A Helpful Guide for Technicians and Administrative Staff to Understand Moisture Transfer and Dehumidification. | CARSI Educational Material

A Helpful Guide for Technicians and Administrative Staff to Understand Moisture Transfer and Dehumidification.

Water: The extra­or­di­nary ver­sa­til­i­ty of water allows it to exist as a sol­id, liq­uid, or gas at ordi­nary tem­per­a­tures. It can trans­form effort­less­ly between these forms as it gains or los­es heat. One of the most sig­nif­i­cant trans­for­ma­tions is the evap­o­ra­tion of liq­uid water into water vapour. We rely on evap­oura­tion to dry any­thing that is wet or moist. How­ev­er, when water vapour con­dens­es back into liq­uid water, it can cause prob­lems such as slip­pery sur­faces and hid­den con­den­sa­tion with­in build­ing mate­ri­als, lead­ing to leaks, cor­ro­sion, decay, and micro­bial haz­ards.

Water Vapour: Although invis­i­ble, water vapour has a pow­er­ful impact on our com­fort and inter­ac­tions with mate­ri­als. Clouds, fog, mist, and steam are not water vapour, but rather accu­mu­la­tions of water droplets or ice crys­tals. Water vapour is sim­ply a col­lec­tion of indi­vid­ual mol­e­cules that have escaped the sur­face of water. These mol­e­cules inter­act with the gas­es in the air and trav­el with them with­in the moist air mass that forms our atmos­phere. The amount of water vapour in the atmos­phere varies depend­ing on loca­tion and tem­per­a­ture, with even the dri­est places con­tain­ing a small trace. Sur­pris­ing­ly, adding water vapour to air makes it less dense and more buoy­ant due to the lighter weight of water vapour mol­e­cules com­pared to nitro­gen and oxy­gen mol­e­cules. The small size of these mol­e­cules also allows them to trav­el through mate­ri­als that would block liq­uid water and air.

Water Vapour Dif­fu­sion: Unlike liq­uid water, indi­vid­ual water vapour mol­e­cules are small enough to pen­e­trate mate­ri­als that would typ­i­cal­ly pre­vent the pas­sage of liq­uids and air. If there is a con­cen­tra­tion gra­di­ent, with more water vapour on one side of a mate­r­i­al than the oth­er, the water vapour will attempt to dif­fuse through the mate­r­i­al towards the low­er con­cen­tra­tion. Some build­ing mate­ri­als are per­me­able to water vapour, and the chart com­pares the rates of water vapour dif­fu­sion through these mate­ri­als. Imper­me­able mate­ri­als like glass and most met­als effec­tive­ly block dif­fu­sion. The dri­ving force for dif­fu­sion is the dif­fer­ence in vapour pres­sure across a mate­r­i­al or between spaces. Each gas in a mix­ture, such as air and water vapour, acts inde­pen­dent­ly and exerts its own vapour pres­sure based on tem­per­a­ture. Water vapour responds to dif­fer­ences in its own par­tial vapour pres­sure, regard­less of the over­all atmos­pher­ic pres­sure. How­ev­er, most water vapour trav­els through acci­den­tal or inten­tion­al gaps and holes, rather than through dif­fu­sion in the build­ing fab­ric.

In humans: When it’s hot, our bod­ies cool down by sweat­ing. But as humid­i­ty ris­es, this cool­ing process becomes less effec­tive. This puts stress on our bod­ies, mak­ing our heart work hard­er to recir­cu­late blood.

4 Effective Methods to Reduce Relative Humidity:

Low­er­ing the rel­a­tive humid­i­ty of moist air can be achieved through four straight­for­ward approach­es, visu­al­ly depict­ed on the psy­chro­met­ric chart.

Ref: Hand­book of Dehu­mid­i­fi­ca­tion | By G.W. Brun­drett.

Increase the tem­per­a­ture: By rais­ing the dry-bulb tem­per­a­ture, the sat­u­ra­tion vapor pres­sure ele­vates, con­se­quent­ly reduc­ing the rel­a­tive humid­i­ty. How­ev­er, the over­all amount of water present remains unchanged.

Har­ness heat pump dehu­mid­i­fi­ca­tion: The heat pump dehu­mid­i­fi­er ampli­fies its effi­cien­cy along con­stant enthalpy lines. This means that the latent heat of the con­den­sate is utilised to warm up the dry air. As cool moist air pri­mar­i­ly stems from the latent heat of water vapour, the poten­tial ener­gy that can be recov­ered is sub­stan­tial. Prac­ti­cal­ly, the dehu­mid­i­fi­er does add sen­si­ble heat to the air pass­ing through it, caus­ing the slope of this line on the psy­chro­met­ric chart to tilt more towards the “heat­ing” side.

Imple­ment mois­ture removal: Sor­bent sys­tems effec­tive­ly elim­i­nate water vapour, thus effec­tive­ly reduc­ing the rel­a­tive humid­i­ty. It should be not­ed that the des­ic­cant heat­ing up may result in a slight increase in dry-bulb tem­per­a­tures. How­ev­er, the ener­gy penal­ty incurred by such a sys­tem is note­wor­thy since the sor­bent needs to be regen­er­at­ed at a high tem­per­a­ture.

Dilu­tion with dry air: The con­ven­tion­al approach to low­er­ing rel­a­tive humid­i­ty involves blend­ing dry air with moist air. If the dry air is cold­er than the moist air, the tem­per­a­ture of the mix­ture will drop. How­ev­er, it’s impor­tant to acknowl­edge that such meth­ods come with a high ener­gy cost.

Refrigerant Dehumidification: Efficient Moisture Removal

Refrig­er­ant dehu­mid­i­fi­ca­tion is a unique method that con­verts the enthalpy of moist air into sen­si­ble heat. By refrig­er­at­ing a heat-exchang­er sur­face below the air’s dew point, mois­ture con­dens­es, releas­ing its latent heat of con­den­sa­tion. The result­ing cold, dry air is then passed over the hot con­denser of the refrig­er­ant cir­cuit. This cycle acts as a heat pump, con­vert­ing the latent heat of con­den­sa­tion into sen­si­ble heat, pro­vid­ing more ener­gy out­put than the elec­tric­i­ty con­sumed.

The effi­cien­cy of the dehu­mid­i­fi­er depends on the rel­a­tive humid­i­ty of the incom­ing air. Cool­ing even slight­ly can cause water to con­dense in 100% rel­a­tive humid­i­ty air. How­ev­er, at low rel­a­tive humidi­ties, the air needs to be cooled to its dew point before any water is released. In this case, most of the refrig­er­ant cool­ing is used to low­er the air tem­per­a­ture, leav­ing less capac­i­ty to remove mois­ture. This makes the cycle less effec­tive at low rel­a­tive humidi­ties. Addi­tion­al­ly, the water extrac­tion rate decreas­es as the room tem­per­a­ture declines, due to a reduc­tion in refrig­er­ant recy­cling with­in the dehu­mid­i­fi­er and a cor­re­spond­ing decrease in elec­tric­i­ty con­sump­tion.

The basic com­po­nents of a refrig­er­at­ed dehu­mid­i­fi­er include the com­pres­sor, con­denser, expan­sion valve, evap­o­ra­tor, refrig­er­ant flu­id, and the fan for air recir­cu­la­tion.

The com­pres­sor takes the refrig­er­ant from the low-pres­sure evap­oura­tor and increas­es its pres­sure, push­ing it into the con­denser. The process con­sists of two con­stant tem­per­a­ture and pres­sure oper­a­tions.

The evap­o­ra­tor, a low-pres­sure heat exchang­er, requires heat from the air to evap­o­rate the refrig­er­ant. This caus­es the evap­o­ra­tor to oper­ate at a very cold tem­per­a­ture, cold­er than the airstream pass­ing through the heat exchang­er. If the evap­o­ra­tor tem­per­a­ture is low­er than the dew­point of the air, some dehu­mid­i­fi­ca­tion occurs.

After com­pres­sion, the refrig­er­ant flows into the con­denser. Since it is hot­ter than the air pass­ing through the con­denser, it con­dens­es and trans­fers its latent heat to the airstream, there­by heat­ing it up.

Refrigerant Authority: Discover the Key to Cooling Success

When it comes to refrig­er­ants, flu­o­ri­nat­ed hydro­car­bons reign supreme. Recog­nis­able by the pre­fix R and a series of num­bers derived from their chem­i­cal for­mu­lae, they are the absolute essen­tials for effec­tive cool­ing.

Rel­a­tive humid­i­ty (RH) is often men­tioned as a mea­sure of water vapour in the atmos­phere, but it is fre­quent­ly mis­un­der­stood. RH tells us the amount of water vapour present in rela­tion to the max­i­mum amount that the cur­rent tem­per­a­ture can hold.

The Impact of Relative Humidity on Comfort and Health

Rel­a­tive humid­i­ty (RH) can pose prob­lems when it’s too low or too high. Low RH lev­els can cause dry­ness of the eyes, skin, and mucos­al mem­branes, lead­ing to dis­com­fort and fatigue. This is often expe­ri­enced by trav­el­ers in pas­sen­ger air­craft cab­ins, where RH is typ­i­cal­ly around 10%. Experts rec­om­mend a min­i­mum RH of 30% and a max­i­mum of 60% for opti­mal human com­fort, with­out con­sid­er­ing tem­per­a­ture. On the oth­er hand, high RH can impede the skin’s nat­ur­al cool­ing process through per­spi­ra­tion evap­o­ra­tion, result­ing in that both­er­some “sticky” sen­sa­tion. Even with­in the rec­om­mend­ed 30–60% RH range, oth­er organ­isms may also thrive, cre­at­ing addi­tion­al chal­lenges.

The con­tin­u­ous air­flow from the air con­di­tion­ing unit to the ther­mo­stat. This image rep­re­sents localised vari­a­tion in sur­face tem­per­a­ture with arti­fi­cial cool­ing.

Thanks for read­ing — we hope this has been use­ful!

The CARSI Team.