Why Does Your LGR Dehumidifier Stop Removing Moisture at 30–35 GPP?
If you’ve been running an LGR dehumidifier on a job, you’ve probably noticed this: grain levels are dropping nicely, and then somewhere around 30–35 grains per pound (GPP), the numbers just flatten out. The LGR keeps running, the airflow is fine, but you’re stuck. What gives?
The Short Answer: It’s Physics, Not a Malfunction
The short answer is thermodynamics and physics. The longer answer is worth understanding, because it will make you a better tech and give you the ability to explain yourself better to all parties involved.
A Note on This Explanation
It is important to note that the explanation used in this article has been simplified in order to keep this from being a long article. This explanation will suffice at comparing the different dehumidifiers and answering the proposed question. The E3 algorithms for evaluating dehumidifier performances in all conditions account for several other variables to arrive at a complete evaluation.
Sensible vs. Latent Load: What Your Dehumidifier Does Before Removing Any Moisture
The answer to this question also explains why LGR’s have a limited chamber temperature operational range between 70-90F (can be higher than this when the grain load is higher). Before any dehumidifier can remove any moisture from the air it must first remove enough energy from the incoming air to drop its temperature to the dew point temperature. This is called the sensible energy load on the DH. Once this is accomplished, any energy removal capacity remaining on the evaporator via the compressor will be used to remove moisture from the air by condensation. This is called the latent energy load of the DH. The less energy required to reach the dew point temperature (sensible load), the more energy the evaporator can remove for condensing/removing moisture from the air or, in other words, the bigger the grain depression the DH will have. Put simply, the smaller the gap (ΔT) between the ambient temperature and the dew point temperature, the more water the same DH will remove from the air.
The BTU-to-Airflow Ratio: Why Every Dehumidifier Has a Limit
Let’s look at the thermodynamics and physics a little deeper to drive this point home. Dehumidifiers in our industry have a ratio of Btu/hr removal capacity of the compressor to airflow (CFM) of 25-30:1. For instance, the Phoenix 250 Max LGR DH has a 9,100 Btu/hr capacity of the compressor and airflow at 365 CFM. This is a 25:1 ratio. The Dri-Eaz 1200 Conventional DH has a 6,800 Btu/hr capacity of the compressor and airflow at 227 CFM or a ratio of 30:1. The energy removal of the evaporators on these DH’s do go up and down based on the overall energy content of the air being pulled through (and all of these variables are factored into the E3 algorithms) but, for the most part, this ratio is set and therefore dictates how much water a given DH will remove (or not remove) in any ambient condition.
LGR vs. Conventional Dehumidifier: Real-World Math at 80°F / 55 GPP
Let’s walk through how both of these DH’s mentioned above would perform in the same ambient/chamber condition of 80F/36%RH @ 55 gpp (sea level elevation). This condition has a dew point temperature of 51F. The Dri-Eaz 1200 is a conventional dehumidifier meaning there is no pre-cooling of the incoming ambient/chamber air. The evaporator must do all of the work in lowering the incoming air from 80F to the dew point temperature of 51F in order to remove any moisture. Here is the math on how it would perform in this condition:
Conventional Dehumidifier Performance: The Dri-Eaz 1200
Like we said, the Dri-Eaz 1200 has a compressor capacity of 6,800 Btu/hr and is pulling through 227 CFM of air through the DH. First we must convert the 227 CFM of airflow into Lb’s of air/hr:
(227 CFM) x (60 min/hr) / (13.9 Ft³/lb) = 980 lbs/hr
The gap (ΔT) between the ambient/chamber temperature of 80F and the dew point temperature of 51F is Δ29F. The dehumidifier will not remove any moisture until this is accomplished first. With this, as well as knowing the specific heat capacity of air at 0.24 Btu/lb-F), we can calculate the sensible energy load of this dehumidifier:
(Δ29F) x (0.24 Btu/lb-F) x (980 lbs/hr) = 6,821 Btu/hr
The Sensible load on this DH in this condition to get to the dew point temperature would be 6,821 Btu/hr and it only has a capacity of 6,800 Btu/hr which means it has reached its limit and has nothing left over for a latent load (or water removal). This means that this 1200 Conventional DH would have a grain depression of 0 in this condition. This also proves and explains why conventional/standard DH’s cannot get the Grain load within a space lower than 55-60 gpp.
LGR Performance: The Phoenix 250 Max
Now let’s look at the Phoenix 250 Max LGR DH in the same ambient condition to see how it would perform. The Phoenix 250 Max LGR has a compressor capacity of 9,100 Btu/hr at 365 CFM:
(365 CFM) x (60 min/hr) / (13.9 Ft³/lb) = 1,575 Lb’s of air/hr
As mentioned before an LGR dehumidifier pre-cools the incoming air either with a heat exchanger or an extra evaporator coil(s). The 250 Max has a heat exchanger which gives free pre-cooling and has a pre-cooling efficiency between 90-95% (92.5% Avg). This means that the DH will use the colder air coming off of the evaporator coil to pre-cool the incoming ambient/chamber air bringing it closer to the dew point temperature, so the evaporator does not have to waste a big portion of its capacity doing so. Here is how it looks mathematically for this LGR DH:
(Δ29F) x (0.24 Btu/lb-F) x (0.575) x (1,575 lbs/hr) = 6,303 Btu/hr
(0.575) = 92.5% efficiency multiplier for heat exchanger
The sensible load on this DH would be 6,303 Btu/hr and it has a capacity of 9,100 Btu/hr. This means that the latent load left over would be 9,100 Btu/hr – 6,303 Btu/hr = 2,797 Btu/hr and this would be used for water removal. With this information we can calculate the exact performance of this DH in this condition. The latent heat of vaporization or condensation is 1,061 Btu/lb. This means, on average, 1,061 Btu’s of energy must be removed from the air stream to condense 1 lb of water. With a latent load of 2,797 Btu/hr we can calculate how much water this DH should be removing:
(2,797 Btu/hr) / (1,061 Btu/lb) = 2.64 lbs of water/hr
Convert this to Grains/hr:
(2.64 lbs/hr) x (7,000 grains/lb) = 18,480 grains/hr
Knowing that we are pulling 1,575 lbs of air/hr through this dehumidifier we can now calculate the expected grain depression of the Phoenix LGR dehumidifier in this condition of 80F/36%RH – 55 gpp:
(18,480 grains/hr) / (1,575 lbs/hr) = Δ11.7 gpp
This means this dehumidifier’s would have a grain depression of 11-12 gpp giving exhausted grains from this dehumidifier at 43-44 gpp.
Why LGR Dehumidifiers Stop Drying at 30–35 Grains Per Pound
Having shown this math between the 2 different types of dehumidifier’s in the same condition, let’s use the same math to drive home why LGR’s quit removing moisture at an ambient/chamber grain load of 30-35 gpp.
Let’s look at the Phoenix 250 Max LGR at an ambient/chamber condition of 80F/21%RH – 32 gpp (sea level elevation). Dew point temperature of 37F. (Δ43F)
Sensible load of this LGR:
(Δ43F) x (0.24 Btu/lb-F) x (0.575) x (1,575 lbs/hr) = 9,346 Btu/hr
With only a 9,100 Btu/hr capacity of the compressor and a sensible load of 9,346 Btu/hr needed to reach the dew point temperature, this means that this dehumidifier would have a grain depression of 0 in this ambient/chamber condition and thus explains/proves why LGR’s bottom/max out at chamber grain load of 30-35 gpp.
Is Your LGR Broken — Or Just Doing Its Job?
So, it is very important to note that it’s not that your LGR is broken or not working properly (in this condition), it is doing exactly what it is supposed to do or capable of. It has just reached its maximum operating/drying capacity. It is also important to note how important this difference is between the conventional dehumidifier’s and LGR dehumidifier’s performances. Because the LGR dehumidifier’s can remove ambient moisture loads down to 30-35 gpp versus 55-60 gpp by conventional dehumidifier’s, this allows LGR’s to be able to dry the same wet building twice as fast as conventional dehumidifier’s with all other variables being the same (and the E3 evaporation metric proves this evaluation). Want to master structural drying performance at a deeper level?
Why the Right Dehumidifier Evaluation Method Matters
It is also important to point out that the E3 algorithms are the only available complete evaluations of ALL dehumidifier types and performances in all ambient/chamber conditions to let the restorer know exactly how their dehumidifiers should be performing. This gives the exact purpose and meaning behind all dehumidifier readings for the first time in all conditions.
Why a 5 GPP Grain Depression Standard Gets It Wrong
Some programs look to evaluate dehumidifier performances in all conditions by looking to have a grain depression of at least 5 gpp. This evaluation is wrong on so many levels. First off, there are many conditions throughout a water loss where an LGR, conventional, and desiccant dehumidifier should have a minimum grain depression of 30-35 gpp or higher. And according to these same programs, they would be fine with a grain depression of 6-7 gpp (because this is greater than 5 gpp) in these conditions even though these dehumidifier ‘s are only performing at a small fraction of where they should be. These same programs would also try to punish restorers in the bill negotiations because their LGR’s do not have the minimum grain depression of 5 gpp even though it has just been proven that this will not/should not be the case when you have done your job correctly and maximized the performance of your LGR’s by pulling the grains per pound in the chamber below 40 gpp (at the best drying temperatures).
The Bottom Line: Know Your Equipment’s True Performance Limits
It is crucial to know how to properly evaluate the performances of all types of dehumidifiers (conventional, LGR’s, HVAC units, desiccants) in all chamber conditions. For industry-recognized drying standards, visit IICRC.org. Currently, E3 (Apple Store and Google Play) is the only set of algorithms that can do this for any restorer.
