Preventative Maintenance Seminar for P/M Presses and Furnaces
Design and Maintenance of Cooling Water Systems for Sintered Metal Plants
Sponsored by Metal Powder Industries Federation, Indianapolis, IN
May 5-6, 1998

Timothy Keister, CWT
Chief Chemist
ProChemTech International, Inc.
Brockway, Pennsylvania
I. INTRODUCTION

Initial design and maintenance of cooling water systems are commonly neglected areas of the sintered metal production process which are often responsible for substantial cost increases due to downtime, equipment damage, and loss of process control. Other items which relate to cooling water systems; such as Legionnaire's Disease, water and sewerage costs, and environmental regulations; can also have substantial impacts on manufacturing operations. The common neglect of cooling water systems results from two causes: first, the manufacturer often does not appreciate that the cooling step is a vital part of the production process; and second, the fact that much of the information available concerning cooling water has a strong element of "hype", being provided by the marketing departments of large water treatment chemical companies and "snake oil" salesmen.

This seminar is intended to provide the sintered metal parts manufacturer with information on both initial design and maintenance of cooling water systems.

II. SYSTEM DESIGN

A. Desired Outcome
Before proceeding with design of a cooling system, the desired results, or performance, of the system must be established.  For a sintered metal parts plant, it is desired to cool the sintered parts from near sintering temperature to about room temperature in a minimum time and space. Due to its high heat transfer capacity, cooling water in a non-contact jacket is normally used.

B. Thermal Load
The amount of heat to be removed from the product being cooled is the primary item in design of a cooling water system.  This is readily calculated using the furnace belt speed, weight of belt, specific heat of the belt material and product, maximum weight of product, and temperature difference (delta T) hot to cold; the result being given as Q in btu/hr.

Q = lbs metal belt+product/hr X specific heat X delta T

Typical specific heats as btu/lb/F :

water 1.00
aluminum 0.23
nickel 0.11
brass 0.09
steel 0.11
copper 0.09
tin 0.55
iron 0.108
zinc 0.09

C. Design Limits
The following are some design limits which apply to cooling systems for sintered metal plants.

1. Cold water temperature, in most areas of the country, "wet" cooling towers can produce a cold water temperature within 5 F of wet bulb, which means that design cold water temperature will be between 80 and 85 F under worse case conditions.

Dry cooling systems, or water to air heat exchangers, can produce a cold water temperature within 5 F of dry bulb, which means that design cold water temperatures will be between 100 and 110 F under worse case conditions.

2. Standard cooling tower fill is limited to a maximum temperature of 120 F, optional fill will go to 135 F.

3. The rate of parts cooling is limited by the thermal processes; conduction, convection, and radiation, taking place within the cooling chamber more than by the absolute cooling water temperature.

D. Special Considerations
1. Water flow fate, to help control deposition within cooling jackets, a minimum flow velocity between 2 and 5 feet/sec should be maintained.  If needed, internal water chamber recirculation should be considered to meet this basic requirement.

2. Maximum water temperatures should be held as low as possible, a maximum of 110 F is suggested. If not possible due to production considerations, a tempering loop must be included in the system design.

3. Effective water treatment chemistry against corrosion, scale, deposition, and biological fouling should be used. Note that the various "snake oil" fixes for this requirement, such as electronic, magnetic, catalytic, and ozone; have been proven to be worthless.

4. For those poor people who live in a cold climate, system draindown to prevent freeze damage must be incorporated into the system design.

5. To prevent equipment damage and product losses, some form of automatic city water backup must be provided for power outages and equipment failures.  Larger or critical

installations can benefit from use of dual water supply pumps and dual cooling towers with automatic switchover.

6. "Wet" cooling towers reject 75 to 80% of the thermal load by evaporation, dry coolers reject heat simply by conduction.  Due to this difference, dry towers utilize much more energy (electric to run the fans) than an equal capacity wet cooling unit will,

E. Typical Design
1. One pump, direct return to the cooling tower. Major limit is the total head which can be placed upon the sinter furnace cooling jackets.  Typical maximum head is between 10 and 151.  Use of a low head, open distribution, cooling tower is required for this design.

2. Hot well/cold well, "standard" design for retrofit to older plants. Uses collection tanks and pumps to return hot water to cooling tower, then a second pump to provide cold water to equipment.

3. The cooling water flow rate required is easily calculated using the calculated thermal 1oad and the difference (delta) between the established hot and cold water temperatures as follows:

GPM =(Q in BTU/hr)/ (delta T F X 8.345 X 60 min/hr)

4. Some properties of these calculations to be mindful of when designing a cooling system:

- higher delta T = lower flows = smaller pumps/pipes

- cooling towers operate more effectively with higher inlet temperatures

II. EQUIPMENT MAINTENANCE

A. An annual cooling system drain down, cleaning, and inspection is recommended. The Cooling tower and system tanks should be cleaned of debris, inspected, and any obvious areas of corrosion cleaned and recoated. A cold galvanize coating is recommended for ".he cooling tower and coal tar epoxy for the tanks.

B. Cooling tower fan and cooling water pump bearings should be lubricated on a routine schedule per the manufacturer's recommendation.

C. Cooling tower fan belts should be inspected once a month. Loose belts should be adjusted, worn belts replaced as needed.

D.If so equipped, bypass filters should be routinely maintained per manufacturer's recommendation.

E.If so equipped, dual pump cooling systems should be manually switched once a month.

III.CHEMICAL WATER

TREATMENT REQUIREMENTS
Any discussion of chemical water treatment must begin with an objective statement of what is expected.  In a sintered metal plant, it is desired to cool the sintered parts from near sintering temperature to about room temperature in a minimum time and space.  The following five basic requirements for a cooling water treatment program are derived from this objective.

1) Water Chemistry Control Must Be Consistent
2) Corrosion Must Be min4-mized
3) Scale Must Be Controlled or Eliminated
4) Deposition Should Be minimized
5) Biological Growth In-system Must Be Controlled

These five requirements will now be expanded upon in sufficient detail to provide a basis for understanding control and treatment of cooling water.

A. Water Chemistry Control
We have observed over the years that operational control of cooling water treatment programs is invariably neglected and is thus the single most common cause of program failure. The best possible chemistry is completely worthless if not consistently and correctly applied.

1. The primary item in water chemistry control is cycles of concentration (C), or the number of times that the dissolved salts in the fresh makeup water (MU) are concentrated by evaporation loss from the system.  This parameter is commonly calculated by measuring the conductivity of the cooling water (CW) and dividing it by the measured conductivity of the makeup water.

C = CW conductivity/MU conductivity

Control of cycles is critical as no chemistry can cope with the increased potential for scale formation resultant from excessive levels of hardness salts in cooling water caused by very high cycles operation. Cycles control is easily automated using either conductivity or proportional control units.

2. Proper dosage of the chemical inhibitors commonly used to control corrosion, scale, and deposition in cooling systems is also critical. The amount of these materials to be fed into a system is based upon the amount and the specific chemistry of the makeup water used. We have found that the best method for feeding inhibitors is to use a separate automatic chemical feed system based on metering the amount of makeup water added to the cooling water system and activating a metering pump via a timer to add a proportional amount of inhibitor.

3. Biocide Materials are added to cooling systems on a routine basis in direct proportion to the amount of water in the system to control biological growth.  We normally recommend automation using a timer and pump for this function.  Reduction in biocide usage is a good reason to minimize the amount of water in any cooling system design.

4. Calculation of the various operating parameters for a cooling system is often of interest and we have found the following equations to be the most accurate for obtaining values for system evaporation (E), blowdown (BD), windage (N), and makeup (MU) as gal/day.

E = (Q thermal load btu/hr X 24 hr/day X 0.75)/ (1040 btu/lb X 8.34Tlb/gal)

Thermal load (Q) in btu/hr can be determined by the following equation if the cooling tower recirculation rate (R) in gpm and delta T across the cooling tower is known.



Q = R X 60 min/hr X 8.345 lb/gal X delta F

Blowdown is calculated using the evaporation determined from the heat load on the cooling system with the following equation.



B = E / maximum C - 1

The maximum cycles value will be set by the supplier of the water treatment program to control scale formation while minimizing blowdown and chemical use.  This value will typically be between 3 and 6.

Windage is commonly calculated using the following equation.

W = R X 1440 min/day X 0.001

Please note that the windage factor varies from 0.001 to 0.005 dependent on the model and type of cooling tower in use.

Makeup is simply the sum of evaporation, blowdown, and windage, and can be checked using standard mass balance techniques.

M = E + B + W

B. Corrosion
Corrosion is an oxidation process which results in chemical destruction of the base metal. Uncontrolled corrosion is often responsible for downtime from physical equipment failure, or plugging of cooling water passages from deposition of corrosion products. More subtle effects, often not linked to corrosion, are loss of production speed and/or process control and decreased efficiency, from deposition of corrosion products on heat transfer surfaces where the deposit acts as an insulator to decrease thermal conductivity.

It is a fact that corrosion cannot be eliminated, only minimized, by the various corrosion control chemicals added to the cooling water.  An acceptable cooling water treatment program should be able to reduce corrosion rates to the following average levels reported as mil/yr:

MILD STEEL 1 to 2
COPPER ALLOYS 0.1 to 0.2
ALUMINUM 1 to 2
ZINC 2 to 4


Sintered metal plants can present a severe corrosion control problem due to high water temperatures.  A "rule of thumb" in chemistry notes that reaction rate doubles for every 10 F increase in water temperature. High water temperatures can thus substantially increase corrosion rates. The highest intentional exit water temperature we have ever observed, 190 F, was in a sintered metal plant.

C. Scale
Deposition of scale is a chemical precipitation process where dissolved salts in the cooling water crystallize on surfaces in contact with the water due to their solubility limits being exceeded.  The most common scale formers, calcium salts, exhibit reverse solubility in that they become less soluble as the temperature of the water increases.  This property causes scale formation in the most sensitive area, the furnace cooling jacket.



Since the thermal conductivity of scale is substantially less than metal, heat removal from the equipment is reduced and production. speeds must be lowered to compensate.  In extreme cases, enough material precipitates to physically block the cooling water passages, resulting in the effected equipment being removed from production for either chemical (acid) or mechanical cleaning.

Scale is controlled by use of scale control chemicals, usually a phosphonate - polymer, which are typically blended in with the corrosion inhibitor. As with corrosion, the high water temperatures often encountered in sintered metal plants can severely test a scale control chemistry.

D. Deposition
We use the term "deposition" to cover the various deposits which can be found within a cooling system which are not due to corrosion, scale, or biological fouling. We find that such deposition can result from scrubbing of airborne material from the ambient air by the cooling tower, process contamination of cooling water by such things as leaking oil coolers, and suspended material in the makeup water.

Deposition effects process operations much like scale, the deposit acts as a thermal insulator to decrease heat transfer efficiency in production equipment.  Deposition can also cause physical blockage of cooling water passages and increase corrosion rates by blocking corrosion inhibitor access to the base metal.

Specific deposit inhibitors, usually based upon polymers and surfactants, are typically included in the inhibitor used for control of scale and deposition. Thus, often a single blended product can be used for control of three problem areas: corrosion, scale, and deposition.

E. Biological Fouling
Microbiological growth within a cooling water system, if not controlled, can result in formation of biological fouling layers on surfaces in contact with the cooling water.  Biological fouling effects process operation much like the previously discussed scale and deposition, the fouling acting as a thermal insulator to decrease heat transfer efficiency in production equipment. Biological fouling also results in substantial corrosion rate increases due to formation of anaerobic areas under the fouling.

Severe cases of biological fouling have resulted in complete cooling system failure due to the biomass physically plugging cooling water passages in production equipment and cooling towers.

Present practice for control of biological fouling is to periodically dose the cooling system with a biocide to kill as many of the organisms present as possible.

Due to the specific chemistry of biocides, none of the effective ones are compatible with effective corrosion, scale, and deposition inhibitors.  Thus, a separate product is needed for biological control of cooling waters.

Control of biocide feed is somewhat subjective in that a visual determination of microbiological growth is generally used to determine if the biocide program is effective.  "Dip stick" indicators to test organism density are available and are being utilized to more effectively control biocide feeds.

IV.SUMMARY

Proper design of cooling water systems followed by routine preventative maintenance will provide reliable cooling of sintered metal furnaces at minimal cost with maximum equipment life.  Errors in initial design and failure to provide proper preventative maintenance will result in production problems and premature equipment replacement.


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