In this sample MEP specification section, Mechanical Refrigeration Plant has been described in detail under construction contract work. Main Highlights of this MEP Specification are: General Requirements, Compressors, Evaporators, Condensers, Condensing Units, Refrigeration Plant Accessories, Pipework, Valves and Ancillaries, Insulation, and Controls.
General Requirements for Mechanical Refrigeration Plant
This section deals with mechanical refrigeration plant and ancillaries for vapour compression refrigeration plant using halogenated fluorocarbon refrigerants.
All items of plant whether supplied separately or as a factory assembled package unit shall comply with the following sub-sections unless superseded by the Particular Specification.
The refrigeration plant shall include compressor(s), evaporator(s) and condenser(s) all as indicated to suit the application. The items shall be correctly matched to ensure satisfactory operation. The heat rejected at the condenser under maximum design conditions shall include the design heat load absorbed at the evaporator, the heat equivalent of the work done by the compressor and any other heat gains of the system.
A full charge of refrigerant and lubricating oil shall be supplied together with all items necessary for the satisfactory functioning of the system.
Complete control and instrumentation equipment including safety devices shall be included.
Packaged assemblies shall be resiliently mounted on a combined base plate and tubular framework requiring only the external water and electrical connections to be made on site.
Complete data of plant operation and maintenance should be made available to the Engineer together with a Schedule of Spare Parts and Repair Information.
Compressors in Refrigeration Plant
Hermetic compressors may be used up to 10 kW; where they are supplied, the entire refrigeration system shall be completed and tested at the Factory.
Semi-hermetic and open type compressors shall be suitable for the type of refrigerant specified in the Particular Specification. Where a direct expansion cooler is employed the refrigerant shall be non-toxic.
All compressors shall be driven by electric motors and may be of the reciprocating, centrifugal, rotary or screw type. Speed shall be such that the noise is kept to a reasonably low level consistent with the application and location of the machine. The complete compressor unit shall be mounted on an anti-vibration base reducing transmissibility from compressor to structure to below 10%. The Contractor shall supply all necessary foundation bolts.
When one compressor is specified it shall be designed for the maximum cooling load; where two or more compressors are required the maximum cooling load shall be divided equally between the number of compressors.
Lubrication shall be automatic and so arranged that the compressor may be started frequently (under thermostatic control) or be run for prolonged periods without inspection. An oil separator shall be fitted to the compressor discharge line and shall be of the float and needle valve type and shall have an automatic return to the compressor crank case. Crank case oil heaters shall be provided and shall operate when the compressor is shut down.
Semi-hermetic compressors shall include the following features:
- All parts of the electric motor shall be proof against contact with refrigerants and the associated lubricating oils. Additional motor protection shall be provided against failure or partial failure of the gas cooling system. All valves on the refrigerant circuits shall be located in a readily accessible position and shall be provided with seal caps.
- A rotary mechanical seal shall prevent loss of refrigerant at the driving shaft. The seal shall be effective for long operating periods and short rest periods without leakage.
- All compressors of duty in excess of 35kW shall incorporate capacity control and shall start unloaded. Hot gas by-pass capacity control shall not be used unless indicated.
- When called for in the Particular Specification the low pressure cut outs shall be used to control the operation of the compressor, in which case it is to be the self restoring type with adjustable working differential.
- Liquid receivers with inlet and outlet service valves shall be provided where required. A liquid sight glass shall be mounted vertically in the liquid line with integral filter.
- Means shall be provided for oil to return to the crank case from the suction gas header and for refrigerant gas (but not oil) to return to the suction manifold from the crank case. Crank case oil heaters shall be provided and shall operate when the compressor is shut down. An oil pressure relief valve or a bleed from the oil pump discharge back to the crank case shall be provided.
Suction gas entry to the compressor shall be designed to reduce gas velocity so as to assist in releasing any entrained oil or dirt; suction gas strainers shall be fitted.
The following fittings shall be provided:
- Suction and discharge stop valves.
- Suction and discharge refrigerant gas pressure gauges with means of isolation.
- Oil pressure gauge.
- Oil sight glass.
- High and low gas pressure adjustable safety controls.
- Semi hermetic compressors shall include the following features.
- All parts of the electric motor shall be proof against contact with halo-carbon refrigerants and the lubricating oil associated with halo-carbon refrigeration systems.
- The oil pump shall operate equally well under either direction of rotation.
- Where the motor rotor is overhung a support bearing of adequate width shall be provided.
- Additional motor protection shall be provided against failure or partial failure of the gas cooling system.
Evaporators in Refrigeration Plant
Evaporators are to be designed in accordance with the current standard practice being suitable for direct expansion, chilled water or brine as required.
Evaporators for water chilling installations unless otherwise indicated shall be of the shell and tube heat exchanger type. The evaporator shell, tube plates and end covers shall be of steel or cast iron; the tubes shall be of copper and may be extended surface piping.
The end covers shall be removable and the plant shall be arranged so that there is adequate space for cleaning and removal of the tubes. Isolating valves and flow and return connections shall be provided. Unless otherwise indicated the fouling factor shall be 0.000088 m2 oC per W.
Evaporators may be of either the flooded type or the dry expansion type. The flow of refrigerant to dry expansion type evaporators shall be controlled by a thermostatic expansion valve; the flow of refrigerant to flooded type evaporators may be controlled by either a float level or a thermostatic expansion valve.
In dry expansion type evaporators the velocity of refrigerant and the design of the refrigerant passages shall ensure that any oil present is carried over to the compressor. In flooded type evaporators special provision shall be made to return oil from the evaporator to the compressor.
Unless otherwise indicated the design evaporating temperature for direct expansion air coolers and for dry expansion shell and tube water chillers shall not be less than -1oC; for flooded evaporators it shall be not less than +1oC.
A thermostatic expansion valve shall maintain the design degree of superheat at the evaporator outlet; the remote sensing bulb shall be securely fixed to the evaporator outlet piping in a position where the degree of superheat can be correctly sensed. Thermostatic expansion valves shall incorporate an external equalizer line.
Refrigerant spaces shall be tested at the maker’s works to 1400 kPa for use with refrigerant 12 and 2100 kPa for use with refrigerant 22. The water side of heat exchangers shall be tested to 700 kPa.
Air cooled condensers in Refrigeration Plant
Air cooled condensers shall be tested at the maker’s works to 2000 kPa for use with refrigerant 12 and to 2750 kPa for use with refrigerant 22. Condensers shall have copper tubes with aluminium fins or, where indicated, they shall have copper tubes with copper fins. Condensers with fins which have been badly damaged during transport or erection or which have become choked or coated with builder's material will not be accepted. Minor irregularities in the fins shall be combed straight.
Where air cooled condensers are sited within a building and serve a compressor larger than 2.25kW the air outlet shall be ducted to the external air; condenser fans shall be capable of overcoming the extra resistance of this ducting. The condenser frame, supports and enclosure shall be of galvanised steel. For saline, sand laden or corrosive atmosphere extra protective finishes shall be provided.
Where air cooled condensers are mounted outside buildings, weatherproof motors shall be provided and the condenser enclosures shall be designed to protect the fan, motor, drive and finned coils from the weather, including solar radiation. Externally mounted condensers shall discharge air vertically upwards unless they are protected by an integral wind deflection shield or a purpose made builders work baffle.
Where an air cooled condenser will have to operate under conditions of much lower air temperatures than the design maximum a means of maintaining condenser pressure shall be incorporated. This may be achieved by modulation of the outlet dampers and/or by-pass dampers by condenser flooding or by fan volume control.
Where there are three or more condenser fans, control may be effected by sequencing of these fans. Where modulation of the outlet damper only is used the fan motor shall be of adequate size to deal with the minimum extra load and shall in any cased be switched off by the dampers when they are closed. Fans shall comply with the relevant Section of the Specification.
Condensing Units in Refrigeration Plant
Condensing units may be water cooled or air cooled as indicated. Sub-components i.e. compressors etc. shall be in accordance with the appropriate sections of the Specification.
The components shall be constructed within a robust steel framework with all internal piping and wiring completed. Where the unit is exposed to weather it shall be suitably treated to avoid corrosion and damage.
A control panel shall be provided and shall include starters for all compressors and fans complete with overload protection. The panel shall be pre-wired but allow for connections for external interlocks and shall also include a control circuit fuse. Anti-condensation heaters and oil failure switches shall also be provided.
Capacity control shall be as indicated in the Particular Specification or as shown on the contract drawings.
Refrigeration Plant Accessories
Every refrigerating system shall be protected by a pressure relief device.
Each system with an air cooled or evaporative condenser shall have a liquid receiver complete with two stop valves, a sight glass, charging connection and a purge valve. Relief valves shall be fitted on liquid receivers and water condensers for plant larger than 17.5 kW. Liquid receivers on plant smaller than 17.5 kW capacity may, alternatively, have a fusible plug.
Water cooled condensers on systems larger than 17.5 kW shall have a liquid receiver capable of holding a complete charge. Either the water cooled condenser or the associated liquid receiver shall have a purge valve. A capped valved connection shall be provided for refrigerant charging.
All major items of plant shall be dried at the factory and filled with a holding charge of refrigerant or inert gas; all openings shall be sealed. During erection care shall be taken to prevent the entry of moisture. A dehydrator shall be fitted in the refrigerant pipework. In addition to the suction gas and lubricating oil strainers fitted to the compressor, a liquid line strainer shall be fitted to protect the expansion valve or float valve; any pressure regulating device fitted in the suction line shall also be protected by a strainer. Where an evaporator pressure regulating device is fitted, an additional gauge indicating evaporator pressure shall be provided, complete with means of isolation.
Each compressor of more than 10 kW capacity shall have a discharge gas silencer.
In dry expansion type systems the liquid leaving the condenser shall be sub-cooled and the refrigerant gas leaving the evaporator shall be super-heated. For compressors greater than 200 kW capacity this shall be achieved by means of a purpose made heat exchanger.
Pipework, Valves and Ancillaries in Refrigeration Plant
All refrigerant pipework and all necessary fittings and ancillaries shall be supplied for the complete erection of the plant(s) at site. Pipework for refrigerant circuits shall be of refrigeration quality copper tube. Joints shall be flanged, brazed, welded or made with soldered capillary fittings. Pipelines shall be firmly secured and measures shall be taken to prevent vibration weakening joints and connections. In particular, the suction and delivery pipes from the compressor shall incorporate an adequate length of flexible piping of the armoured bellows type.
Pipework shall be designed and run so that any oil in the compressor discharge refrigerant which passes through the oil separator (where fitted) is carried through the system and returned.
At any point where a large quantity of oil may accumulate an oil separator and means of returning the oil to the compressor shall be provided.
All necessary suction, delivery, liquid, gauge and charging valves shall be provided so that all major portions of the system may be isolated for maintenance. In particular the refrigerant circuit shall be arranged so that any thermostatic expansion valve may be removed without the necessity of "pumping-down" the system.
Valves required for compressors, liquid receivers etc. shall be of either diaphragm or bellows type or be packed valves complete with a back seating and a seal cap.
The liquid refrigerant regulating valves shall be of the thermostatic expansion type preferably multi-outlet with pressure distribution and shall be provided with thermal elements of the "liquid charged" type and removable fine wire-mesh liquid strainer.
Magnetic liquid valves shall be installed in the liquid line before the expansion valves. The magnetic valves shall be provided with manual lifting screw or alternatively a valved by-pass.
Compressors shall not be connected to run in parallel unless specified in the Particular Specification, in which case all major plant items that are cross connected shall be valved for isolation.
When the refrigeration plant is used to provided a storage of chilled water or brine, the temperature of the liquid is to be maintained and controlled by capillary tube immersion type thermostats. A thin walled metal sleeve phial pocket is to be fixed securely in the side of the tank to receive the phial and isolate it from the contents of the tank.
Insulation in Refrigeration Plant
Refrigerant discharge and liquid lines need not normally be insulated, but where exposure to solar radiation is likely they shall be shielded or insulated with a 19mm thickness of material having a thermal conductivity of less than 0.036 W/moC and weatherproofed with a material having a reflective or light coloured finish.
The suction line from the evaporator to the compressor shall be insulated with a material having a thermal conductivity of less than 0.036 W/moC and least 19mm thick for pipes of up to 76mm diameter and 25mm thick for pipes of 108mm diameter and above.
This insulation shall be protected by a vapour barrier which shall not be broken between joint of sectional material or at discontinuities at valves or brackets.
Shell and tube evaporators shall be insulated with at least a 25mm thickness of material having a thermal conductivity of less than 0.036 W/moC. This material shall be protected by a vapour barrier and further protected by a painted sheet metal casing. The insulation and protection shall be so arranged that the end covers can be removed without damage to the insulation.
Controls in Refrigeration Plant
A solenoid valve shall be fitted in the liquid line to each evaporator, except high pressure float controlled flooded evaporators, to prevent the flow of liquid refrigerant to the evaporator when the compressor is not running.
Each compressor of 35 kW capacity and above shall be provided with an oil pressure failure control.
Compressors in tropical latitudes subject to time controlled operation shall be fitted with suction pressure regulating devices to limit the maximum suction pressure.
Where the stopping of a compressor of more than 35 kW capacity (or less where indicated) is not controlled by suction pressure, i.e. it is controlled by air or water temperature criteria, and the evaporator is at a higher level than the compressor, a pump-down cycle shall be incorporated. The control signal shall initially close the liquid line solenoid valve and the compressor shall then pump down to a predetermined suction pressure.
For each shell and tube evaporator a flow switch shall be fitted in the chilled water pipeline. The flow switch shall prevent the compressor(s) starting unless the chilled water is flowing and shall stop the compressor(s) if the flow falls below a safe minimum.
Additionally a chilled water low temperature limit thermostat shall be provided and shall stop the compressor(s) if the leaving water temperature drops below a set value. The flow switch and the thermostat shall each be connected in a separate circuit independent of all other control circuits; they shall not be wired in series.