How to boost your expansion device assembly's capacity
& overall system energy efficiency

Figure 1

Energy Savings

The expression above, Figure 1, was introduced in the July 2003 publication, to show that overall system power consumption will be reduced by some 40% of design power consumption in the event of moderate head pressure reductions, the said savings were expected when saturated condensing temperature is permitted to drop from 43°C to 23.5°C affected by continued condenser fan operation when ambient temperatures drop from the summer design of 28°C to the UK annual average of 10°C. The reduced compressor energy consumption, against the sustained fan energy consumption, brings about the overall 40% energy savings. However, the primary obstacle to maintaining system stability during reduced head pressure operation is the subsequent reduction in expansion device capacity occurring predominantly as a result of the reduced liquid pressures available ahead of the device.

TEV Capacity Loss

Then, in last months publication, the TEV capacity change expression was introduced, Figure 2, predicting TEV capacity change when, for constant suction side conditions, high side liquid conditions change. For an R22 system operating with a low side saturated temperature of 2°C, the high side reduction, mentioned above, brings about a 25% reduction in TEV capacity.

Figure 2

The expression of TEV capacity change predicts that if compressor capacity is caped at 66% or 75% of full load, during periods of energy saving reduced head pressure operation, the reduced available TEV capacity should still cope in the efforts to maintain designed saturated suction conditions, maintaining almost design evaporator superheat. A controls philosophy incorporated to achieve this compressor capacity limiting during low ambients may very well work, especially if system loads reduce proportionately.

TEV Capacity/Superheat Curves

Figure 3, below, illustrates a theoretical TEV capacity/superheat curve with dotted lines mapping compressor staging, TEV capacity and TEV superheat. For constant inlet and outlet pressures and inlet temperatures the TEV will open or close in response to sensed superheat through the 2K range of opening superheat, which perhaps could be called the "proportional band", existing between 2K and 4K of total superheat. In other words, this theoretical TEV has a static superheat of 2K, through which the orifice will remain perfectly shut, thereafter occurs the 2K opening superheat in combination giving the valve a total operating superheat of 4K. At 25% compressor capacity the total operating superheat would be 2.5K, 2K static superheat plus 0.5K opening superheat, then at 50% it would be 3K and so forth.

Figure 3

Figure 4, below, illustrates the modified TEV capacity/superheat curve associated with the energy-saving lowering of saturated condensing temperatures from the said 43°C to 23.5°C. The illustration shows that after the full span of 2K opening superheat the valve's modified capacity is only 75% of design and even the residual capacity, seen available at slightly higher superheats, would not match the compressors full load capacity, which we could assume still approximates the original design capacity of 100kW or more. Without finely set low side protection, such as LP switches, a new system component balance establishes at pressures considerably below design. Reduced instantaneous system power consumption may be witnessed but the increased run time will generally more than compensate for this, negatively, which results in an overall reduction in both system capacity and efficiency. In the case of standard water chillers and DX air conditioning systems, the resulting misbalance and evaporator starvation means that evaporator icing is the expected result with or without nuisance LP trips.

Figure 4

Optichiller TEV Supplement

Typically, in response to the above-described TEV troubles, many engineers opt to use either slightly oversized balanced port expansion valves or various types of electronic expansion valves. A few years back I thought of another potential solution, a concept I called OptiChiller, which is meant to be a simple, inexpensive and an easily retrofit-able solution. The idea is to supplement TEV capacity with a fixed orifice system comprising perhaps a ¼" liquid line solenoid valve and a cut-to-measure capillary tube, sized by trail and error, to offset the said TEV capacity shortage occurring at 100% compressor staging.

Figure 5

Figure 5 illustrates how the OptiChiller concept alters the TEV's superheat/capacity curve bringing total capacity back up inline with full load compressor capacity, maintaining the original design total operating superheat. The 4K operating superheat, seen in Figure 5, is being achieved at both 75% and 100% compressor staging. Interestingly, this TEV capacity supplement could be introduced even during design summer ambients when, with design liquid pressures, it would reduce the TEV's operating superheats such that the summer opening superheat "proportional band" might be caped at approximately 3.5K, increasing system capacity and efficiency somewhat. Functioning along side the supplementary fixed orifice expansion device the TEV would adjust to maintain superheat more stably than would two TEV's competing in parallel for control of a common suction line superheat.

Liquid Delivery

Like all expansion devices, the OptiChiller concept assumes good quality liquid is delivered ahead of the combined TEV and OptiChiller assembly. Proper liquid delivery is in fact probably the most complex of problems associated with reduced head pressure operation, regardless of expansion device type. The simplest means to maximise the chances of good liquid delivery to the TEV inlet is to configure the system for close coupling, minimizing distances between the condenser or receiver outlet and the TEV inlet. When close coupling is not possible then giving the liquid an increased gravity assisted downhill path to the TEV inlet does help. Where elevations permit, installing an inline flow-through liquid bulge after the condenser would allow for seasonal liquid line liquid mass alterations without too much disruption to available subcool established by liquid head if not in the condenser.