In Hvac Systems
Buildings consume over 20% of the world’s water and HVAC systems form a bulk of the use. An effective water treatment system ensures that minimum amount of water is used in the HVAC system to obtain the desired outputs...
- Aneesh Kadyan
Water is a critical resource in building operations – it is used in a number of building systems such as domestic water systems, Drinking water, landscaping and horticulture, and the major utilities such as Diesel Generators and the HVAC system.
The quality requirements are different for different uses in the buildings and adherence to the laid down water quality is essential to ensure correct functioning of the equipments as well as to enhance the life of the machinery and systems.
While the need for quality water is understood by the operations' team, in practice, there is often not enough understanding of the water quality as well as importance of the correct water hygiene, which leads to a number of maintenance related issues, which can impact building services uptime and availability. HVAC systems in buildings consume large amounts of water and the optimum operation of these systems is linked to the quality of the water in the system.
Overview of Water Quality in the Building Environment
In a water cooled HVAC system, there are two types of water systems that are used.
Closed Loop Water System
This system is used for the chilled water circulation within the evaporator and AHUs. As the name suggests, closed loop systems are not exposed to the external environment and once a system is charged, there is usually very little water that is needed to be added to the system. As the system is isolated from external contamination agents, the water quality at the time of filling up the system is critical and thereafter only a water quality is usually maintained to reasonable levels. The closed loop system does not have problems of scale formation as there is no evaporation from the system. In addition, the system being a closed loop eliminates contact with air, which further prevents formation of microbiological contaminants in the system.
Corrosion of system pipe internal surfaces is also drastically reduced due to the absence of oxygen in the system. Care is thus required at the time of initial charging of the system and when there is need to add water due to a leak in the system of during routine draining of the system for preventive maintenance.
Open Loop Systems
The condenser water system is an open loop system where water used in the condenser is passed through the cooling tower in contact with air for cooling. Due to the external contact, there are additional challenges for water treatment and quality. The key issues that the operations team encounters related to water treatment and condition in open loop systems arepresence of dissolved solids, corrosion and microbiological contamination.
Water evaporates in the cooling tower, which in turn increases the concentration of salts in the condenser water circuit. These dissolved solids end up as scale formation on the piping surface and condenser heat exchange surfaces. The effect of scale on the pipe surfaces is an increase in the resistance to flow, resulting in higher pumping power requirements. Scale formation on condenser tubes reduces the heat transfer and consequently the condensing temperatures increase, impacting the efficiency of the HVAC system as a whole. To offset the loss, 'make up' water is used which has a better water quality. Thus, introduction of makeup water in addition to replenishing the lost water in the system also lowers the concentration of solid particles in the system. To improve water quality, a pre-determined quantity of water is also removed from the system as 'blow down' so that the concentration of solids is reduced further. The effectiveness of the water management programme is seen in the water hardness and alkinity of the circulating water.
Various factors can cause corrosion in the piping of an open loop system. Localised Catholic and Anodic sites form due to impurities in the metal or defects on the piping surface. The electrolyte to complete the transfer of ions from the cathode to the anode is the cooling water, hence, the quality of water is very important to prevent corrosion. Corrosion of pipes causes degradation of the metal surface and can lead to leakages – and if not checked, leads to failure.
Micro Biological Contaminants: Bacteria and Algae enter into the system through the open cooling tower as well as the makeup water. Interaction of these contaminants with the cooling water results in formation of slime, which creates a film over the piping surfaces. The impact of this film of slim is called fouling of the surfaces and leads to a loss of heat transfer effectiveness. In addition to fouling, micro biological contaminants also cause corrosion, which is referred to as Microbiological Induced Corrosion (MIC), which can be more severe than the metallic corrosion. The legionella bacteria are another biological contaminants that can have serious consequences. The legionella bacteria can grow in the cooling tower water and infect humans if not treated.
Water Treatment Program
Poor water quality has a direct impact on the effective operation of the HVAC system, leading to higher operating costs due to increased maintenance requirements and lower plant operating efficiencies. A robust water management plan is thus essential to keep the system operating at design efficiency levels as well as manage operating costs within budgets. The first step in developing a water treatment programme is to understand what 'good water' is. Table 1 lists some of the key parameters that constitute good water.
A robust water treatment programme consists of both a preventive approach and a measurement process. The key approaches used for keeping water quality to the desired levels are described hereafter.
There are three ways in which corrosion can be prevented in piping systems:
- Passive Inhibitors which form an anti-corrosive layer over the metal surface
- Perceptive Inhibitors such as Zinc form complexes, which precipitate out of the water and prevent corrosion
- Absorption Inhibitors are organic compounds that attach with the metal surface and prevent corrosion through complex mechanisms.
Microbiological Contaminants: Since the contaminants enter the system from the environment, the first step in control of such contaminants is to have a clean system. This includes regular draining of the system as well as effective cleaning of the cooling tower sump where the contaminants form. Chemical inhibitors are also effective in reducing the incidence of micro biological contaminants.
A key requirement of keeping this type of contaminant is to ensure that inhibitors are in the system at all times as once the system gets contaminated, removal is very difficult. Thus, a continuous dosing system is essential to keep microbiological contaminants within limits.
Dissolved Solids: This type of contaminant impacts the system most as there is a direct impact on the system efficiency through scaling and increased pumping requirements. Blowdown of cooling water from the cooling tower is used to keep the level of dissolved solids to acceptable levels. The amount of blow down is arrived at based on the 'cycles of concentration,' which is a ration of the makeup water and the amount of water bled from the system. The cycles of concentration is usually kept between 5 – 10 to keep a balanced mix between the amount of makeup water and the dissolved solids.
The blowdown can be either undertaken manually in a constant amount at laid down frequencies or through an automated system which is linked to a measurement system that measures the water hardness in the system.
Annual Maintenance Programmes
In addition to the chemical prevention approaches, annual physical cleaning of the system is essential to clear the pipe surfaces of accumulated solids and scale. Cleaning is usually undertaken using mechanical systems or chemical solutions.
Water is a scarce resource and it is essential to conserve water both from a sustainability point of view as well as cost perspectives.
Buildings consume over 20% of the world’s water and HVAC systems form a bulk of the use. An effective water treatment system ensures that minimum amount of water is used in the HVAC system to obtain the desired outputs.
Lower blowdown leads to lesser water use, and also reduces the amount of contaminated water from going into the drain water system. Operations and maintenance teams thus need to understand that water quality is very important to not only ensure that the system works effectively – but also to reduce the impact of the HVAC system note environment.
Aneesh Kadyan is Director - Operations, CBRE South Asia Pvt Ltd., Asset Services - India.
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