Vol 17 – Shell & Tube Evaporators (Continued)

Author Mike Creamer, Business Edge Ltd

 

AIR CONDITIONING TECHNOLOGY

Volume 17       

In the last article we commenced a study of various types of liquid chiller evaporators and their construction and features.  This month, Part 17 continues with shell and tube evaporators.

 

Tube Surface Enhancement

The performance of a 2 pass evaporator, where the refrigerant travels in one direction through 50% of the tubes and returns to the same end of the bundle through the remaining 50% of tubes, suffers where, after the first pass, a large proportion (approximately 50%) of the refrigerant has been evaporated and even distribution of the remaining 50% is more difficult to attain, especially as the balance of refrigerant also has to turn 180 degrees to return through the remaining tubes.

Whilst this halves the length of the overall bundle that would otherwise be required, thereby leading to a more practical design for integration within a packaged water chiller, the partial loss of performance must be offset by increasing the length of the bundle accordingly.

Tube surface enhancement,  for improved heat transfer, would offset the aforementioned disadvantages and the 2 pass shell and tube evaporator incorporating tube enhancement is therefore an ideal arrangement.

The use of a tube-within-a-tube, and a spirally wound copper crimped fin insert promoted annular flow, and led to a great improvement in heat transfer unmatched by others at that time.  Since then, heat transfer development has continued throughout the air conditioning industry and a number of other enhancement techniques have been used.  The most recent of these, the use of rifled microbore DX evaporator tubing, has allowed many manufacturers to offer compact shell and tube heat exchangers that can provide similar capacities to “Inner Fin” evaporators.

The need to protect the environment against the one of the causes of ozone depletion, i.e. CFC’s, and to a much lesser extent HCFC`s, has led to the development of HFC refrigerants such as R407C and R134a.  These refrigerants, although they have an ozone depletion potential (O.D.P.) of zero, required package chiller manufacturers to replace the mineral oils normally used with CFC’s and HCFC’s with polyolester lubricants, in order to maintain satisfactory oil transport within their chiller systems. It has been established, through extended factory testing, that the use of ester oils in evaporators having small bore rifled tubing can lead to a very significant reduction in capacity, as the grooved micro-rifling can become logged with oil, partially transforming rifled tubes into smooth bore tubes. Evaporators that rely on performance enhancement via grooved tube may therefore be unable to translate the calculated performance specification into reality, resulting in a high evaporator approach, reduced efficiency and possible shutdown due to occasional tripping of safety devices.

The “Inner Fin” shell and tube evaporator does not suffer from loss of capacity when operating with high viscosity ester based lubricants.  The comparatively coarse nature of the “Inner Finning” prevents oil logging, and a low approach is maintained.  For this reason, many manufacturers have for some years used at least one viscosity grade higher than the mineral oil it replaces when selecting ester oil for a particular application. This provides adequate protection against thinning of the oil by the HFC refrigerant used, preventing accelerated compressor wear rates. The ability of “Inner Fin” evaporators to maintain their performance when operated with HFC refrigerants and thicker oils offers an assurance that the specified performance will be attained.

 

Construction

The capacity range of “Inner Fin” direct expansion evaporators available can extend to 1200kW cooling capacity with the ability to specify up to four refrigerant circuits. All evaporators feature single refrigerant and water passes in counter-flow configuration for optimal heat transfer and the greatest protection against freeze up.  The smaller “CH” and “DCH” type evaporators feature copper tubes brazed into brass tube sheets.  The tube bundle is brazed into a steel shell, and steel end caps are then welded to the shell.  Finally, the threaded or flanged water spigots are welded to the shell before pressure testing.  Vessels may be supplied with the chilled water connection spigots arranged for vertical or horizontal connection.

Where potable water is to chilled, shell and tube evaporators can be supplied with a standard copper tube bundle but with a stainless steel shell.  Where more aggressive fluids are to be chilled, the entire tube bundle, shell, baffle plates, tube plates and connections can be constructed from 316 grade stainless steel.

The use of HFC refrigerants and ester oils places the manufacturer under a number of obligations. To ensure long term reliability of a chiller, it is vitally important that a proper pressure test and evacuation of the product is carried out to eliminate leaks.  Typically, a DX evaporator would be pressure tested in accordance with BSEN 378:

	Max. Working Pressure (M.W.P.)	Test
Shell Side (chilled liquid medium)	14 bar	21 bar
Tube Side (refrigerant)	21 bar	32 bar

 

Thorough evacuation and dehydration between 200 and 250 microns provides additional reassurance that the vessel is totally leak free.  The use of a helium leak detection system may be employed if a vacuum shows signs of deteriorating over the 15 minute hold period.  Finally, each vessel is charged with dry nitrogen at 0.5 bar pressure to prevent corrosion or moisture ingress during storage or transit.

As HFCs do have a reasonable global warming potential, and ester oils have a great affinity to absorb moisture, the importance placed on achieving a leak free and dry vessel cannot be overstated or this could lead to cross leaks, tube leaks and gasket problems.

Most chiller manufacturers insist on the installation of a paddle type flow switch to protect the evaporator against loss of chilled water flow.  This must be installed properly in a straight length of pipe in accordance with the switch manufacturer’s instructions. The use of a pressure differential switch in place of a flow switch will void the vessel guarantee.  For those evaporators operating with either R22 or R407C where water is the chilled media, a minimum saturated suction pressure of 3.5 bar gauge must be ensured via the use of a pressure switch or microcomputer/pressure transducer combination.  A minimum saturated suction pressure of 1.7 bar applies in the case of R134a evaporators. A minimum chilled media temperature of 3.3°C is permitted, where water is the chilled media.  For glycol solutions, a minimum solution temperature 3.3°C above the freezing temperature of the glycol concentration is permitted. These limits, in combination with the use of a flow switch, ensure that there is no danger of evaporator freezing. When R407C is used, a minimum mean evaporation temperature of 2.5°C is used during evaporator selection to compensate for the glide in the evaporator.  This prevents nuisance time delayed low pressure trips and provides further protection against freeze up.

 

Water Quality

Once installed in a packaged water chiller, there is a need to ensure proper control of water quality on site.  The services of a local water treatment specialist may be justified in order to maintain a neutral pH value on the waterside of the vessel.  The use of brines may require the use of special evaporator materials.  In particular, the use of Calcium Chloride on the chilled media circuit should be checked with the manufacturer before ordering.

Shell and tube evaporators will occasionally require cleaning.  In the event of a failure where the tube bundle may have to be removed or repaired it is essential to ensure that adequate space exists on site to facilitate withdrawal and re-insertion of tubes, an entire bundle or cleaning rods.

 

Chilled Water Flow Pumps

Two pumps are normally installed; one for normally running operation (Run) and one to bring into operation should the Run pump fail (Standby).  When designing a chilled media circuit, the constant flow pump should always be located to discharge into the evaporator inlet spigot, downstream of a 40-mesh strainer.

 

Evaporator Anti-Freeze Protection

To reduce the possibility of freeze up, the chilled media pump starter, flow switch contacts and anti-freeze thermostat should be electrically interlocked with the chiller control panel, so that if any of these detects a flow failure, each compressor is immediately stopped.

Many DX evaporators are not provided with sockets to permit the installation of suction pressure relief valves, but a relief valve or valves should be included in the suction piping of the chiller unit, to comply with the European Standard, and to protect the evaporator against over-pressurisation by unqualified site personnel during the commissioning process. This has been known to be particularly important if the hot and cold water piping circuits become accidentally crossed during system installation.

 

Refrigerant Flow Control

The variation of refrigerant flow and consistent close control of superheat in DX evaporators has a dramatic effect on their performance.  The use of electronic expansion valves offers a number of benefits. By pushing the refrigerant liquid dry out point (where all liquid refrigerant has evaporated and superheat begins) nearer to the suction end of the evaporator, more tube surface is used for evaporation and less in achieving the suction superheat necessary to protect the compressor against liquid slugging. This is only possible if the expansion valve control system is both sufficiently well advanced and correctly commissioned.  Another vital ingredient is the use of properly engineered refrigerant distribution systems within the evaporator.  The use of mild steel angle to create a splash plate is considered insufficient to create a homogenous liquid/gas mixture of equal quality across the inlet tube plate. Oscillations of the liquid dry out point across the tube bundle cross section would prevent the expansion valve(s) from maintaining a consistently low suction superheat.

Electronic expansion valves incorporate the use of pulsed solenoid valve devices to provide a reliable means of minimising suction superheat while maintaining stable suction pressure and evaporating temperature.  Rapid reaction to changes in evaporator / compressor load, and the ability to provide stable operation with low discharge / suction pressure differentials that allow efficient chiller operation at reduced head pressure are two advantages offered by this control technique.  The pulsed nature of the flow leads to a high level of refrigerant turbulence in the evaporator refrigerant inlet plenum, further improving distribution, thermal performance and oil return.  Additionally, the electronic control system allows the rapid alteration of refrigerant type, eliminating the need to change thermostatic elements within a wide capacity range whilst reducing package chiller manufacturers valve inventory.

 

NEXT MONTH: Vol 18 –  Water Chiller Evaporators continued.

DISCLAIMER:  Whilst every effort is made to ensure absolute accuracy, Business Edge Ltd/EvoMart 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.