Chapter 117 - Definitions - Heat Exchangers For Refrigeration Systems

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Heat exchangers are

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equipment in which heat is transferred from one fluid to another without mixing the fluids.

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two types of heat exchangers are used in a refrigeration system, they are:

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- evaporator

- condenser

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An evaporator is

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the part of the refrigerating system n which liquid refrigerant is vaporized by the absorption of heat from the medium to be cooled.

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two types of evaporators are

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- dry/direct expansion

- flooded

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in a dry evaporator the amount of liquid refrigerant in the system at any given time is

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- a small amount, varies with load demand

- the amount of liquid refrigerant entering the evaporator balances with the amount leaving as vapour. (no recirculation of liquid or gas takes place)

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in flooded evaporator the liquid

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- keeps the evaporator almost completely filled (flooded) regardless of load.

- as vapour is drawn off fresh liquid is brought in to maintain the level.

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Advantage of flooded evaporator

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the heat exchange surface in a flooded evaporator is almost always completely wetted with liquid therefore it has a higher heat transfer rate than dry.

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Disadvantage of flooded evaporator

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require a relatively large refrigerant charge that is often quite bulky.

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construction classifications of evaporators are

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- bare tube

- plate surface

- finned tube

- shell and tube

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bare tube evaporator

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- consists of either a single coil bent in various shapes or a number of coils placed in parallel and connected to common headers.

- submerged in the liquids that are to be cooled

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bare tube evaporator is made of

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steel pipe or copper tubing

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bare tube evaporator can be used as

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- direct expansion evaporator

- flooded evaporator (in this case the coil would be attached to an accumulator)

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bare tube evaporatord are used where

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space temperature is maintained below 1°C and frost accumulation cannot be readily prevented

ex: coolers, freezers

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plate surface evaporator are made by

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- embossing two flat steel plates to form a serpentine coil between them when fused together.

- using a regular coil attached between two plates

- can be formed into various shapes

- can be used individually or in banks

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plate surface evaporators are widely used in

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- refrigerators

- freezers

- display cases

- locker plants

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advantages to plate surface evaporators

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can easily be cleaned and defrosted by manual scraping without interrupting the cooling process

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plate surface evaporators are usually applied as

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direct expansion evaporators

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Finned tube evaporator are

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basically a bare tube evaporator coil with fins attached by crimping or bonding.

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fins in a finned tube evaporator serve what purpose?

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provide a large surface area for heat exchange

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the most common method of attaching fins to the tubing of a finned tube evaporator is

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- by placing the fins over the tubing and running a mandrel or expander down the tube causing it to expand.

- the connection between the fins and the tub is referred to as pressed

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Finned tube evaporator is applied as a _______________ evaporator and is used for:

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direct expansion used for many applications where space and temperature are held well above freezing.

- walk-in coolers

- display cases

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the formation of frost on a Finned tube evaporator could

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- clog air passages

- reduce the refrigerating capacity

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the most widely used in air conditioning applications is the

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Finned tube evaporator with forced air circulation

*aka blast coil

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when does a Finned tube evaporator use natural convection and when does it use fans?

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natural convection - low air velocities and minimum dehydration are required

one or more fans - to increase air flow and cooling capacity

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how does fin spacing vary between low and medium temperature applications?

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- smaller spacing for medium temperature applications (more heat transfer surface)

- larger spacing for low temperature applications (less heat transfer surface)

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shell and tube evaporator is used for

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almost any type of liquid-chilling application

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shell and tube evaporator is also known as

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a chiller

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describe a shell and tube evaporator

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a cylindrical steel shell in which a number of straight bare tubes are arranged in parallel and held in place by tubesheets which also close the shell. (like the fireside of a firetube boiler) outside the tubesheets are hollow covers fitted with baffles (presumably to direct flow of coolant)

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what type of evaporators are chillers?

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shell and tube evaporators that are either direct-expansion or flooded

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in a direct-expansion evaporator

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- the liquid refrigerant enters the lower tubes and as it passes back and forth through the tubes it evaporates by absorbing heat from the liquid (medium to be cooled) surrounding the tubes.

- vapour is drawn from the unit by the compressor suction after it leaves the final upper pass.

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the flow of liquid to be cooled in a direct expansion evaporator

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- enters at one end of the shell

- is forced to follow a path through baffles across the tubes

- liquid reaches the outlet at the other end of the shell and leaves at a reduced temperature

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in the flooded type evaporator the liquid to be chilled passes through the tubes arranged in ______ or ___________ configuration.

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single or multi pass

*haha multipass

**if the direct expansion is like a firetube boiler this one is like a watertube boiler if you replace combustion gases with coolant.

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in a flooded type evaporator the level of the liquid in the shell is maintained by

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a float controlled valve

*at least 75% or the tubes are submerged at all times.

** vapour is drawn off by nozzles at the top of the shell.

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to aid in the separation of refrigerant liquid from the vapour most evaporators are also equipped with

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a bank of chevron shaped mist eliminator plates

*similar to drift eliminators in boilers

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The function of refrigeration condenser is to

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- cool the hot compressed refrigerant vapour until it condenses into a liquid.

- it must remove the heat absorbed in the evaporator by the vapour and the heat of compression added in the compressor.

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three types of condensers are

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- air-cooled

- water-cooled

- evaporative

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air cooled condensers remove heat from the refrigerant vapour by

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refrigerant vapour flows through finned tubes while cooling air is circulated by fans or blowers around the outside of the tubes.

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a remotely mounted air cooled condenser is usually installed

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- either indoors or outdoors

- with larger capacities

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to ensure an indoor air cooled condenser receives adequate cool air:

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small units - opening a window

large units - ducts must be used

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large condensers are commonly mounted

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outdoors - roof being the most popular (can be vertical or horizontal)

indoors tends to require extensive space for ductwork. generally they are not mounted here.

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3 basic designs of water cooled condensers

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- double tube

- shell and coil

- shell and tube

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double tube water cooled condenser

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- consists of a small tube contained concentrically within a larger tube.

- cooling water flows through the inner tube

- refrigerant flows in the opposite direction through the outer tube

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disadvantage of double tube water cooled condenser

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- has lowest efficiency of the 3 designs

- needs a larger condenser for a given capacity

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double tube water cooled condenser is sometimes used as

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a booster condenser with air-cooled units during periods of peak loading

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shell and coil water cooled condenser

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- consists of a horizontal or vertical steel sheet which contains a coiled tube.

- coil is either bare or finned

- cooling water circulates through the coil

- refrigerant enters near the top of the shell and collects at the bottom as it is cooled.

- bottom portion of the shell serves as a receiver

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Advantages of shell and coil water cooled condenser

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- very efficient

- very compact

- easily adaptable to packaged refrigeration units

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Disadvantages of shell and coil water cooled condenser

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- difficult to clean

- when leaks develop repair costs often exceed cost of new condenser

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to clean tubes in a shell and coil water cooled condenser

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using an acid solution which is then neutralized

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shell and tube water cooled condenser

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- consists of welded steel shell containing a number of straight tubes fastened to tubesheets

- fitted with waterboxes which direct the water from inlet to outlet in two or more passes (water in the tubes)

- refrigerant enters at the top of the shell and condenses as it passes through the tubes and collects at the bottom of the shell

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shell and tube water cooled condenser is used extensively in

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- ammonia installations of all sizes

- medium and large sized AC installations using other types of refrigerants

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Advantages to horizontal shell and tube water cooled condenser

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- high cooling efficiency

- reasonable cost

- easy cleaning

- low routine maintenance costs

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what is special about the evaporative water cooled condenser

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it uses both air and water to provide cooling

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evaporative water cooled condenser

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- contains condensing coils mounted in a cabinet

- refrigerant vapour enters at the top of the condenser and flows through a set of coils over

- water is sprayed from nozzles and a current of air is drawn in counterflow to the water

- thus some of the water evaporates and has a cooling effect on the remaining water

- refrigerant liquid collects and leaves at the bottom where it flows to receiver and then to evaporator cooling coils

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how is water circulated in an evaporative water cooled condenser

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water is continuously cirvulated by a sump pump located at the bottom of the condensers

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the fan in an evaporative water cooled condenser may be

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induced or forced draft

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as air leaves the evaporative water cooled condenser it flows through

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drift eliminators - entrained water droplets precipitate out and fall back over the cooling coil

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advantage of evaporating water in an evaporative water cooled condenser

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-large amount of heat is absorbed

(approx 2257kJ/kg of water)

- the amount of water used is a very small % compared to that required for other water cooled condensers of the same capacity

- the amount of air circulated is also a very small % compared to other air cooled condensers of the same capacity

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evaporative water cooled condenser are favoured in locations where

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it is necessary to conserve water

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evaporative water cooled condensers may save ____ to _____ % in water consumption compared to conventional water cooled condensers

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80-90%

*conventional water cooled condensers also generally discharge the water as waste

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why is makeup water required in an evaporative water cooled condenser

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part of the spray water that evaporates is carried off by the air leaving the system

*a float valve is used to maintain a constant water level

** will also have a bleed valve in the sump will continuously bleed some of the water to help manage the concentration of dissolved solids in the water supply

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when an evaporative water cooled condenser is located indoors

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- air inlet and discharge are connected to the outside via ducts that are closed by louvers when the condenser is not in operation

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when the evaporative water cooled condenser is located outdoors

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- care should be taken to prevent freezing of the water during cold weather.

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preventing water freezing in an evaporative water cooled condenser located outside is done by

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- placing a heating coil in the sump of the condenser. this coil keeps the temperature of the water above freezing when condenser is not in operation

- or the condenser sump drains to an indoor tank that the spray pump (also indoors) draws from

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the liquid refrigerant receiver in an evaporative water cooled condenser is sometimes placed in the sump to provide

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- subcooling of the liquid refrigerant

- results in an increased refrigerating effect when supplied to the evaporator.

*exterior of the tank should be protected against the corrosive properties of the spray water and air

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the air intake of an evaporative water cooled condenser should be provided with a _____ to prevent dust or sludge formation in the sump or water tank

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filter

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Defective condenser operation may result from

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- dirty cooling water on waterside

- coating of oil or oil-derived material on refrigerant side

*use proper compressor lubricating oil to keep refrigerant side surface, coil, and tubes clean

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the presence of non-condensable gases in the condenser

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- reduces capacity of the condenser causing high compressor discharge pressure and temperatures

- therefore condenser must be purged periodically

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if cooling water contains scale-forming solids

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- scale may be deposited on waterside surfaces of condenser in shell and tube, double tube, and evaporative condensers

- scale reduces heat transfer rate and seriously reduces efficiency of the condenser and refrigeration system

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internal and external surfaces of condenser tubes or coils should be inspected at __________ and cleaned __________.

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frequent intervals, whenever deposits become discernible

*a water softening system may be economical especially where water is exceptionally hard.

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the interior surfaces of shell and coil and double tube condensers can be cleaned by

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taking the unit out of service and circulating an acid or other chemical solution through it

*such an operation will require the advice from a qualified chemist or the manufacturer of the unit

**shell and tube condensers with straight tubes can also be cleaned by passing mechanical cleaners through the tubes