COLIN HEWETSON ON CONCRETE COOLING

CONCRETE COOLING SYSTEMS

DESIGN GUIDELINES FOR PRE AND POST CONCRETE COOLING

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RECIPROCATING & SCREW COMPRESSORS

PC CONTROLS & SCADA MONITORING

CONTAINERIZED FLAKE ICE PLANT & SUPERIOR ICE DRUMS

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THE NEED FOR CONCRETE COOLINGC.J.HEWETSON 22/11/04

1.0. HOT WEATHER ISSUESIn Saudi Arabia batching plants and civil building operations face increased problems due to the high ambient summer time dry bulb temperatures which can rise in excess of 50°C with associated lower RH values of 20%. Concrete mixing, transportation and placement under extremely high temperature conditions, low humidity and high winds, requires an understanding of the effects such environmental factors have on concrete properties and construction operations. Understanding these effects and factors provides a means of counteracting and eliminating or minimizing the undesirable effects.

The recommendations to reduce placement temperature are a) to precool the concrete mixture using flake ice at -8°C and chilled water at 0.5°C to pre cool the aggregate – sand – cement and b) for large mass concrete constructions like dams to use post placement chilled water circulation to slow the rate of exothermic heat generation and to ensure the curing rate is controlled over time to provide the design concrete compressive and flexural strength.The main problem with concrete mixed, placed and cured at high temperatures is it develops higher early strengths than concretes produced and cured at lower temperatures, but at 28 days or later, strengths are generally lower as per figure 1.

Additionally plastic shrinkage cracking is a major problem with hot weather concrete placement, whenever the rate of evaporation of the mix water is greater than the rate at which the water can rise or bleed to the surface. High winds, high air temperatures combined with low humidity, cause rapid evaporation which increase the likelihood of plastic shrinkage cracking occurring. Plastic shrinkage cracks are virtually impossible to close. Precautions to avoid plastic shrinkage cracking during hot weather placement when the rate of evaporation approaches 0.98kg/M2/hr, consist of dampening sub-grade and forms, placing concrete at the lowest possible temperature, erecting wind breaks and sunshades, fog spraying with potable water. Higher cement temperatures effect the rate at which the cement hydrates in the mix, the rate of stiffening and water content.

Generally the concrete placement temperature for heavy construction where the exothermic heat of hydration is greatest should be < 18°C or lower. If the contractor or consultant specifies a given maximum placement temperature, which is the temperature in the forms, then the concrete mix must be batched at a lower temperature still, say 2-3°C to allow for the temperature rise from the batching plant, transport to the site and the forms as well as the hydration heat arising during transport and placement.

compressive strength versus temperature

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compressive strength PSI compressive strength at 1 day

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However as can be seen from figure 2 it is impossible to achieve a temperature of <18°C purely by the addition of chilled water at 0.5°C plus flake ice at -5°C as the weight is approximately only 50% of the cement, < 9% of the aggregate & pozzolon's per M3. Too, the water requirement of a concrete mix increases with higher mix temperatures, as the water content effects the concrete strength, its durability and cured permeability, it is important to reduce the amount of water in the mix, another reason why cooling cannot be satisfactorily achieved using chilled water alone or with flake ice.

Admixtures meeting ASTM specifications may be beneficial in offsetting some of the undesirable effects of concrete placement during hot weather to obtain a uniform setting time regardless of temperature changes. Water reducing retarders will reduce the chilled water requirement and may also increase the compressive and flexural strength of the concrete.

2.0 AGGREGATE COOLING ISSUESGenerally the aggregates, sand and cement are stored outdoors next to the batching platform either in silo's or under open sheds. The temperature of these will certainly likely approach the summer time ambient dry bulb temperature which could be >50°C. Therefore where the civil design requirements require a concrete mix placement temperature < 18°C, cooling of the aggregate, sand, cement is essentially the only way such requirement can be met. It cannot be met by using chilled water and flake ice alone and the solution for such cooling can be met by chilled water immersion, chilled water spray fogging or chilled air blast cooling depending on the size of the batching plant and number of batches/hr or day. For smaller plants batching100 -200m3/hr, air blast cooling of the aggregate in the storage silo is the most economical solution, since cooling takes place in the storage silo and no additional cooling or material handling equipment is required.

For larger plants chilled water spray cooling of the aggregate and sand, either in purpose designed insulated storage silos or conveying elevator (from storage silo to batching tower) provides the most economical solution. However with chilled water spray cooling of sand and aggregate, the higher water or moisture content of the sand and aggregate needs to be taken into account, by sample trial analysis, and the concrete mix chilled water – flake ice quantity adjusted accordingly. Mix formulaes are generally specified in terms of dry weight of each ingredient. Therefore it is necessary to know the approximate amount of surface or retained water of the various grades of aggregate and sand used. It is common practice to assume that the uncooled aggregates contain approximately 2-5% surface moisture and the sand contains 5-10% surface moisture depending on storage methods and ambient temperature.

The cement also should be pre-cooled as it constitutes up to 20% of the mix, for small plants where the cement is bagged, the cement should be kept in an air conditioned warehouse, for larger plants where the cement is stored in vertical silos, the silos should be insulated double skinned type and air conditioned. The cement should not be introduced into the batcher higher than 25°C.For chilled air cooling, a packaged stand alone air chiller is located adjacent to the storage silos, chilled air at around 0-5°C is ducted into the underside of the storage bins or silos above the stone gates or so called fill valves, blows up through the aggregate and is collected at the top of the bins and returned through a common return air duct to the air cooling system. The storage silo should therefore be airtight and provided with a roof with hinged gates for filling the silo with the various aggregates, the sides and top would need to be insulated. For multi compartment aggregate silos storing the full range of aggregate from say 6" to 3/4, individual air ducts can supply chilled air in to the individual compartments, ducts would be sized for a minimum velocity of 20m/s, the ducts can be galvanized spiral lock formed circular form. Where the aggregate is not prewashed to remove dust, a spray washer can be incorporated into the system to remove the dust.

Assuming the aggregate moisture content amounts to 2% and is precooled, the amount of flake ice required at -8°C would be approximately 22kg/m3 and the weight of chilled water at 0.5°C would be approximately 18kg/m3.

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FIGURE 2

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CONTAINERIZED FLAKE ICE PRODUCTION AND DELIVERY PLANT

The ice storage silo is specially modified from new double 40' ISO high cube containers to provide storage capacity of 30 – 70 tonne of flake ice. It is stiffened and reinforced to accept the dead & live weights of the equipment and ice as well as the plant room above. All necessary structural steel galvanized or painted support steelwork is supplied for field erection. The ice store is supplied fully painted, with separate low temperature air cooling system, automatic ice rake, lighting and insulation. It is equipped with specially-designed internal double thermal layers, which ensures cold air circulation around the flake ice to maintain dry flake ice at -8°C regardless of the ice store volume. In this way, the ice flakes can remain dry and reduces the problem of rat holing and bridging and we can guarantee that no problems will appear when ice flakes are raked from the ice storage container into the discharge screw auger and into the electronic weighing system and from the weigher into the flake ice delivery screw or pneumatic flake blower rotary valve.

The main features of the system are 2-3 x brand new ISO containers (not secondhand)- 1 plant room containing the ice generating drum/s, compressor/s, liquid receiver, starters and controls- 1 or 2 plant rooms containing weighing and conveying plant- Evaporative condenser roof mounted- Plant air conditioning air cooled condensing unit roof mounted- Separate air or water cooled concrete mix water chiller & Flake Ice generator water precooler Containers are stiffened and reinforced to accommodate the live and dead loads of the equipment, ice, plant

room and condenser above. Containers are insulated, air conditioned by a separate air cooled R134a or R404a air conditioning system

Container supporting steelwork & access ladders are supplied in CKD for site installation. The steelwork and ladders can be painted or galvanized.

The ice rake always raises to its maximum height to ensure it does not become embedded in the ice.

The internal structural steel and rake are hot dipped galvanized after manufacture or stainless steel (optional).

Ice store comes complete with delivery screw auger.

Ice height & ice production is automatically controlled via a maximum level sensor at the top of the ice bin.

Ice storage capacities from 20 – 200tonne

Pneumatic or screw auger delivery system

Electronic batch or continuous weighing system

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MODEL # STORAGE CAPACITY TONNES

DIMENSIONS

AIS 50 50 12190 x 4876 x 3200

AIS 75 75 12190 x 4876 x 4200

AIS 100 100 12190 x 4876 x 5480

AIS 120 120 12190 x 4876 x 5100

AIS 150 150 12190 x 9752 x 4200

AIS 200 200 12190 x 9752 x 5480

FLAKE ICE DRUMS

POWER SUPPLY： 380/3/60HzSTANDARD CONDITIONS: Water supply temperature 10°C ambient temp 45°CICE THICKNESS： 1.8-2.2mmICE CAPACITIES: 1.5 to 100 tonneREFRIGERANTS: R717/R404A/R22ICE DRUM SPECIFICATION: Carbon steel chrome plated or Aluminium Alloy or Stainless Steel.

The Snowkey ice drum is insulated with 100mm Polyurethane foam injected under polished Aluminium or Stainless steel sheet cover. The drum is stationary and the ice scraper blade or auger plus water spray jets rotate around the inside of the drum. There are no seals or potential leaks as with systems where the drum rotates around a stationary blade.The flake ice thickness is adjusted by changing the speed of the rotating ice blade or auger, the slower the rotation the thicker the ice. The ice drum surface area is 1750M2/tonne which is far higher than competitor ice drums and therefore provides superior efficiency. The flake ice is delivered to the ice storage container completely dry at -8°C, there is no surplus water thanks to the superior design of the drum which catches unfrozen spray water in a trough at the bottom of the drum and returns it to the circulating tank. The ice drum is completely packaged and factory tested, complete with water pump & distribution system, drive motors, connecting hoses, insulation, refrigerant controls and safety controls.

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SCREW ICE DELIVERY SYSTEMS

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Screw auger ice delivery systems perform better for short-distances between the weighing system and the concrete batching plant and is more economic.

Screw auger ice delivery systems can perform vertically and horizontally upto an angle of repose of 30°. Screw augersice can deliver the flake ice to two or more target hoppers. The length of ice delivery screw auger can be upto 25 meters. Screw auger ice delivery units are the most effective for vertical and horizontal delivery.

model TSL10 TSL15 TSL20 TSL25

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Delivery

capacity(ton)10 15 20 25

Ø(mm) 250 300 300 350

Angle 0°~+30° 0°~+30° 0°~+30° 0°~+30°

A(mm) 16686 13686 9686 16686 13686 9686 13686 9686 13686 9686

B(mm) 15000 12000 8000 15000 12000 8000 12000 8000 12000 8000

power（kw） 7.5 7.5 7.5 11 11 7.5 11 7.5 11 7.5

PNEUMATIC FLAKE ICE DELIVERY SYSTEMS

The pneumatic blowing system is ideal for transporting ice flakes over long distances. The blowing system is installed into its own 20 or 40' ISO container which is airconditioned with a separate R134a or R404a airconditioning system. The container is fitted out with Aluminium chequer plate floor, lights, air intake louvers and filters and air extract louvers with necessary sand traps. The pneumatic system incorporates all necessary automatic controls including air lock rotary valve feeder, discharge line air cooler to reduce the blower discharge temperature. If ice flakes need delivering to several concrete batchers, then the blowing unit is the most ideal choice. Automatic diverting valves can be provided to accomplish this or alternatively separate blowers can be supplied for each batcher. The system is delivered complete with all necessary 4" or 6" HPDE piping and bends which transports ice flakes pneumatically from the ice store – weigher to a cyclone feeder above the mixer or to a small ice storage bin over the concrete batcher to remove air. The discharge line air cooler ensures the ice is delivered without excessive heat pick up or ice melting so the ice flakes are delivered to the batcher dry. The maximum weight of ice delivery for our systems is 30 tonne/hr.

MODEL DELIVERY MAXIMUM HORIZONTAL HDPE PIPE DIA

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CAPACITY

TONNE/HR

DELIVERY DISTANCE

(MTRS) (mm)

AID-6 6 200 100

AID-10 10 200 100

AID-12 12 200 100

AID-15 15 180 125

AID-18 18 160 125

AID-20 20 160 180

AID-25 25 150 180

AID-30 30 150 180

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IN ORDER TO PROCESS YOUR ENQUIRY PLEASE PHOTOCOPY THIS PAGE, COMPLETE ALL DATA IN THE QUESTIONAIRE AND RETURN

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FALLING FILM & FLOODED WATER CHILLERS

FALLING FILM CAPACITIES: 15 – 4000KwPAC SCREW CHILLER CAPACITIES: 130 – 8000KwTEMPERATURE LEAVING FALLING FILM: 0.5 - 5°CTEMPERATURE LEAVING PAC SCREW CHILLER: 2°CINSULATION: 50-80mm Polyurethane foamFALLING FILM MATERIALS OF CONSTRUCTION: 304 Stainless steelEVAPORATING TEMPERATURE: -10 to -3°CREFRIGERANTS: R717/R22/R134a/R404A

Falling film water chilling system For concrete batching plants where a requirement arises to provide concrete at < 25°C normal potable water at ambient temperature cannot be used, chilled water at the lowest possible temperature is necessary. Standard air conditioning type semi hermetic air cooled water chillers are sometimes used in concrete batching plants however they cannot provide chilled water below 5°C. Air-conditioning chillers do not have a long life in these applications, they are not suitable for batching plant applications. Batching plants are industrial areas, where cement, dust and sand tend to clog the condenser and commercial semi hermetic compressors cannot provide the reliability required in a demanding process application.

Falling film water chillers are installed with a separate refrigeration plant, independent of the flake ice system. The systems are packaged or site installed including industrial refrigeration reciprocating or screw compressors, evaporative condensers, HP liquid receiver. The chillers can provide chilled water from 0.5 to 2°C, the water is chilled by flowing over a number of stainless steel plates and collected in an insulated tank underneath. The stainless steel plates are fully flooded with either R22/R717 or R404A refrigerant. The plates can if required operate with a thin film of ice unlike conventional shell & tube evaporators. 

Alternatively ice bank chilled water systems should be considered, comprising similar plates as above but installed in an insulated tank and immersed in the water to be cooled. The ice bank can be operated overnight to build up ice on the plates, which then provides the means of chilling the water required for the following day. Systems from 40000 to 40000ltrs are available.

PAC industrial factory packaged chillers use either industrial reciprocating compressors or screw compressors. The chillers operate on a flooded Thermosyphon principle using R717/R22/R404A/R134a, the package is complete with plate heat exchanger evaporators + LP liquid accumulator, plate heat exchanger water cooled condenser, starter and control panel, structural steel base frame and all necessary safety and operational controls and stop valves.

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TYPICAL 50 TONNE ICE STORE AND DUAL PNEUMATIC ICE DELIVERY SYSTEM

FIGURE 3

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Containerized Modular Concrete Cooling Systems

Water chilling plants 1. The system is designed on the basis of modules, which can be added or reduced according to actual use2. The product is containerized for cost effective and quick installation on site.3. The equipment can easily be relocated to different plants around the Kingdom4. The refrigeration systems use environmentally and efficient refrigerants such as R717 5. Designs are optimized to provide the highest efficiency for the lowest cost.7. Intelligent design and all-automatic electrically controlled system.

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Even under the most severe climatic conditions in Saudi Arabia, it is possible to cool water from an average supply temperature of 40°C down to as low as 32°C depending on the local Wet bulb temperature, using an evaporative condenser without the use of a refrigeration system. This reduces the water chiller load and improves the system overall efficiency of the system.Using a Plate exchanger operating on direct expansion, further water cooling from 35°C to 10°C is possible taking the refrigeration load as a side load when the system utilizes screw compressors. This cooling is virtually free as the compressor shaft power only increases by some 1.5-2% when the load is taken as a side load.

When reciprocating compressors are used, a separate independent falling film type exchanger or PHE water chiller can be used which can reduce the water temperature to 0.5 and 2°C respectively. The falling film system consists of a stainless steel water tank in which a stainless steel plate extended surface exchangers is installed. The plate heat exchanger surface builds up a thin film of ice over which the water is circulated.

Plate heat exchanger characteristics:1. High heat conduction efficiency and small delta t2. Low pressure drop3. Safety and reliability in use, easy maintenance4. Compact

PHE water chillers boast high efficiency and adopt 3 or 4-stage cooling.

The screw compressor

The twin helical screw compressor is an industrial heavy duty compressor with stepless capacity control and variable volume ratio control. Screw compressors are more liquid refrigerant tolerant than reciprocating compressors (but contrary to popular belief are not liquid tolerant in terms of rotor and bearing wear) and provide extended operating life and mean time between failures. Screw compressors exert fewer shaking, inertial, torsional moments providing good dynamic balance and low vibration, stable in operation, have a small footprint. Drive may be electric or gas or diesel engine drive.

Cold water storage tanksEvery water cooling system requires a well-insulated cold water tank, whose dimensions depend on the length of the non-production period of the batching plant. There are 4 different possibilities for installing such a tank:1) The tank is made of concrete and can be manufactured on site to meet the customer's requirements;2) The tank is containerized and internally insulated inside a 20 or 40-foot high cube container;3) The tank is made of stainless steel and insulated4) The tank is made of Fibreglass and insulatedIn the above-mentioned 3 cases, it is recommend to install the water chiller on top of the tank to save space and reduce installation cost.

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