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Design/Selection
Process Cooling Bookmark and Share

Basic Process Cooling Design

Why Ice Thermal Storage for Process Cooling Projects?

BAC has been involved in the design and manufacture of ice thermal storage for over 25 years.
The BAC team can provide in-depth knowledge on the design and operation of your air-conditioning system.

Basic Information

Getting Started

A daily load profile is the hour-by-hour representation of cooling loads for a 24-hour period. Most District Cooling applications use a daily load profile to determine the amount of storage required. For conventional systems, chillers are selected based on the peak-cooling load. For ice storage systems, the chillers are selected based on the ton-hours of cooling required and a defined operating strategy.

The Air-Conditioning & Refrigeration Institute (ARI) has published Guideline T, “Specifying the Thermal Performance of Cool Storage Equipment.” The purpose of Guideline T is to establish the minimum user-specified data and supplier-specified performance data. Design data provided by the customer includes: System Loads, Flow Rates and Temperatures. Table 1 details the user-specified data.

Selection Optimization

BAC has done extensive research and testing on the building and melting characteristics of ice storage. This research and testing has resulted in selection capabilities unmatched by any other company in the industry.

BAC can predict the temperatures required on an hour-by-hour basis for building ice on standard and custom products, over a variety of conditions and build times. BAC can optimize the design of an ice storage system to meet load profile, discharge temperatures, chiller capacity, operating sequences and layout restrictions.

Operating Strategies

The next step in selecting thermal storage equipment is to define an operating strategy. Choices include either full or partial storage. Partial storage operating strategies can be categorized as either demand limiting or load leveling. The operating strategy used is dependent upon the load profile, utility rate structure, energy cost and equipment first cost.

Full storage systems eliminate the need to operate the chiller during the utility on-peak period by storing the required cooling capacity during off-peak periods. This strategy shifts the largest amount of electrical demand and results in the lowest operating costs. However, the equipment first cost is considerably higher than partial storage systems due to larger chiller and storage requirements.

Unlike full storage systems, the chiller must operate during the on-peak period when a partial storage operating strategy is used. There are two types of partial storage operating strategies. The first strategy is demand limiting. With the demand limiting operating strategy, the non-storage system loads establish the peak demand for the facility. Items that contribute to the non-storage system loads include lights, equipment, appliances, fans, motors etc. The thermal storage equipment is selected so the chiller operation does not increase the facility’s non-storage demand. This operating strategy provides the lowest operating costs for partial storage systems. This strategy also requires less storage capacity and smaller chillers than a full storage design. The disadvantages of the demand limiting operating strategy are that the storage requirement and chiller capacity is larger than required for a load leveling operating strategy. This results in a longer payback period.

The second partial storage operating strategy is load leveling. By distributing the cooling load equally over a 24-hour period, this operating strategy reduces the size of the thermal storage equipment and chiller when compared to either full storage or demand limiting strategies. This results in the lowest possible first cost and shortest payback period. Since the chiller operates fully loaded during the on-peak period, operating costs are higher than either demand limiting or full storage.

Melt Discharge Strategies

External Melt:

When ice is built on a coil using glycol or refrigerant inside the coil, and melted by water on the outside of the coil with direct ice contact. The warm return water is in direct contact with the ice and a constant low leaving water temperature is provided during the entire melt cycle. The external melt design is shown below.

TES Design External Melt.JPG

Typical melt temperatures for a BAC external melt system are graphed below. System designs which require 34°F (1°C) to 36°F(2°C) supply water or rapid melting of the ice are best suited for external melt.

External Melt.JPG

Schematics

Modes of Operation

External Melt

The BAC ICE CHILLER® Thermal Storage unit for external melt can operate in five distinct operating modes. These modes of operation provide the flexibility required by operators to meat their cooling loads.

ICE BUILD –

In this operating mode, ice is built by circulating glycol or refrigerant through the ice storage coils. The glycol or refrigerant temperature required to build the ice is dependent on the length of the ice build and the thickness of the ice build. The “Typical Ice Build Performance for Different Build Times” graph above illustrates the typical chiller supply temperatures for 8-, 10- and 12- hour build cycles. The graph below, “Ice Thickness vs. Chiller Supply Temperature” shows typical chiller supply temperatures for 1.5, 2 and 2.5 in (38, 51 and 64 mm) ice builds with a 12-hour build cycle.

Ice Thickness

ICE BUILD WITH COOLING –

When cooling loads exist during the ice build period, it is best to handle these loads with a base water chiller.

COOLING – ICE ONLY –

In this operating mode, the glycol chiller (or refrigeration machine) is off. The warm return water is cooled to the desired set point temperature by direct contact with the ice.

COOLING – CHILLER ONLY –

In this operating mode, the glycol chiller (or refrigeration machine) supplies all the building cooling requirements. Either a base water chiller or the glycol chiller through a heat exchanger cools the warm return water. Temperature set points are maintained by the chiller.

COOLING – ICE WITH CHILLER –

In this operating mode, cooling is provided by the combined operation of the base water chiller, glycol chiller (or refrigeration machine) and ice thermal storage equipment. The base water chiller and glycol chiller (through a heat exchanger) pre-cools the warm return water. The partially cooled warm return water then passes through the BAC ICE CHILLER® Thermal Storage Unit where it is cooled by direct contact with the ice.

Below are examples of piping schematics tor ice thermal storage systems.

External Melt

  • EM-1A Single Loop, Open System
  • EM-2A Closed System, Optional Base Water Chiller
  • EM-3A Closed System, Chiller Upstream, Optional Base Water Chiller in Parallel
  • EM-4A Closed System, Dual Evaporator Chiller Upstream, Optional Base Water Chiller in Parallel

BAC Equipment

cooling_thermal_storage_silo

Construction Details

AIR BLOWER

Centrifugal regenerative blower for field mounting to supply low pressure air for agitation of the water. Blower is furnished with an inline air filter, check valve and rain shield for field installation.

COVERS

Sectional insulated tank covers are provided with a thermosetting hybrid polymer. Covers are interlocking and rain shedding.

COIL

The coil is constructed of multiple prime surface serpentine steel circuits and tested at 375 psig air pressure under water. It is encased in a steel frame, and the entire assembly is hot-dip galvanized after fabrication. For ammonia systems, purge connections are provided on each coil for oil maintenance.

ICE-LOGIC™ ICE THICKNESS CONTROLLER

An electronic, multi-point adjustable ice thickness control is mounted on the unit.

A control relay is provided for deactivating the refrigeration system when a full build of ice is reached.

EXTERIOR PANELS

Exterior panels sealed at all seams provide a complete vapor barrier and protect the insulation. They are furnished with a thermosetting hybrid polymer.

INSULATION

Expanded polystyrene insulation is provided between the tank and the exterior panels. The insulation is three inches thick (R-13) on the tank sides and ends, two inches thick (R-8) on the bottom and one inch thick inside the covers.

TANK

The tank is constructed of heavy gauge, hot-dip galvanized steel reinforced with full-length structural steel angles beneath and on all four sides. All seams are welded to ensure watertight construction. A zinc rich coating is applied to all exposed edges and welds.

AIR DISTRIBUTOR

Low pressure air from the air blower is distributed below the coils through multiple perforated Schedule 40 PVC pipes.

Unit Selection

Heat Rejection Equipment

Cooling Towers

Cooling towers use an environmentally compatible evaporative cooling process to produce significantly lower water temperatures than achievable with air-cooled equipment for maximum energy savings and operating efficiency in recycled cooling water systems.

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Evaporative Condensers

Evaporative condensers offer energy savings by providing lower system horsepower than conventional air-cooled and water-cooled condensing systems.

Condenser_G2b.JPG

Closed Circuit Cooling Systems

Closed circuit cooling towers use a clean closed loop in a highly efficient evaporative heat transfer process to cool water and other process fluids while isolating the cooling fluid from the outside airstream.

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