A Desuperheater Regenerator

     The most common way to regenerate a liquid desiccant is to heat it.  As a desiccant’s temperature increases, its equilibrium vapor pressure with water increases exponentially.  Since regenerating a liquid desiccant is equivalent to evaporating the water (i.e., water converts from a condensed state to a vapor state), higher equilibrium water vapor pressure leads to higher regeneration rates.

     As described elsewhere on this website (and on the websites of others), a desiccant can be regenerated using the warm air exiting a vapor-compression air conditioner’s (VC-AC) condenser.  Although the regeneration Coefficient of Performance (COP) for this process will be low—typically less than 0.5*—the quantity of available condenser heat is large.  This combination of low COP but large available heat can produce a meaningful Water Removal Rate, but it comes with a price—a condenser-air regenerator must handle a large volume of air.

     AILR’s patent-pending desuperheater regenerator will be an attractive alternative to a condenser-air regenerator.  Just how attractive is illustrated by the following example in which we use a liquid desiccant to increase the latent cooling of a 30-ton roof-top unit (RTU) by 40%. 

     At the AHRI A rating conditions and a 12,000-cfm supply, our modeled R410a RTU delivers 360,000 Btu/h total cooling, 76,400 Btu/h of which is latent cooling (i.e., the Sensible Heat Ratio—SHR—for the supplied cooling is 0.788).  Saturated refrigerant temperatures within the evaporator and condenser are 52 F and 120 F, respectively, and the temperature of the refrigerant flowing from the compressor into the desuperheater is 170 F.

     For nominal RTU operation, the latent cooling is equivalent to 72.8 lb/h water removal.  A 4”-deep pad of corrugated contact media located behind the evaporator, with a face area that matches that of the evaporator, and flooded with 1.8-gpm of 19% (by weight) lithium chloride will absorb 28.8 lb/h of water from the nearly saturated air leaving the evaporator—a 40% increase in latent cooling.  The heat released by this absorption will raise the air’s temperature from 58 F to 62 F.  There is a small loss in total cooling due to the effect of the released chemical heat of mixing between water and the warm desiccant supplied to the pad, but the overall effect is close to a lossless conversion of sensible cooling into latent cooling with the SHR for the overall cooling decreasing to 0.688. 

     The relatively weak desiccant required in this example can be supplied by an extremely compact desuperheater regenerator.  With an 60% effective desuperheater, desiccant will be supplied to the regenerator at 137 F.  A scavenging-air regenerator composed of a desiccant-wetted pad of contact media that is 24” tall, 5” deep and 7” wide through which flows 220 cfm of condenser cooling air will provide the required water rejection.  For many RTUs now on the market, this desuperheater regenerator could be retrofit into the RTU’s condenser/compressor cabinet.  In a preferred design, the scavenging air is first filtered before passing through the desiccant-wetted contact media with a small blower exhausting the air to ambient.  Although located within the outdoor condenser/compressor cabinet, this forced-air assembly would be ducted to protect it from rain intrusion. 

     The alternative conventional approach to increasing the latent cooling for the RTU would be to lower its evaporator temperature. With no change to the evaporator and condenser coils, increasing compressor tonnage from 27-tons to 33-tons will decrease the evaporator temperature at full load from 52 F to 50 F. The lower evaporator temperature increases latent cooling by the required 30,200 Btu/h--but sensible cooling also increases 21,600 Btu/h, so the RTU’s SHR remains high—0.741—unless a reheat coil is used.  A reheat coil can reduce the SHR to the LD RTU's 0.688, but the loss in total cooling reduces the EER for the overcool/reheat DX RTU to 9.0--a 21% efficiency penalty compared to the LD RTU.  

  

* The thermal COP for regeneration compares the amount of energy driving the regeneration with the amount of energy for converting an equivalent amount of water from liquid to vapor; a thermal efficiency of one indicates one pound of water is released by the desiccant for every 1050 Btu of thermal energy driving the process.