Neal Walsh

The Paths to Leadership in Energy and Environment Design Water Conservation Credits for Evaporative Cooling Towers

Non-Potable Drinking water as an Substitute Resource

In which offered, non-potable drinking water sources can be a excellent way to conserve potable water, and a route to earning 2 LEED credits.   Let’s seem at 4 typical classes of non-potable water: HVAC condensate, rainwater and stormwater, recycled municipal drinking water, and gray water.

HVAC Condensate. Especially in the Southeastern U.S., substantial humidity and significant cooling loads throughout most of the yr affords a superior probable for condensate seize. In Washington, D.C., for example, the condensate capture could be as a lot as 10 gal/cfm of OA every calendar year and in Miami as much as 31gal/cfm of OA every single calendar year, according to a 2021 ASHRAE Journal posting.

HVAC condensate is an great source of make-up h2o for cooling towers for two good reasons. Initially, the timing of the era of condensate from air conditioning units aligns well with the timing of need to have for make-up water for the cooling towers. This alignment suggests that a storage tank may possibly not be required. 2nd, condensate h2o is pure with a very very low dissolved mineral articles. However, a prospective draw back of HVAC condensate is that it in some cases consists of significant metals, this sort of as copper or direct, which could require procedure prior to use as make-up water.

Rainwater. Rainwater and stormwater are typically harvested from roofs and hard surfaces, such as roadbeds or parking tons. Rules change by condition on the use of this sort of drinking water.  This map provides an preliminary evaluation of the feasibility of implementing rainwater and stormwater seize. A 2012 study released by the University of Tennessee concluded that a higher selection of COC can be accomplished with rainwater due to the fact dissolved solids are noticeably reduce than in tap water.

Nonetheless, rainwater pH routinely is all around or down below 6 and hence needs to be mitigated in advance of use in a cooling tower to limit the danger of corrosion and contamination.   Also, regulate of microbiological development have to be included in any water treatment approach where by harvested rainwater is being employed. The level of procedure essential for harvested rainwater depends on the resource. Two typical challenges are bird droppings if the rain is harvested from a roof and oil if harvested from roadbeds and parking plenty.

Recycled Municipal Water. Local municipalities are more and more developing the functionality to reclaim and market handled wastewater (at a appreciably decreased price than potable h2o) relatively than discharging it into a lake or river.  “Purple pipe,” along with appropriate signage, is utilised to distinguish these types of distribution devices from potable drinking water strains.

This water is generally fantastic good quality, despite the fact that the concentration of minerals is normally higher than potable drinking water. An edge is that the greater silica, alkalinity, hardness, and phosphate material in reclaimed h2o are usually significantly less corrosive than faucet h2o. When employing recycled municipal water, h2o excellent management teams will need to assess how corrosion inhibitors from the municipal procedure could influence h2o procedure approaches for cooling tower make-up h2o.

Gray H2o. Unfortunately, normal business sources of grey h2o –- e.g., urinals and laundry –- are not ideal for use as a immediate non-potable h2o source devoid of significant even further cure. Soaps discovered in laundry can be problematic simply because they act as a meals resource for microbiological growth.

Addressing water high-quality difficulties

Depending on the excellent of the h2o out there, and centered upon screening and tips of drinking water therapy experts, the approach for conserving drinking water may possibly involve the implementation of a single or more mitigation techniques. These solutions divide into two standard types: (a) improving upon the water with chemical therapy and filtration, and (b) shielding the technique with supplies of construction that offer you superior protection from corrosion.

Dissolved Minerals. Water with substantial mineral information can be significantly demanding due to the fact high levels of calcium, magnesium, alkalinity, and silica improve the possibility of scale on warmth transfer surfaces, which can swiftly degrade technique overall performance. On the other hand, superior levels of chlorides and sulfates improve the threat of corrosion on many metals made use of in cooling h2o systems, which could lead to enhance upkeep expenditures and minimized asset everyday living. Restricting dissolved solids is vital to attaining boosts in the variety of cycles of focus. Three mitigation techniques are commonly utilized:

Chemical remedy. Scale inhibitor chemical compounds bring about a approach referred to as crystal modification to take place, which softens the hard edges of the crystalline precipitate into rounder content that stays in remedy extended and is much less most likely to type scale. Chemical treatment plans also consist of dispersant polymers that coalesce and agglomerate these softer particles, and by hydrophilic and hydrophobic forces make these particles less very likely to bond to the heat transfer surfaces. These chemical remedies work, but they have limits based on modern chemistry, and are often inadequate to tackle h2o with high mineral written content.

Mechanical Pretreatment. Most services need to contemplate mechanical pretreatment, possibly drinking water softener units or partial reverse osmosis programs or a mix of the two. Water softeners use an ion exchange resin, which collects calcium and magnesium, together with a brine tank, which makes use of salt as a regenerant. These devices are relatively uncomplicated and have a low full price tag of ownership, but they only remove calcium and magnesium and not other dissolved solids like chlorides and they do not reduce alkalinity. Water softeners can enhance cycles of focus from roughly 2 to 4, but seldom allow for the huge leaps required to reach 10 COC.

Partial reverse osmosis (RO) is much more effective.  Water is pushed by way of a membrane, taking away 95-98% of all minerals. Typically RO is used to produce pure drinking water, but that would be much too corrosive for cooling tower apps.  Thus partial RO is applied, which blends the RO h2o with municipal make-up h2o.  For h2o with incredibly high mineral material, the most value-powerful layout is to use a h2o softener to eliminate the hardness minerals prior to the RO process.

Suspended Solids. The focus of suspended solids from various h2o sources and from airborne particulates collecting on the cooling tower facilitates biological fouling and challenges hazardous bacterial expansion.  For this software, cyclonic filtration is extra successful and much easier to sustain than sand filtration.

The most expense-efficient remedy is to style and design and install a facet stream filtration program with basin sweeper piping. The procedure must be manufacturing facility assembled and shipped with the new cooling tower.  (Aftermarket systems involve taking away the fill pack which improves the hazard of hurt to the fill.)

Corrosion. Water conservation approaches often contain elevated corrosiveness of the program drinking water, particularly when utilizing partial RO or large purity non-potable h2o these types of as HVAC condensate and rainwater.

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