Vol 18 – Shell & Tube Water Chillers, and Others Types of Water Chillers.

Author Mike Creamer, Business Edge Ltd

 

AIR CONDITIONING TECHNOLOGY

Volume 18

In Volume 17 last month we studied the construction, testing and protection of shell and tube evaporators.  Part 18 continues with information on shell and tube water chillers and other types of water chiller evaporators.

Shell and tube direct expansion water chillers have the following advantages over other types of water chiller evaporators:

1. Their design generally allows for oil to be entrained in the refrigerant gas for adequate return to the compressor without the need for complex and expensive oil return systems.

2. At a cooling capacity of less than 1000 kW they are generally smaller than flooded vessels.

3. As a consequence of their simple oil management and compactness they are generally cheaper than flooded vessels below 1000 kW capacity.

The disadvantages of shell and tube direct expansion water chillers are:

1. Unless suitable filters are used upstream in the water circuit and/ or water treatment is employed where necessary they can become clogged with water- borne solids which subsequently lead to a reduction in cooling capacity and the possibility of damage through freezing as a result of reduced water flow rate.

The design of shell and tube direct expansion water chillers usually makes them suitable for cleaning by chemical methods only.

2. The compact design of the tube bundles is one of the reasons for the relatively high efficiency attainable as a result of having low water/ brine contents in the shell.  It is for this reason they have to the protected against damage through freezing by incorporating and accurately calibrating a high quality and reliable low limit and freeze protection thermostat.

The capacity range for this type of water chiller can be from as low as 2 kW up to 2000 kW.   However, for financial and practical reasons they tend to be limited to 1000 kW with flooded shell and tube evaporators generally being used for larger capacities.

Refrigerants

Shell and tube direct expansion water chillers are generally suitable for use with the traditional CFC and HCFC refrigerants such as  R12, R22, R500, R502, etc. as well as R134a and R717.

They are suitable for use with the Zeotropic refrigerant blends such as R407c although caution must be exercised when deciding upon operational parameters due to the varying temperatures at which the individual constituents of the blend evaporate, a phenomenon known as the glide factor.

Flooded shell and tube evaporators, where the refrigerant is in the shell surrounding the tube bundle and the water / brine runs through the tubes, have already been covered in reasonable detail, but the advantages and disadvantages of these are now considered in relation to shell and tube direct expansion water chillers.

Advantages:

1. Higher efficiency than shell and tube direct expansion water chillers.
2. Above 1000 kW they are generally more compact than shell and tube direct          expansion water chillers and are therefore of lower cost.
3. Water / brine tubes are relatively easy to clean.
4. The capacity rang is much larger than DX (direct expansion) vessels, ranging from 1000kW to 6000 kW.
5. The refrigerant distribution difficulties associated with shell and tube DX water chillers, particularly on the second pass do not apply to the flooded design.

Disadvantages:

1. Oil is generally retained in the vessel on the boiling surface of the liquid refrigerant making oil recovery a more complicated and expensive affair.
2. It is vital to ensure that only dry and adequately superheated refrigerant vapour leaves the evaporator for the purposes of compressor protection.
3. The introduction of replacement zeotropic refrigerant blends types dictates that     these should only be considered for use with flooded evaporators with extreme caution.
4. The oil recovery and suction gas filtration requirements generally make them        un-competitive against DX vessels below 1000 kW capacity.

Flooded shell and tube evaporators are normally suitable for use with the traditional CFC and HCFC refrigerants including R11, R12, R22, R113, R114, R123, R500 and R502 as well as R134a and R717.

Evaporator selection

In order  to select an evaporator  of the correct capacity for a given duty, the following  criteria  must be known:

• Required cooling duty –                         kW
• Chilled water inlet temperature –           Deg C
• Chilled water outlet temperature –         Deg C
• Chilled water flow rate –                        kg/s or l/s
• Refrigerant type
• Evaporating temperature –                     Deg C
• Saturated discharge temperature –         Deg C
• Extent of sub-cooling –                         K
• Number of refrigerant circuits

Where an additive to the water for anti-freeze purposes is involved, the type must be known together with the degree of concentration as this causes a variation to the value of several physical properties of the solution including specific heat, thermal conductivity and viscosity.

One single factor which affects the size and model selected for a given duty is the Approach Temperature.  The Approach Temperature is the difference between the Chilled Water Outlet Temperature and the Evaporating Temperature. Within reasonable parameters, the larger the approach temperature the smaller the vessel required for a given duty.

In order to select a shell and tube direct expansion water chiller utilising normal selection tables, the following procedure applies:

1. Calculate the chilled water Range (inlet temperature – outlet temperature)
2. Calculate the Approach temperature (chilled water outlet temperature – evaporating temperature).
3. Locate the selection table for the calculated chilled water Range and then plot the required cooling duty against the Approach temperature to obtain the vessel which must closely matches the requirement.
4. Calculate the design water flow rate (kg/s or l/s) using the following equation:

Q

water flow rate         =        ——————           =         kg/s or l/s

Cp x  (T2 –  T1)

Where:

Q         =          design cooling capacity          –           kW

T1        =          entering water temperature    –           Deg C

T2        =          leaving water temperature      –           Deg C

Cp       =          specific heat of water             –           4.187  kJ/kg K

kg/s or l/s     =     design water flow rate        –           kg/s or l/s

5. Plot the design water flow rate onto the pressure drop curves for the selected vessel to ascertain the water-side pressure drop through the vessel.

6. When an additive such as glycol is used, the revised specific heat of the solution must be used.

 

Shell & Tube Evaporator Selection Procedure

Two key factors which have a major influence on the performance of a the shell & tube dry expansion evaporator are:

1. Cooling Range (K) = Water Inlet Temperature Deg C – Water Outlet Temperature Deg C.
2. Terminal Temperature Difference (K) (Approach Temperature) = Water Outlet   Temperature Deg – Evaporating Temperature Deg C.

The above therefore very often form the basis of manufacturer’s capacity data for equipment selection purposes.  Certain products provide the optimum performance when a temperature difference of at least 10 K exists between the evaporating temperature and the water inlet temperature, very often defined as the superheat approach temperature.

Selection example:

The essential information that will always be required includes:

• evaporator load in kW
• evaporator water flow rate in litres per second
• chilled water inlet temperature Deg C
• chilled water outlet temperature Deg C

(any 3 of the above items will be required)

• evaporating temperature Deg C
• single or dual refrigerant circuit evaporator required

The above information will then allow a selection to be made. For example:

• evaporator load = 55 kW
• evaporator water flow rate in litres per second (Not given)
• chilled water inlet temperature = 16 Deg C
• chilled water outlet temperature = 8 Deg C
• evaporating temperature = 2 Deg C
• single refrigerant circuit evaporator required

 

Step 1

From the information, calculate the range and approach for your application.

Range:            16 Deg C – 8 Deg C = 8 K

Approach:       8 Deg C – 2 Deg C = 6 K

 

Step 2

Refer to selection table example which relates to a Range of 8 K and use the Approach column for 6 K, locating the model with a capacity most closely related to the requirement (select CH548B).

TABLE No: 1 HERE

 

Step 3 

Calculate the design water flow  rate from the following equation:

Q

Water flow rate     =     ——————       =      kg/s or l/s

Cp x  Range

 

 

55 kW

                                =   ——————        =       1.642 kg/s or l/s

4.187 x  8 K

 

Step 4

From the curves showing water pressure drop in relation to water flow rate, determine the water pressure drop for 1.642 l/s. (11 kPa).

TABLE No: 2 HERE

Simple Do’s and Don’ts for protection of evaporators in operation:

DO

• Check and maintain glycol strength regularly.
• Install a strainer in the water pipework immediately upstream of the evaporator.
• Install a flow switch in the water pipework downstream of the evaporator, interlocked with the safety controls governing the running of the compressor(s).
• Ensure the materials of construction of the evaporator are suitable for use with the fluid to be cooled.
• Ensure the evaporator is correctly sized to provide an adequate level of turbulent flow over the tubes to ensure good heat transfer (approximately 0.6 m/s). Generally speaking, the lower the pressure drop, the lower the level of turbulent flow.
• Ensure the fluid to be pooled does not carry any small particles in suspension which could be trapped within the vessel leading to eventual blockage.

DON’T

• Use shell and to evaporator with a continuous pump-down cycle.
• Evaporate below 0 Deg C without using a glycol solution of adequate concentration to ensure a freezing point below the minimum operating leaving water temperature.
• Reduce the fluid flow rate through the vessel below the design figure.
• Stop the fluid flow rate through the vessel prior to stopping the refrigeration system.
• Operate the vessel beyond the design parameters.
• Weld any items to the vessel shell without consulting the manufacturer.
• Set low limit and anti-freeze thermostats below the manufacturer’s recommendations.

 

Chilled Water Pipework

It is essential to install a variety of key components in conjunction with Shell & Tube DX water chillers in order to be able to commission the system correctly, maintain a check on performance and protect the chiller against freeze up due to inadequate water flow rate.

The following devices should therefore be included:

• Flow Switch
• Binder Point (for measuring temperature)
• Isolating Valves (for service, repair and replacement)
• Regulating Valve (for balancing water floe rates to design levels)
• Flow Measuring Device
• Strainer (for collection of particulate matter)
• Pressure Tappings
• Vibration Isolators

A typical arrangement for the positioning of the above items is shown in the following illustration:

Figure 1

 

Baudelot Cooler

A variety of applications require a fluid to be cooled to a temperature very close to the freezing point.  Baudelot coolers are therefore commonly found in within industrial refrigeration, dairy and a variety of food cooling applications.  Wine and wort cooling can also be achieved with this type of cooler.

The Baudelot cooler comprises a set of horizontal refrigerant tubes very similar in format to a conventional air to refrigerant evaporator coil.  However, this type of liquid cooler exposes the liquid to the atmosphere.   The refrigerant passes through the tubes in the same way as heat exchangers described previously.  The liquid to be cooled passes over the outside of the refrigerant tubes in the form of a thin liquid film thus causing aeration of the liquid which can be advantageous in certain food cooling applications.

If freezing of the liquid should occur, there will be no damage to the evaporator tubes as these are not confined within a shell which would otherwise cause collapse and fracture of the tubes due to the anomalous expansion of water.

The liquid to be cooled is fed to the upper section of the Baudelot cooler via a distributor arrangement to ensure equal supply of liquid over the bundle of evaporator tubes.  A collection tray at the base of the evaporator collects the chilled liquid.  The cooler can be insulated by surrounding walls in order to maintain high efficiency.  Ammonia is very often used with this type of cooler.  This requires a gravity feed system from a surge drum located above the chiller.  The level of refrigerant within the drum is maintained by a simple float control.

 

Shell & Coil Cooler

The shell and coil cooler is very basic and simple in concept and construction.  The refrigerant runs within a simple helical coil submersed in an enclosed vessel containing the liquid to be cooled.   The refrigerant coil can also be attached to the exterior of the vessel.  Although this will reduce the heat transfer efficiency, fracture of the refrigerant coil with subsequent release of refrigerant will prevent injection of the refrigerant into the liquid to be cooled which may be a concern in food related applications.

Unlike the conventional shell and tube evaporator where the volume of water is relatively low in relation to the tube bundle, the shell and coil cooler has a large body of liquid compared to the size of the refrigerant coil.  This large body of water can serve as a useful buffer to meet transient heat loads or to provide a limited amount of cooling time in the event of refrigeration system failure.

The cleaning of the liquid chamber in a dry expansion shell and tube evaporator is not possible unless the tube bundle is removed or cleaned by chemical means.  Cleaning of the shell and coil cooler is normally simple to achieve.  This type of liquid chilling heat exchanger is usually confined to small capacity applications.

 

Co-Axial Evaporators

This type of heat exchanger consists of a tube concentrically positioned within a larger tube.  The entire arrangement is then normally wound into a helical coil to form a compact heat exchanger assembly.  This type of heat exchanger is also very often used as a condenser, particularly in small unitary reverse cycle heat pump products.  The refrigerant usually flows through the inner tube and the water/fluid through the outer annulus, although this arrangement is reversed on certain products.

When used as an evaporator, the liquid refrigerant is injected at the base of the helix thus allowing the vaporised refrigerant to leave at the top connection.  This is reversed when applied as a condenser.  It also possible to arrange the water/liquid flow to run parallel with the refrigerant flow or contra flow.

The external tube is rated for 10 bar maximum and -20 Deg C to +100 Deg C.  The internal tube is rated for 26.5 bar maximum and -30 Deg C to +100 Deg C.

NEXT MONTH:  Volume 19 – Liquid Receivers.

 

DISCLAIMER:  Whilst every effort is made to ensure absolute accuracy, Business Edge Ltd will not accept any responsibility or liability for direct or indirect losses arising from the use of the data contained in this series of articles.