Using Control Valves To Optimize Cooling Water System Efficiency
The expense of treating water for use in cooling towers, boilers, and other plant applications is rapidly increasing. In addition to high costs, plants often face the need to comply with regulations governing effluents, including cooling water, sent through treatment facilities before being discharged.
Optimum operation of cooling water systems means minimum use of water while maintaining proper temperatures to limit algae growth and cool all equipment properly. One way to help achieve these goals while significantly reducing energy consumption is to install cooling water (CW) control valves.
A well-balanced system
Operating a cooling water system efficiently requires balance. A well-balanced system is one from which short-circuiting is eliminated. Short-circuiting occurs when excessive cooling water flows through one cooler causing insufficient flow through the others. This starving often occurs at the end of a system or in units at higher elevations.
Achieving a balanced system is a detailed and complicated process. Pressure drops must be figured for each piece of equipment and its associated piping, and for each branch of the cooling water circuit. Even if these calculations are done when a plant is new, conditions change over time. Deposits build up on surfaces altering the heat transfer coefficient and resistance to flow (pressure drop). Adding or removing equipment from the system also changes the balance and can lead to short circuiting.
Manually balancing a cooling water system using orifice plates is difficult and time consuming. Safety concerns often dictate that an orifice be sized for maximum demand. As a result, the cooling water pump must be sized for, and often must operate at, excessively high flow rates.
Sometimes attempts are made to balance a system by using a globe valve and manually throttling the flow. Unfortunately, this approach often leads to an operator opening the valve fully when maximum flow is needed, then never readjusting it. Again the result is high flow when the system requires average or minimum flow.
Too much flow is indicated by a cooling water outlet temperature only a few degrees above the inlet. This unbalanced flow condition leads to higher pump energy consumption and distant or elevated areas that are often starved for water.
Higher cooling water return (outlet) temperatures result in lower cooling water consumption. Under these conditions, cooling water temperatures should be increased to the maximum permitted by the process. This increase is accomplished by minimizing the flow. But before this action is taken, other conditions must be evaluated.
Higher temperatures (above 120 F) can cause calcium to precipitate out of water at a high rate, resulting in scaling and leading to increased pressure drop and reduced heat transfer. Increased temperatures also promote algae growth. The rate varies with quality of water and type of treatment.
Distributing cooling water throughout a system requires proper controllers that maintain outlet temperatures within a specified range, even during partial cooling. If outlet temperatures cannot be increased, controllers can still reduce the flow when water requirements drop.
Control valve configurations
Control valves are successfully applied in a variety of cooling water systems. In most systems, a proportional CW control valve can be installed in the return line (Fig. 1). The valve, which controls the water flow rate in direct proportion to the outlet temperature, should be located as close to the cooler as possible.
When the cooling water is cold, the valve reduces the flow rate to a slight bleed. As the outlet temperature rises, the valve opens and regulates the flow to maintain a constant discharge temperature. The CW valve should be designed to maintain a constant bleed flow. Without some flow, the valve sensing element cannot tell what is going on.
Use of CW control valves ensures automatic balancing of the cooling water system, because the valve uses only as much water as the cooler requires. Reduced water use ensures an adequate supply of cooling water is provided, even to areas far from the cooler or at higher elevations.
Maintaining a process temperature at a precise value requires a different control scheme. A temperature sensor (thermocouple), controller, and pneumatically or electrically actuated control valve can be used. Another option is a self-acting control valve with a capillary tube (Fig. 2) inserted in the process stream. Either arrangement controls process stream temperatures with varying degrees of accuracy. In many cases, the self-acting valve offers reasonable accuracy at a lower installed cost.
In applications that have an open discharge to a drain, the CW valve discharge line should always be full. This condition can be ensured with a loop seal at the outlet piping at an elevation above the valve that then goes to grade (Fig. 3). Without a liquid seal, lines may empty when equipment is shut down. Liquid or wax-filled thermostatic seal elements can dry out and fail prematurely.
A strainer may be installed upstream of the CW valve if water quality conditions require. Dirt and debris affect proper closing and stroking of the valve. If this problem exists, a pneumatic override (Fig. 4) can be used to purge the valve of dirt.
The best way to control algae growth and buildup is to maintain high water quality. Other factors contributing to algae growth include temperature and velocity. Keeping the temperature below 120 F is desirable. Sizing the flow rate to achieve a higher velocity also tends to hinder algae growth. Algae slime has been known to form in lines with velocities that can reach as high as 10 ft/sec.
The cooling water side of a process is often overlooked as uncontrollable. However, cooling water control valves can promote savings by reducing the use of water, pump energy requirements, and water treatment costs. In new construction, smaller pipe and pump sizes can lower capital equipment costs. CW valves also provide better process control by maintaining a fixed temperature difference across the cooling water inlet and outlet. In most cases, analyses justify the installation of such controls.
— Edited by Jeanine Katzel, Senior Editor, 847-390-2701, email@example.com
The author will answer technical questions about this article. He may be reached by phone at 201-403-1556 or by mail in care of his company, 10 York Ave., West Caldwell, NJ 07006.
Optimal operation of cooling water systems helps limit algae growth and cool equipment properly while maintaining proper temperatures.
Cooling water control valves reduce water use, pump energy requirements, and water treatment costs.
Water and energy savings typically provide rapid payback on the valve system investment.
Justifying the costs
Savings in cooling water and energy typically provide a rapid payback on the valve system investment. Cooling water control valves also reduce capital costs of a new installation by allowing use of smaller pumps and filters, and, in some cases, reduced pipe sizes.
An example of savings achieved in a retrofit system is shown below.
Q = heat removal rate of cooler, 700,000 Btu/hr
T(sub i) = inlet cooling water temperature, 50 F
T(sub o) = outlet cooling water temperature without control, 59 F
C(sub p) = specific heat, 1 Btu/lb/deg F
m = mass flow rate, lb/hr
v = volumetric flow rate, gpm
m = Q/C(sub p) (T(sub o) – T(sub i) ) = 700,000/1(59 – 50) = 77,777 lb/hr
v = 155.5 gpm
(For v, to convert lb/hr to gpm divide by the conversion factor of 500, which is arrived at by multiplying 8.33 lb/gal. of water by 60 min/hr.)
After a CW valve is installed, the discharge temperature can be set to 82 F. Inserting the new T(sub o) into the equation yields:
m = Q/C(sub p) (T(sub o) – T(sub i) ) = 700,000/1(82 – 50) = 21,875 lb/hr
v (new volumetric flow rate) = 43.7 gpm
In a system in which water is not recirculated, water use drops 72%. In a closed loop system, a certain amount of water is lost to evaporation in the cooling tower and during blowdown. Water treatment costs also must be taken into account in the analysis.
In addition to water savings, energy is conserved because less power is needed to pump less water. The pump energy savings figure shows three discharge head charts. Efficiency and power consumption of a typical centrifugal pump are plotted against volume displacement. Note that even with a reduction in efficiency, power consumption is 6.5 kW before the control valve is installed and 3.5 kW afterward, a 46% energy reduction.
At a treated water cost of $0.50/1000 gal. and energy cost of $0.05/kWh, annual savings total $7190. The figure assumes a 10% water loss from blowdown evaporation. Annual savings for an open discharge system are more than $60,000.