Many factors should be considered when choosing a system for cooling an induction process, including cost, space availability, existing utilities, energy and water usage, equipment location, potential freezing, reliability, and maintenance.

 

In this article we will introduce the various methods typically used to cool an induction system.

 

Water Quality

Cooling water circulating through much of an induction system is exposed to electromagnetic fields and high voltage potentials, resulting in prime areas for erosion and corrosion. Poor water quality can quickly lead to clogged cooling passages, causing equipment to run hot, arcing and significant replacement costs.

Therefore, it is necessary to use clean, low-conductivity water and monitor it regularly.

The water quality for an induction machine will vary slightly from one OEM to another, but most commonly we see the following recommendations:

  • Conductivity from 50-300 micro-siemen/cm
  • Total dissolved solids < 200 ppm
  • A pH of 7.0-7.8
  • Minimal sulphates and chlorides

Typical water quality:

  • Deionized water < 1 µS/cm
  • Steam distilled water < 10 µS/cm
  • Reverse-osmosis water < 20 µS/cm
  • Drinking water < 300 µS/cm

On smaller cooling systems, customers will often add bottled distilled water and periodically change it out to make sure it is clean. For larger installations, customers may use deionized water or reverse osmosis (RO) water with a low-conductivity corrosion inhibitor or glycol. A good low-conductivity inhibitor will only raise the conductivity 100-200 micro-siemens. Avoid using straight deionized or de-mineralized water because it can be an aggressive scavenger and will leach metals off your equipment. Also avoid using city water or well water.

 

Cooling-System Design

A dedicated closed-loop cooling system is required with all components constructed from nonferrous materials (stainless steel, plastic, PVC, bronze, or non-conductive hoses). Never use steel or iron in the system. Also, the water supply temperature is usually controlled between 25-35°C to provide adequate cooling and yet keep the temperature above dew point to prevent condensation on the equipment. More sophisticated systems will monitor conductivity and may include filtration and mixed-bed deionizer for polishing the water.

 

Methods of Cooling

Smaller induction machines commonly use water-to-water plate heat exchangers and point-of-service air-cooled chillers.

Larger induction installations include open evaporative tower, air-cooled heat exchanger (dry cooler), closed-circuit cooling tower and hybrid cooling systems using closed-tower with free-cooler or a combination of chiller with free-cooler for reducing energy consumption and minimizing water usage.

 

Nonferrous Pump Station with Plate Heat Exchanger

NFP PID (proportional-integral-derivative)
Fig. 1. NFP PID (proportional-integral-derivative) 

Every induction machine requires a nonferrous pump station for circulating the low-conductivity water (or glycol). Figure 1 illustrates the principle of operation of the nonferrous pump station with stainless steel plate heat exchanger.

The plate heat exchanger is a very effective means of transferring heat between the existing plant water system and the induction cooling water. The plate heat exchanger is made from a series of flat corrugated plates that are nested against each other. One side of the plate is in contact with the clean induction water, and the other side is in contact with the plant water. When piped in counterflow configuration, the plate heat exchanger is a very effective means of transferring heat.

The gap between plates can range from 3-6 mm. For this reason, it is very important to have a protective strainer upstream of the heat exchanger on the plant water side. Also, plate heat exchangers can be made as bolted, cleanable assemblies (plate heat exchanger) or as a brazed-plate pack (brazed-plate heat exchanger). For especially dirty plant water systems, it is recommended to use a cleanable plate heat exchanger that allows disassembly and cleaning. A brazed-plate heat exchanger is a low-cost alternative normally used for lower-flow applications, usually replaced instead of cleaned.

The water temperature is controlled by means of a water solenoid valve or modulating valve on the plant water side of the system. More sophisticated systems will monitor conductivity and may include filtration and mixed-bed deionizer for scavenging ions from the water.

 

Indoor Air-Cooled Chiller

Chiller PID
Fig. 2. Chiller PID 

For small induction installations (less than 70 kW of cooling) where the customer does not have plant water, a small indoor air-cooled chiller is often used (Fig. 2).

An air-cooled chiller uses a mechanical refrigeration circuit to reject heat from the induction water to the ambient air. The refrigeration circuit allows the water temperature to be controlled to 25-30°C even when the surrounding ambient air is >35°C. Combine the chiller with a nonferrous tank and pump, and it becomes a very attractive solution for cooling induction machines.

The chiller offers stand-alone point-of-service cooling for an induction machine, is easy to install and can be mounted near the induction machine, which is convenient for the operator.

An air-cooled chiller uses a mechanical-refrigeration circuit to reject heat from the induction water to the ambient air. The refrigeration circuit allows the water temperature to be controlled to 25-30°C even when the surrounding ambient air is above 35°C. Confirm the chiller is constructed from all nonferrous materials.

There are a few important considerations and hidden costs associated with using a chiller.

1.  All the heat from the induction process must pass through the refrigeration circuit before being rejected to the ambient air. This means that the chiller must be sized accurately. A refrigerant chiller cannot reject more heat than it is sized for and will trip out a high-pressure alarm if it is undersized. It is also important not to undersize a chiller because it can short-cycle. It is best to size the chiller so that it will run for extended periods of time to ensure good oil circulation through the refrigeration loop and ensure the chiller reaches a stable operating temperature.

2.  Be sure the location of the chiller has sufficient fresh cool air to recirculate. The heat from the chiller will be rejected into the air. Do not put the chiller in a small room without proper ventilation because it can add a lot of heat to your HVAC load.

3.  Electricity usage: The mechanical refrigeration cycle uses relatively large amount of electricity to reject heat. As a rule of thumb, it requires 1 kW electricity per ton of refrigeration or roughly 3 kW of electricity for every 10 kW of cooling from an air-cooled chiller. This can add up to several thousands of dollars in electricity costs over the course of a year. By contrast, this is about eight to 10 times more electricity usage than from a similarly sized air-cooled heat exchanger or evaporative cooling tower. For energy savings, consider a hybrid outdoor chiller integrated free-cooler. Discussed later in this article.

4.  Maintenance: The compressor is the “engine” of the refrigeration system and draws the most power from the system. It will be constantly turning ON/OFF as it controls temperature for your induction process. Chillers need periodic maintenance and replacement parts, both of which can be expensive. Service on a chiller must be done by a qualified refrigerant technician, which most customers do not have on staff. Discuss with your chiller supplier what costs may be incurred over the life of the chiller.

 

Open Evaporative Tower with Plate Heat Exchanger

An open evaporative-tower water system is commonly used for cooling multiple induction machines in the 100-1,000 kW range (or higher) and can produce 30°C water in every climate. Many large plant-wide water systems are open evaporative-type.

As shown in figure 3, water is pumped from the tower water reservoir to the plant and to the cooling tower. The cooling tower uses a fan to draw air through the tower where it comes in direct contact with the water that cascades down through the tower. Heat is rejected to the air by evaporation of a small amount of water (1%) in the cooling tower as it comes in contact with the air. The rule of thumb for a cooling tower is that 2 gpm of water is evaporated for every 1,000,000 btu/hour (290 kW) of cooling. A cooling tower will cool water near to the wet-bulb temperature, which is a function of the humidity. In many hot, dry locations the water in an evaporative tower can be cooled down to +30C even though the ambient air is much warmer (say +40C).

PID of evaporative-tower water-cooling system
Fig. 3. PID of evaporative-tower water-cooling system 

The evaporated water must be made up using city water or well water. Also, the cooling tower tends to “scour” the air and will result in accumulation of debris in the tower water. As a result, a good water monitoring and treatment program must be implemented to regulate water quality and to maintain good corrosion control for protecting the equipment and piping system.

Water treatment and water blow-down (discharge) are required to prevent corrosion, scaling and biologicals in the system. Freeze protection is also required in northern climates.

An open evaporative tower requires about 1 square meter (10 square feet) footprint for 100 kW of cooling. This is a very effective use of space and more than 50% smaller than required for an air cooler or air-cooled chiller.

An evaporative system produces a large amount of cooling at relatively low capital cost and low energy cost. However, it requires a moderate amount of maintenance, continually consumes and discharges water, requires filtration and treatment, and must be freeze protected.

 

Air-Cooled Heat Exchanger

Air-cooled heat exchangers are most commonly used on induction systems up to 500 kW and mainly in northern climates where the ambient rarely exceeds 30°C.

For some OEMs, the induction coil can operate at 50°C, which makes the air cooler very attractive. The air-cooled system requires the use of glycol for freeze protection for the outdoor heat exchanger. OEMs will often use uninhibited glycol or glycol with a low-conductivity inhibitor.

The air cooler uses high-efficiency propeller fans to draw ambient air over a large-finned tube radiator that rejects heat to the atmosphere. The air coolers are usually sized to cool the glycol to within 4-5°C (7-9°F) of ambient air.

About 2 square meters (20 square feet) footprint are required for 100 kW of cooling, which is about two times more than a cooling tower. The footprint of the air coolers can be reduced by stacking. Stacking the air coolers with a center diverting plate is beneficial for some customers. The warm air from the lower air cooler is diverted around the upper air cooler.

To control temperature, the fans on the air cooler are cycled on/off with a temperature controller or the fans can be speed controlled with a variable-speed drive. Since there is a large exposed area on the air cooler, it becomes very efficient in the winter and may only require 1-fan running. This makes the air-cooled heat exchanger an excellent choice for northern climates and for “free-cooling” chillers in the winter. In fact, in northern climates, the air cooler can actually over-cool the glycol in the winter if the induction machine is idle. For this reason, it is necessary to include a winter bypass valve to prevent over-cooling (Fig. 4).

PID of air-cooled heat-exchanger system
Fig. 4. PID of air-cooled heat-exchanger system

To obtain more cooling from the air coolers during hot summer days, some customers resort to adding a sprinkler beneath the air cooler. The water on the coil will quickly cool off the feed coolant. In most cases, the sprinkler ends up being left on for long periods of time, and the cooling coil quickly becomes clogged with debris and scaled up from minerals in the water. At this point, the air cooler is operating at a very-reduced capacity and needs a serious cleaning. Cleaning a coil must be done carefully so as to not damage the fins.

As an alternative to a sprinkler system, some manufacturers have added fogging or misting nozzles to the upstream face of the heat-exchanger coil (Fig. 4a). As the water evaporates, the surrounding air temperature is reduced prior to entering the finned coil. You can get 100 kW of additional cooling by misting only 0.68 gpm (5.6 lpm) of water. On a hot summer day, this evaporation can reduce the coolant temperature by an additional 3-5°C. By controlling the evaporation, you can reduce water usage and maintenance on the air cooler. In some dry locations, the misting system also has the added benefit of reducing the size of the air cooler. More sophisticated systems monitor humidity level and offer replaceable pads in front of the air cooler to further protect the finned surface.

An air-cooled system has a moderate capital cost, very low maintenance and requires no make-up water and no continuous chemical treatment.

 

Closed-Circuit Tower

PID of closed-circuit cooling tower
Fig. 5. PID of closed-circuit cooling tower

A closed-circuit evaporative tower (Fig. 5) is often used for midsized installations (70-500 kW) where the cooling required is too much for a small chiller but not large enough for a full open-tower water system. The closed-circuit tower is similar to an open evaporative tower but with a heat-exchanger coil inside the cooling tower.

Glycol flows through the inside of copper tubes, and raw water cascades over the outside. Similar to the open tower, a small portion of the external water (1%) will evaporate to produce the cooling effect. The inside of the tubes remains isolated from the raw tower water and will keep your equipment clean. The tower water located in the basin of the cooling tower requires water treatment similar to an open evaporative tower.

The tower must be constructed with a nonferrous heat-exchanger coil and should have a stainless steel or fiberglass basin for additional corrosion protection. An immersion heater is required to prevent freezing the tower water, and glycol is required for internal freeze protection. Some customers will drain the basin of the tower, run the tower in a “dry” mode or use a free-cooler in conjunction with the closed-circuit tower.

Since the tower is sized based on the evaporative effect of water (1000 Btu/pound of water evaporated), it is really not very effective running dry. As a guide, a tower running dry may need to operate at an ambient of 0°C (or below) to get the same cooling effect that you would get from evaporation at 95°F ambient. For this reason, some customers prefer to use a free-cooler in conjunction with the closed-circuit tower.

 

Hybrid Air-Cooled Heat Exchanger with Closed-Circuit Tower for Summer Trim Cooling

PID of hybrid air-cooled heat exchanger with closed-circuit cooling tower
Fig. 6. PID of hybrid air-cooled heat exchanger with closed-circuit cooling tower

Using an air-cooled heat exchanger upstream of the closed-circuit tower allows the tower to be turned off and drained for a large part of the year (Fig. 6). The air cooler can be the primary mode of cooling with the cooling tower only operating in the summer, thereby minimizing water and chemical usage. As previously mentioned, air coolers are very efficient in cool weather. Additional benefits include eliminating the immersion heater and associated freezing-water issues in the winter.

 

Hybrid Air-Cooled Chiller with Free-Cooler

PID of outdoor air-cooled heat exchanger with integrated free-cooler
Fig. 7. PID of outdoor air-cooled heat exchanger with integrated free-cooler

In locations where water usage must be eliminated, an outdoor air-cooled chiller with integrated free-cooler (IFC) may be a good option.

The IFC is enabled/disabled using an ambient thermostat (Fig. 7). When the ambient air drops below the fluid temperature, the IFC is enabled, allowing the coolant to flow through the air-cooled heat exchanger. This is ideal for an induction process because the circulating water temperature only needs to be 30°C or less.

The IFC provides cooling for the majority of the year and allows the chiller to be disabled, which results in substantial energy savings. This can save thousands of dollars per year on even small chillers. The added benefit is that the compression system turns off for most of the year and is not required to operate in the winter, which is harsh on the refrigerant system. This substantially reduces the energy usage for the chiller and reduces wear and maintenance associated with the chiller.

As a rule of thumb, a chiller system requires about 1 kW of power per 3.5 kW of refrigeration (1 chiller ton). A 100-kW chiller can cost $10,000-15,000 in electricity over the course of a year. This is about 10 times more energy than similarly sized air coolers or cooling towers. Using an IFC can reduce the electricity costs by 50% and associated chiller maintenance.

A chiller with an IFC has a high initial capital cost, relatively high electrical costs and low maintenance. It also requires no make-up water and no continuous chemical treatment.

To estimate the energy costs, use this equation:

Energy Usage = XX kWcooling x 0.3 kWelec /kWcooling x Hours of operation x $/kW-hour x Duty cycle (%)

Example:

  • 100 kW of cooling requires 30 kW of electricity to operate the chiller
  • 300 days x 16 hours/day = 4,800 hours
  • Energy Costs = 100 kW × 0.3 kW/kW × 4,800 hours x $0.1 / kW-hour x 1.0 duty cycle = $14,400 per year

The free-cooler allows the chiller to be turned off for a large part of the year. So if you can turn the chiller OFF for 50% of the year can potentially save up to $7,200 (14,400×0.5). Adjust this for your location, duty cycle and cooling requirement, and you will see that the integrated free-cooler can provide fabulous cost savings.

A chiller with integrated free-cooler has a high initial capital cost, relatively high electrical costs, low maintenance, requires no make-up water, and no continuous chemical treatment.

 

Summary

See figure 8 for a summary of the methods of cooling discussed here. Good-quality water is critical to maintaining a high-performance, efficient induction machine. Monitor the conductivity regularly and replace when needed. Also, choose a system that provides a balance of reliable, effective cooling along with minimal energy and water consumption.

Summary of Various Cooling Systems
Click to view full-size

 

For more information: Contact Matthew Reed, P.E., director of sales and technology; Dry Coolers, Inc., 575 S. Glaspie St., Oxford, MI 48371; tel: 248-969-3400 x151; e-mail: matt.reed@drycoolers.com; web: www.drycoolers.com