As we introduced our Parker Sporlan webinar series we realized that we couldn't possibly answer all the questions in that short amount of time. We decided to create Climate Control blogs to answer some of the more pressing questions. This is the third of three blogs answering questions from our Supermarket Seminar Series: Metering Devices, TEVs.
Q: What is the minimum pressure difference across the TEV?
A: The existence of pressure-drop helps to facilitate flow through the thermostatic expansion valve or TEV. At some point, flow won’t occur if the pressure drop is too low. Manufacturers will typically provide ratings for expansion valves with a minimum pressure drop of 30 psid across the TEV. This does not include the pressure drop that would occur across the distributor if present in the system and the pressure drop across the evaporator.
Q: The pressure drop across TEV in the Correction Factor table ranges widely, 30 to 275 psig. How does a TXV with a widely varying condensing pressure act, say a freezer that condenses from 150 psig winter to 250 psig summer?
A: The ratings in the capacity tables for Sporlan TEVs are in accordance with ANSI/ARI Standard Number 750. The proper valve selection is critical to offering a good balance of control over the varying conditions. The TEV will ultimately attempt to control superheat at the bulb location even under varying conditions. As conditions vary, a situation may occur that is beyond the ability of the TEV to control superheat at the bulb location. Adjustment or replacement of the TEV may be required. In more extreme cases, there may exist a need for the hot gas bypass to control low side pressures and a head pressure control system to control high side pressures in order to stabilize system conditions. You stack the odds in favor of the TEV being able to control superheat with stable conditions. We recommend using our Virtual Engineer program to properly select valves based on those varying conditions.
Q: We need a TEV rated for R449A, but we don't have it in stock. Could we use a similar TEV rated for R22 as a replacement?
A: The mass flow rate for R449A is 7 to 10% higher compared to R22 depending upon system conditions. The Net Refrigerating Effect (NRE) of R22 is slightly higher compared to R449A as expected. In general, the existing R22 TEV will serve as a suitable replacement for the R449A application; however, this is no guarantee. It is good practice to evaluate the existing components with appropriate selection software or rating tables using the new system conditions.
Q: In the age of energy savings and reduced head pressure during cool weather conditions, what should we watch out for? How would we detect a problem with the TXV?
A: The TEV is intended to control the superheat at the sensing bulb location. Determining the superheat at the bulb location is one way to determine if something is amiss. Careful product selection and system commissioning is ever important today. System monitoring during various conditions is key with these new reductions in head pressure (walk-in coolers, etc.). There are many system parameters that should trigger an alarm condition and ultimately indicate some control problem has occurred. Low or High superheat at the bulb location would be one such condition. Unfortunately, a problem may not be detected until the case is warm and the product has been lost. Utilization of Sporlan’s Virtual Engineer program can assist with proper valve selection and help get things started correctly.
Q: On the MOP charge migration concern, wouldn’t heating the element affect valve function?
A: Yes, it will but in a good way. If the thermostatic charge constituents have all migrated to the diaphragm housing, the TEV will not be operational and it will not be controlling superheat at the bulb location. By warming the diaphragm housing or element, the charge constituents will be forced back into the bulb, once again making the TEV operational. A warm rag on the diaphragm housing can be used to determine if charge migration has occurred. The diaphragm housing should always be warmer than the bulb, especially with MOP-style thermostatic charges.
Q: If hot gas for capacity control is being utilized and introduced before the evaporator, how does that affect the bulb and valve?
A: In this instance, the hot gas will be mixed in the evaporator with the refrigerant being introduced from the TEV. The TEV will simply do its job of controlling superheat at the bulb. It will be influenced by the load on the evaporator and the hot gas that has been introduced at the inlet of the evaporator. In this scenario, the TEV will continue to act as the same superheat control that it did prior to the introduction of the hot gas. However, it will also act as a desuperheating device to temper the discharge gas.
Q: Why don’t manufacturers use bleed ports more often since it helps start the compressor?
A: Good question, maybe we should ask the equipment manufacturers. Bleed Ports are handy for restarting a unit against a pressure differential, for fine-tuning TEV capacity, for maintaining minimum suction pressure during startups when the system is equipped with a micro-channel condenser and the list continues. However, bleed ports prevent TEVs from seating tightly and this can complicate the rating process for manufacturers.
Q: Why do compressor manufacturers generally expect a higher superheat at the compressor inlet as compared to the evaporator outlet?
A: The TEV is intended to control the superheat at the sensing bulb location. As the superheated refrigerant travels through the suction line on the way to the compressor inlet, ambient conditions can contribute to the superheat of the refrigerant. This can happen if the suction line is uninsulated or if it is routed through high-temperature areas on the job site. This becomes a balancing act. Too little superheat at the compressor inlet and a flooded condition may damage the compressor. Too much superheat and the compressor may overheat.
Article contributed by Jim Jansen, senior application engineer, Sporlan Division of Parker Hannifin
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