Quick Topics |Liquid Refrigerant Pumping||Superheat Suppression||

Pumping liquid refrigerant through a finned coil placed downstream of the direct expansion coil in an air conditioning system substantially increases the dehumidifying capacity of the system.

Author is a Professor of HVAC&R Engineering Technology at Ferris State University's College of Technology, Big Rapids, Michigan, 49307-2287.

INTRODUCTION Liquid refrigerant pumping technology has given refrigeration systems capacity boosts while at the same time saving electrical energy.

Liquid refrigerant entering the liquid line is pressurized with a small centrifugal pump shown in Fig. 1, by an amount equivalent to the pressure loss between the condenser outlet and the thermostatic expansion valve inlet on receiverless systems, or the receiver and the TEV inlet on TEV/receiver systems. The liquid becomes subcooled and will not flash if exposed to pressure drops in the liquid line.

Figure 1
Liquid centrifugal pump used in the liquid refrigerant pumping system.

The system is shown in Fig. 2 along with a normal system Fig. 3. Thus, by increasing the pressure of the liquid refrigerant, the associated saturation temperature is raised because of the pressure increase, while the actual liquid temperature remains the same.

Because the centrifugal pump's motor is external to the refrigeration system, and the impeller is driven by revolving magnetic field, negligible energy and heat is added to the system.The liquid is pressurized with negligible addition of temperature or heat, and allows for a completely sealed system because no drive shaft protrudes through the pump case. The centrifugal pump can increase the pressure of the liquid by approximately 55.15kPa to 137.8kPa (8 to 20) psi.

LIQUID REFRIGERANT PUMPING SYSTEM
	Since subcooling exists in any liquid below its saturation temperature for a given pressure, there are really several ways to subcool liquid. One way is to sensible cool the liquid in the bottom of the condenser to give it a sensible heat reduction to prevent flashing from liquid line pressure drops.However, subcooling this way will take up valuable condenser volume with subcooled liquid at its bottom, since condensing cannot occur in this area. This will cause higher head pressures and compression ratios, thus lower efficiencies.
If we consider only the subcooling of the liquid without regards to decreasing condenser surface area, we will see a gain of 1/2% of capacity for every degree of liquid subcooling**. However, if we consider the reduction of condenser surface area due to the liquid subcooling, there is a net loss in capacity due to increased condensing pressures and temperatures.

This type of subcooling, often called ambient subcooling, has been practiced for years and was thought to be a free method of subcooling. This simply is not the case. Ambient subcooling is usually accomplished at the cost of increased head pressures. It was used in refrigeration systems simply as a liquid seal in the condenser's bottom, and to prevent liquid line flash gas. This will keep a solid column of liquid supplied to the metering device.

Also, ambient subcooling cannot be maintained at a given level with only air side controls. Condensing pressures are directed related to the temperature of the condenser cooling medium and the useful condensing area in the condenser. Equation 1 defines useful or effective condensing area as total condensing area minus the area used for desuperheating and the area used for subcooling.

Equation 1-
Useful or effective condensing area = (Total condenser area) - (Condenser area used for desuperheating and subcooling)

**ASHRAE Fundamentals Handbook, Chapter 16, Tables 9 & 10.

As one can see, the more desuperheating and liquid subcooling that is done by the condenser, the less useful condenser area there will be. This will raise condensing pressures and compression ratios and cause inefficiencies with higher power draws.

A more efficient way to subcool liquid can be to increase the pressure of the liquid without raising the temperature. This will put the liquid at a higher pressure, thus it will have a higher associated saturation temperature, but will not change its actual temperature. This liquid is subcooled in an amount equal to the difference between the saturation temperature and the actual temperature. You now have liquid below its saturation temperature for that new pressure. By increasing the pressure of the subcooled liquid to overcome any pressure losses that occur in the liquid line, condensing pressures can be allowed to fall to their lowest pressures attainable. Another term for attaining the lowest possible head pressure is called floating the head pressure. Condensing temperatures of -6.66^oC (20^oF) are not uncommon in low temperature systems incorporating liquid pressure amplification.

However, if one tries to float the head pressure with the ambient, these lower head pressures will require more subcooling for the same pressure drops in the liquid line to prevent flashing. This phenomenon happens because the pressure vs. temperature graph of refrigerants is non-linear. The graph of pressure vs. temperature is much flatter at the lower pressures, meaning that the same amount of liquid subcooling is needed to overcome less of a pressure drop at these lower pressures and temperatures Fig 4.

Figure 4
Non-linear pressure vs. temperature curves of a halocarbon refrigerant.

This is one of the reasons why liquid refrigerant pumping is incorporated in the system when the head pressure is floated with the ambient. It subcools the liquid by increasing the pressure of the liquid and forces the liquid to have a new higher saturation temperature. Thus, flash gas is prevented when head pressures are allowed to float because liquid refrigerant pumping insures that the liquid line pressure (and saturation temperature) are always higher than the actual liquid temperature.

Also, as head pressure is reduced without liquid refrigerant pumping and the liquid experiences the same pressure drop through the liquid line as it did at the higher condensing pressures, the flash gas will now occupy more volume in the liquid line because of the higher specific volume of the flashed vapors. The thermostatic expansion valve will begin to hunt, starve the evaporator, and system capacity will be reduced. Consider the curve for a hydro chlorofluorocarbon (HCFC) refrigerant Fig. 5.

Figure 5
Temperature vs. percent Weight vs. percent Volume of a hydrochlorofluoocarbon (HCFC) refrigerant.

As the pressure in the liquid line drops, progressively more liquid will flash into vapor to cool the remaining liquid to the saturation temperature corresponding to the progressively lower pressure. With an 55.15 kPa (8 psi) pressure drop, the flash gas, by weight, will be 2% with a 37.77^oC (100^oF) condensing temperature. The vapor bubbles in the liquid line are now very compressed and occupy only 20% of the volume in the liquid line.

However, reduce the condensing temperature to 10.0^oC (50^oF) and the flashing vapor will occupy 38% of the liquid line volume. This vapor reduces the flow through the expansion valve, has little refrigeration effect, and must be recompressed after doing work. Again, system capacity will suffer, the evaporator will starve, and the thermostatic expansion valve will begin to hunt. This is the primary value of the liquid refrigerant pumping system - to insure solid liquid to the thermostatic expansion valve, so the valve can supply adequate liquid to the evaporator. In the past, designers of air conditioning and refrigeration systems picked an outdoor design condition for the system. This outdoor design condition typically was a temperature that will not be reached any more than 2% of the time in the system's life.

This design condition also occurs only a couple of hour at a time when reached. The selection of the condenser is usually made, however, based on this seldom reached condition.

Some years ago, when energy was much cheaper, designers would select condensing temperatures at 11.1^oC to 16.6^oC (20^oF to 30^oF) above the ambient. This was done because it was thought the higher condensing temperatures and pressures would enhance the flow through the metering device to outweigh any inefficiencies from the high compression ratios. This would force condensing temperatures and pressures higher, causing high compression ratios and lower efficiencies. With today's escalating energy costs, designers are specifying larger condensers with condensing temperatures 5.5^oC to 8.3^oC (10^oF to 15^oF) above the ambient. The significant energy savings from lower compression ratios and possibly increased subcooling of liquid negate the higher costs of the larger condenser.

After much research with metering valve suppliers, it was found out that thermostatic expansion valves would work with much less pressure drop across them than expected in the past, as long as pure liquid was supplied to them. The balanced port TEV design today is noted for its low pressure drop performance. With this new knowledge, condensing pressures and temperatures were allowed to float downward with the ambient temperature. In fact, a majority of the outdoor temperatures in the USA are below 21.1^oC

(70^oF) more of the time than they are above. Liquid pumping allows the systems to take advantage of lower ambient temperatures rather than force the condensing temperatures higher. The compressor capacity increases about 6% for every 5.5^oC (10^oF) drop in condensing temperature***. However, pressure drops across the expansion valve of less than 206.8kPa (30 psi) should be avoided for proper control of feeding of liquid to the evaporator. For an evaporator to operate at peak efficiency, it must operate with as high a percent of liquid to vapor ratio as possible entering the evaporator.

***ASHRAE Fundamentals Handbook, Chapter 16, Tables 9 - 10.

Advantages of a Liquid Refrigerant Pumping System

• Eliminate liquid line flashing by overcoming line pressure losses.
  
• Reduce energy costs because pumping liquid refrigerant is up to 40 times more efficient than using head pressure from the compressor to do the same work.
  
• Evaporator capacity will be increased along with the net refrigeration effect.
  
• Lower compression ratios and less stress on compressors, meaning longer compressor life.

Like it or not, the days of the fixed elevated head pressures are fading.No longer are consumers willing to pay for inefficiencies. Today's market demands efficiency. Customers are looking at life cycle costs - equipment costs plus the operational costs for the life of the equipment.

• Reduce the superheated vapor's heat intensity (temperature) so the pressure and volume of these superheated vapors will decrease.
  
• The superheated vapors will now reach saturation faster and thus begin to condense more quickly.
  
• Condensing of the vapors will occur closer to the inlet of the condenser. This will result in lower condensing temperatures and possibly more ambient subcooling.
  
• The overall condenser heat transfer will be higher because of the increased liquid/vapor mixture heat transfer and increased effective condensing area.

SUBCOOLING AND REHEAT COIL
In an air conditioning system, the evaporator removes moisture from the air. Therefore, it would seem reasonable that if the efficiency of the evaporator were increased, more moisture could be removed. As air is passed through an air conditioner's evaporator it reaches dew point and condenses into a liquid. The liquid is then removed as condensate. This process is called dehumidification.

Even mullion heaters loads can be reduced with lowered relative humidity in the supermarket. Mullion heaters simply keep the surfaces of the refrigeration cases above dew point so condensate will not form. Equipment for this moisture control has been expensive to buy, operate, and maintain. This system will reduce the energy usage on both the air conditioning and refrigeration systems. One of the key benefits of this system is the use of the internal energy of the system to accomplish reheating the air. No external heat source is used for reheat. Subcooling is also accomplished without any external energy source.