4. ICE PLANTS

4.1 Block Ice
4.2 Flake Ice
4.3 Plate Ice
4.4 Tube Ice
4.5 Selection
4.6 Capital Costs
4.7 Area Requirements
4.8 Direct Running Costs
4.9 Power
4.10 Labour
4.11 Water
4.12 Maintenance

Types

There are many methods of producing ice and the selection of the most suitable plant for a prospective buyer is not simple. The manufacturing process of one may involve efficiencies or advantages over another or the ice produced by one process may be better suited to the intended application than another. The plant types are usually referred to by the type of ice they produce; common examples being block, flake, plate and tube.

4.1 Block Ice

In the manufacture of block ice, tapered rectangular cans filled with water are immersed in a tank of refrigerated brine as shown in Figure 9.

Figure 9. A brine type block ice plant

The brine which is cooled to about -5�C by a refrigeration process extracts the heat from the water and produces block ice within the can. The cans are then removed from the tank and thawed for a short time in a tank of water to release the block from the can. The blocks are then stored in a cold room and can be crushed on demand. The freezing period is typically between 16 and 24 h although plants known as rapid-block are available that have freezing periods of only a few hours. The quick freeze is achieved by the direct evaporation of the refrigerant in a jacketed mould fitted with finger evaporators. The blocks are released by a hot gas defrost and are subcooled to a temperature of -8�C. Rapid block plants require far less floor space than brine tank systems. Figure 10 shows a rapid block plant.

Figure 10. Rapid block plant

Typical block sizes and weights are given in Table 5.

Table 5 Block sizes and weights

4.2 Flake Ice

Flake ice is formed by spraying water over the surface of a refrigerated drum to freeze it and then mechanically removing it with a blade as shown in Figure 11 overleaf. In some models the drum rotates against a stationary scraper on its outer surface; in others the scraper rotates and removes ice from the inner double-walled stationary drum (as shown). No water is sprayed on the drum immediately in front of the scraper, so that the water is completely frozen and dry on removal. Ice produced by this method is commonly 2-3 mm thick.

Figure 11. Flake ice plant

4.3 Plate Ice

In the manufacture of plate ice, water is sprayed and frozen onto the outer surface or surfaces of a refrigerated plate to form a sheet of ice which is usually released by an internal hot gas defrost.

Figure 12. Plate ice plant

Thickness of the ice can be varied, within limits, by alteration of the defrost cycle, but is commonly set between 8 and 15 mm. To produce ice thinner than this would be inefficient due to the refrigeration energy required to repeatedly account for the heat of defrost. As the ice falls it is chopped by a rotating cutter. The process is automatic.

4.4 Tube Ice

Tube ice is formed in a vertical shell and tube vessel by passing water down the Inside of tubes which are cooled by the circulation of refrigerant on the outer surfaces as shown in Figure 13.

Figure 13. Tube ice plant

When the ice reaches the desired thickness the water flow is stopped automatically, the refrigerant is removed to a surge drum and hot gas is circulated around the tubes causing the ice tubes to melt and slide down. As the ice falls from the tubes it is broken by a rotating knife blade to a desired size. The ice tubes are about 50 mm in diameter with a wall.thickness of 10-12 mm.

4.5 Selection

In the selection of an ice plant, many considerations need to be made relating to capital costs, running costs, availability of skilled servicing, water supply and the preferred type of ice. No plant is universally superior to all others and the selection of plant needs to be made with knowledge of local conditions and requirements. Manufacturers will meet a customer requirement from a given range of equipment and can manipulate specification to suit a particular market or to meet competition. In the selection of equipment or comparison of competitive tenders, it is essential to consider the total costs of production; both running and capital. It is possible that a plant of low capital cost could have high running costs resulting in a higher cost of ice than a plant of higher initial cost but lower running costs. It is essential to fully understand and appreciate a manufacturers specification and exclusions.'

It is usual at the appraisal stage of an investment to prepare a technical/economic analysis based on estimated capital and running costs. The analysis should help not only in financial planning and pricing but in selection of plant as the relative local costs of electricity, water, finance and labour will influence the design specification.

As an example, the following analysis provides a costs estimate of ice production. The analysis is based on a flake ice factory of 150 t/24 h. The capital and running costs used are 1979 costs in the UK The analysis assumes a location on the dockside and includes no allowance for distribution.

A Cost Estimate of Ice Production

Flake ice plant - 150 t/24 h
Total years production = 120 t/24 h x 5 d x 52 weeks = 31 200 t (at 91% capacity utilization)
Capital cost of plant = US$ 1 000 000

It is clear from the analysis that the correct selection of capacity of plant and store to meet the size and pattern of demand is of importance. If the plant is overspecified and underutilized the fixed overheads will be carried by a disproportionate volume of sales and the unit costs will be high.

The ice store should be sized according to both the volume and pattern of demands. Ice production and demand are seldom in phase and the store acts as a buffer helping to overcome daily fluctuations between production and demand. It also acts as a buffer in the event of a temporary breakdown of plant or interruption of services. Stores are commonly 2-4 times the daily production capacity.

To assist the supplier or refrigeration contractor in selection and design, the buyer should make available as much information as possible regarding local costs and site conditions, etc. The following lists the more important considerations:

Production capacity desired under local ambient conditions
Ice storage capacity desired
Purpose for which ice is to be used
Preferred type of ice, and in the case of block, the block size
Maximum ambient temperature and humidity and annual fluctuations
Cost and availability of local labour including skilled
Temperature, pressure and purity of ice make-up water
Cost of make-up water
Temperature, pressure and type of condenser cooling water
Cost of cooling water
Information on electricity supply: voltage, cycles, phase, maximum installed power, maximum starting current allowable. Cost of electricity/unit and details of any reduced rate for off-peak use.
Detail of any physical or planning restrictions of the intended site
Detail of site soil tests
Particulars of grant aid, or soft loans (if any) toward capital or running costs of plant and conditions attached to aid
Cost of capital (interest)

From the information provided selection of plant can be made that will satisfy any site restrictions and produce ice at a minimum cost.

4.6 Capital Costs

It is often necessary for purposes of financial planning that budget costs are known before a detailed analysis is available. To assist in financial and technical planning the following tabulated data is an attempt to provide an order of costs and physical data for different types of ice plant inclusive of store. The figures shown in Table 6 are based on information provided by plant operators in the UK and assume UK cost levels (1979) and ambient conditions. It must be stressed most strongly that these figures cannot be used universally without considerations to local site conditions and costs, but they do provide a datum from which rough budget costings may be estimated given a general appreciation of local costs compared with those of the UK.

Table 6 Capital costs (US$)

Automatic operation and conveying is assumed on production of 10 t/24 h and above for tube, flake and plate. Manual operation and discharge is assumed for 2 and 5 t/24 h. Three days storage capacity is included for tube, flake and plate and two days for block. The figures represent a turnkey cost and are inclusive of plant, erection, delivery, soil investigations, commissioning, freight and insurance, etc., but exclusive of ground cost. The costs reflect a site that requires no exceptional civil works in piling or clearing with services near at hand.

As a rule of thumb the plant items (ice generators, refrigeration machinery, motors, conveying equipment, weighing equipment, etc.) for tube, flake and plate ice, plants over 10 t/24 h, amount to approximately 50 percent of the total cost.

4.7 Area Requirements

The required overall ground area for an ice plant will largely depend on the type of: ice-maker, condenser and store and the configuration of these units. It is usual in the case of plate and flake ice production that the ice-maker be located over the store enabling gravity feed of the ice to the store which results in a low ground area requirement overall. This is not feasible for the block type of plant because of the heavy loads imposed on the structure and unusual in the case of a tube plant because it would result in an excessively high building. Table 7 shows the area requirements of the ice-maker and refrigeration plant, not including storage, office accommodation, electrical sub-station or condensers.

Table 7 Area requirements for ice-maker and refrigeration plant (m�)

a/ Packaged units with allowance for access

4.8 Direct Running Costs

The direct running costs of labour, electricity, water and maintenance will largely depend on the type and size of plant, location and the degree of automation. As mentioned before, the design of plant should reflect the local capital and running costs such that the cost of ice per ton is minimized. In some instances there will be a trade-off of component costs. For example, the selection and design of the condenser requires a knowledge of local costs of electricity, water and labour (ignoring space as a cost). A shell and tube condenser using sea water which is dumped would have low water and electric costs in comparison to other condensers but higher labour costs due to the cleaning required. An air-cooled condenser would have no associated water cost but higher electric cost (particularly if it was silenced). An evaporative condenser or shell and tube with cooling tower, using fresh water would compromise the costs of electricity and water.

4.9 Power

In addition to the type of ice plant and its associated refrigeration machinery, the power consumption will depend on local ambient conditions and feed water temperatures. Table 8 shows the approximate electricity consumption per ton of ice produced for the icemaker and refrigeration plant for temperate and tropical areas. The figures do not include requirements for handling, crushing or storage.

Table 8 Approximate electricity consumption for ice production (kWh/t)

The flake ice plant has a slightly higher overall requirement due to the low evaporation temperature used to produce subcooled ice even though it has no defrost load requirement. Large ice plants are often more efficient than smaller ones and are sometimes specified with precoolers particularly in tropical areas, which sometimes enables the use of a smaller icemaker. The precooler is specially designed to lower the temperature of the feed water to the ice-maker and does it more efficiently than the ice-maker. Automatic plants (flake, tube and plate) are better suited to take advantage of any tariff reductions for off-peak use of electricity. Where penalties are incurred for high peak demands such that occur on start-up, the plant can be supplied with a current-limiting device that will limit the power demanded.

4.10 Labour

Labour costs will depend on local rates of pay, social security payments, the degree of automation and the type and size of plant. Generally the automatic plants (flake, tube, plate) require far less labour than block plants, particularly for the larger capacities. The cycle and harvest of automatic plants requires little attention and it is possible for one man to discharge from the store a measured quantity of ice. Table 9 compares the labour requirements for different sizes of automatic and block type plants, but it must be stressed that in both cases the labour requirement will be dependent on design. No inclusion is made for trimming of vessels at the quayside and the block plant is assumed to be of the continuous harvest type. Twenty four hours availability of ice is assumed.

Table 9 Labour requirements (man hours/24 h)

4.11 Water

The basic water requirement for ice making will be slightly more than the equivalent quantity of ice allowing for losses and bleeding to drain to prevent the build up of salts in circulated water systems. Additional water will be required for domestic purposes and for condenser cooling. The quantity required for condenser cooling depends on the amount of heat to be rejected, the temperature of cooling water and whether the system recycles the water or not. The requirements of a shell and tube condenser in tons of water per ton of ice produced is given in Table 10.

Table 10 Requirements of cooling water for a shell and tube condenser

An evaporative condenser will use between 0.25 and 0.5 t/t of ice depending on design. Precise requirements can be calculated from manufacturer's figures making allowance for windage, evaporation and bleeding. Shell and tube condensers employing a water tower and recirculation will have similar requirements to the evaporative condenser.

Water used for ice making should meet microbiological standards of potable water and be free of excessive solids or salts, which may detract from the physical properties of the ice produced. Problems may also be experienced when using pure water for flake ice production as the ice produced adheres to the drum and is difficult to remove. This problem may be overcome by the addition of salt to the make-up water.

4.12 Maintenance

Although maintenance costs for financial planning are often budgeted as a fixed percentage of the capital cost the actual costs are obviously dependent on the type of ice plant, its delivery system, the quality of servicing it receives and the age of the plant. Maintenance costs are often higher in the very early life of a plant and in its later years. Between 2 and 5 percent of the capital cost is commonly allocated per annum but actual costs can vary greatly year to year. Block plants generally have higher maintenance costs due largely to repair or replacement of moulds which may have an average life of only a few years. The cost of replacing the moulds of a 100-t/d plant would be in the order of US$ 60 000 to 100 000. Block plants also require more attention to the building structure due to evaporation of the brine and its effect on the steelwork although this can be reduced by the use of inhibitors.