb setpoint, the depression associated with that air becomes only 5/F, producing anW estern Dry Kiln Association May, 200621EMC of 14.8%.   This confirms that a relatively small drop in coil surface temperature cansignificantly impair drying performance, exactly as represented by the collected data.Additional Observations and CommentsThe authors believe that the waterlogging identified in this study is primarily due toelevated condensate piping downstream of the coil, and its associated back pressure.Some additional speculation can be made about quantifying the effect of this backpressure.As stated earlier, the calculated static head above the coil is about 5.6psi.   Therefore, atleast 5.6psig within the coil must be present to purge the condensate.   W hen you relatethat pressure to steam temperature, you can conclude that the steam temperatureinside the coil must be at least 229/F in order to purge the coil of condensate.  Our testmeasured coil surface temperature, which is expected to be slightly lower than the coil’sinternal temperature.   Nevertheless, it is reasonable to state that the last half of thisdrying schedule didn’t continuously require an internal coil temperature of 229/F in orderto reach its 180/F dry bulb setpoint.   Otherwise, the coil wouldn’t have waterlogged asobserved.Therefore, we predict that, in this kiln, if you attempted to eliminate waterlogging bymoving the coil’s RTD to near the bottom of the booster coil, you would produceexcessively high air temperatures above the RTD, resulting in overdried lumber.   For kilnschedules that permit higher dry bulb setpoints, however, it may be possible to reducewaterlogging by lowering the booster coil’s RTD.  Each kiln and schedule must beevaluated separately for such a determination.Hopefully, this study will help eliminate the myth that a high steam supply pressureto the heating valves will prevent waterlogging.   Modern kiln controls will modulatethose valves to produce very low coil steam pressures, which are frequently insufficientto purge the condensate.Although not directly related to its effect on the lumber’s moisture content, we’recompelled to stress that waterlogging of a coil will frequently increase the corrosivity of thecondensate, resulting in irreversible coil damage.  The severity of this corrosion isinfluenced by a number of factors such as water chemistry and pretreatment equipment.Any suspicion of waterlogging should be given serious consideration by a treatmentconsultant. The authors feel that this study produced remarkable and credible information.   Even so,we realize that this topic will benefit from additional testing, which could includemeasurement of the coil’s internal temperature and pressure, as well as pressuremeasurement on the outlet side of the steam trap.   Comparison tests that involvechanges to condensate piping and steam traps will certainly be useful.   Ultimately, wehope that the reader will become alert to the causes of waterlogging, and take steps toavoid them.

Detection of waterlogging in kiln coils and its effect on moisture content

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Detection of waterlogging in kiln coils and its effect on moisture content

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