Dehumidification >> How To Dehumidify A Flooded Crawl Space

Intuitively, reheating a cooled air stream using hot discharge gas from a refrigerant compressor makes sense. However, it mitigates only the last of the four penalties of ordinary reheat systems mentioned in the sidebar. The refrigeration plant still must be larger by the amount of reheat. Also, the owner must pay the increased operating cost of the amount of How To Dehumidify A Flooded Crawl Space cooling canceled by the reheat. 

Refrigerant hot gas or condenser water reheat is inferior to the other dehumidification enhancements, How To Dehumidify A Flooded Crawl Space particularly when reheat is required on peak design days. This is because it requires a larger cooling-generation system and increased cooling-system operating costs. 

If reheat is required only on mild days and reheat from liquid refrigerant is not a viable solution, How To Dehumidify A Flooded Crawl Space this enhancement can be considered along with the recuperative-dehumidification enhancements discussed previously. Designers of refrigeration systems using hot gas for reheat are cautioned that the design is not as simple as it first appears. 

When the reheat coil is reheating, the cycle is obvious. When the control no longer requires reheat, How To Dehumidify A Flooded Crawl Space the reheat coil will be at a low refrigerant pressure and the coil will fill with condensed liquid refrigerant. When the control again calls for reheat, this liquid refrigerant must be displaced for the reheat coil to function. 

The system must incorporate a larger refrigerant storage vessel and additional refrigerant charge to accommodate the liquid refrigerant that is alternately stored in the reheat coil. Designers have tried numerous ways, How To Dehumidify A Flooded Crawl Space including check valves at the reheat coil outlet, to circumvent this problem. If the check valve holds, the reheat coil will be under very low pressure and invite leaks. 

Most valves have a small leakage rate, How To Dehumidify A Flooded Crawl Space which, in this case, means that the system still must be designed with the extra refrigerant charge and receiver. Some designers suggest bleeding a small amount of hot gas to the reheat coil when reheat is not required. This accomplishes nothing. The coil remains filled with liquid refrigerant. 

Some designers vent the reheat coil to compressor suction pressure when reheat is not required. Although this can be an acceptable solution, How To Dehumidify A Flooded Crawl Space it adds the risk of refrigerant-liquid flood back damaging the compressor when the system changes from reheat to no reheat. A variation of hot-gas reheat involves the use of the warm water leaving a water-cooled refrigerant condenser for reheat. 

This simplifies the refrigerant cycle and is a better alternative if the refrigerant cycle is water-cooled. However, penalties 1, 2, and 3 from the sidebar still apply. CONCLUSION In many cases, How To Dehumidify A Flooded Crawl Space the solution to indoor-air quality and energy efficiency problems can be found in 60- year-old technologies. 

The inventors of these technologies were motivated not so much by the cost of energy, but by the need to reduce compressor size to the limited range of available refrigeration compressors How To Dehumidify A Flooded Crawl Space and to keep electrical motor size to a minimum because of the limited peak power available from the electrical utilities of the 1930s. 

REFERENCES 1) Carrier, Willis H., Realto E. Cherne and Walter A. Grant. "Modern Air Conditioning, Heating, and Ventilating."1940. Pitman Publishing Corp. New York. 2) Ramsey, Melvin A. "Tested Solutions to Design Problems in Air Conditioning and Refrigeration."1966. Industrial Press Inc. New York. 3) Goodman, William. "Air Conditioning Analysis With Psychrometric Charts & Tables."1944. Macmillan Co. New York. 4) Goodman, William (ed.). Trane Air Conditioning Manual. 1939. The Trane Company. La Crosse, Wis. 

The third and final installment of this series will cover passive desiccant enthalpy heat exchangers, heat-powered active desiccant dehumidifiers, and self-contained or How To Dehumidify A Flooded Crawl Space packaged vapor-compression dehumidifiers. Ordinary reheat is not an efficient technology. It is included here to establish a basis of comparison for dehumidification technologies that are more energy-efficient. 

To comprehend the advantages of "enhanced"dehumidification systems, it is vital that owners and HVAC designers understand the four penalties of ordinary reheat systems, which are: 1) The first cost of the cooling generation plant, associated auxiliaries, How To Dehumidify A Flooded Crawl Space and electrical service is increased. 2) The first-cost premium of reheat coils includes increased electrical service and/or heating-distribution piping. 

3) The owner pays the annual operating cost of the additional sensible cooling of air. 4) The owner pays the annual operating cost of reheating air. Although ordinary reheat may be necessary as a final supplement to other forms of reheat, How To Dehumidify A Flooded Crawl Space it should not be used alone or first as a means of humidity control. Review applicable building-energy codes prior to designing or installing a "new-energy"reheat system. 

Reheat systems using "new"energy are restricted under many states building energy codes. Figure 1 is a psychrometric chart for a room with a 12-ton sensible-heat gain, a 7.7-ton latent-heat gain (87 lb or 84 pt of water per hr), a 19.7-ton total-room heat gain, How To Dehumidify A Flooded Crawl Space a 60-percent sensible-heat ratio, and 5000 cfm of outside air, which equates to a 21-ton outside-air load. 

The grand-total load is 40.7 tons, assuming that the HVAC equipment can condition 10,000 cfm of air to a 62-F dry bulb (DB) temperature and a 49-F dew point (DP). Unfortunately, How To Dehumidify A Flooded Crawl Space the constraints associated with the physics of cooling and dehumidifying air result in a cooling coil with a 50-F DB temperature and a 49-F DP. 

Although air at this temperature has enough moisture to absorb 87 lb of water vapor per hr, the 50-F air will over cool the space, causing the thermostat to shut off the cooling and stop further moisture removal. To absorb both the sensible and latent (moisture) gains of the space, How To Dehumidify A Flooded Crawl Space an ordinary-reheat system must utilize 39 KW of reheat. 

This 39 KW causes an 11-ton increase—from 40.7 to 51.7 tons (39 KW [3412 Btu per KWH] / [12,000 Btu per ton-hr] = 11.09 tons)—in the grand total cooling load. At an incremental cost of $550 per ton, How To Dehumidify A Flooded Crawl Space the 11-ton increase adds $6050 in first cost (penalty No. 1). The other three penalties associated with ordinary reheat for this example.

The 39-KW reheat coil (assuming electric) requires electrical circuits and possibly an incremental increase in switch-gear and distribution equipment. At an incremental cost of $50 per KW, this adds $1950 in first cost. n If the reheat coil is energized for dehumidification purposes 1500 hrs per year, an annual operating penalty of $3510 will result, How To Dehumidify A Flooded Crawl Space assuming an average electricity cost of 6 cents per KWH. 

The cooling-generation plant must produce an additional 16,634 ton-hrs of cooling per year, How To Dehumidify A Flooded Crawl Space which results in an annual operating penalty of $998, assuming an average electricity cost of 6 cents per KWH and a plant cooling factor of 1 KW per ton. The load in this example could be managed with either of the following technologies.

Vapor-compression (mechanical) cooling and reheat or (2) drying with a desiccant followed by cooling. At dew-point temperatures above 45 F, cooling with reheat generally is preferred to desiccant dehumidification because of the lower first cost, simplicity, longer life, How To Dehumidify A Flooded Crawl Space and nearly equal or lower operating cost. 

It is important to note that actual annual operating costs could double or triple, particularly if the electric-rate structure involves high demand charges or How To Dehumidify A Flooded Crawl Space ratchet provisions. The preceding analysis did not include the costs of additional cooling-coil heat transfer surface, the reheat coil, or the additional fan energy required because of extra air friction. 

Although these should be included in a detailed analysis, they were omitted here for clarity and How To Dehumidify A Flooded Crawl Space because enhanced-dehumidification options also involve additional heat transfer elements and associated air friction.


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